Posted online July 24, 2003
Management of the major postharvest decays of pears in California, gray mold caused by Botrytis cinerea and blue mold caused by Penicillium expansum, by the registered thiabendazole (TBZ) has become problematic in recent years due to fungicide-resistant populations. In addition, efficacy of the registered biocontrol Bio-Save is inconsistent. New alternatives for postharvest decay control were evaluated in studies that simulate commercial harvesting and handling practices. Using TBZ-sensitive and -resistant isolates in post-inoculation treatments, the new reduced-risk fungicides fenhexamid, fludioxonil, and pyrimethanil were highly active against gray mold, whereas blue mold was effectively controlled only by fludioxonil and pyrimethanil. In pre-inoculation treatments, fenhexamid was very effective against gray mold, whereas fludioxonil and pyrimethanil were effective against blue mold. For the first time in 30 years, new postharvest fungicides will be registered on pears. Their use in rotations and mixtures will reduce the development of resistant pathogen populations.
Branch dieback of Raywood ash in California. B. J. AEGERTER (1), T. R. Gordon (1), B. Slippers (2), and M. J. Wingfield (2). (1) Dept. Plant Pathology, University of California, Davis, CA 95616; (2) FABI, University of Pretoria, Pretoria 0002, South Africa. Publication no. P-2004-0002-PCA.
A branch dieback of unknown etiology has been observed in ornamental Raywood ash (Fraxinus oxycarpa ‘Raywood’) in northern California for over a decade. In 2002, a survey of 2640 Raywood ash in 21 cities in the Sacramento Valley and the San Francisco bay area revealed incidences of branch dieback ranging from 12 to 68% of trees. An association was observed between the disease and the presence of pycnidia of a Diplodia sp. formed within the bark of dead branches. Based on morphology and DNA sequence data this fungus was identified as Botryosphaeria stevensii (Diplodia mutila). Inoculations on branches of healthy Raywood ash resulted in sunken cankers which affected the cambium and underlying wood and contained characteristic pycnidia. The same fungus was isolated from the margin of affected tissues. Thus, B. stevensii appears to be involved in the etiology of branch dieback in Raywood ash. Studies are underway to document the progression of the disease both in observational plots of street trees as well as in experimental inoculations.
MIDAS™ (iodomethane), a new alternative for methyl bromide in strawberries and tomatoes. M. A. Allan. Arvesta Corporation (formerly Tomen Agro, Inc.), San Francisco, CA 94105. Publication no. P-2004-0003-PCA.
MIDAS™ (iodomethane) is a superior soil fumigant for the control of soil borne diseases, weed seeds, nematodes and insects. First market entries include strawberry, tomato, pepper, ornamentals, and tree and vine crops. Testing since 1999 has demonstrated control of soil pests and disease equivalent to methyl bromide with the following value added benefits: MIDAS offers efficacious control at reduced rates per acre (100 - 235 lbs/ treated acre) and may be applied using conventional equipment (shank - flat/raised bed or drip fumigated). MIDAS, once it has been injected into the soil, distributes itself through the soil profile bringing control of target pests in a variety of soil types and conditions. Its slower transition from a liquid to a gas compared to methyl bromide creates a longer concentration over time and less pounds per acre for equivalent control. MIDAS offers enhanced worker safety and environmental safety. US EPA has accepted the MIDAS ozone depletion potential of less than (0.002). MIDAS presents no threat to ground water. MIDAS is expected to receive US EPA registration in advance of the 2005 phase out.
Cucumber seed coating with abamectin guards against early root damage by root-knot nematodes. J. O. BECKER (1), B. Slaats (1), and D. Hofer (2). (1) Dept. Nematology, University of California, Riverside, CA 92521; (2) Syngenta Crop Protection, Basel, CH. Publication no. P-2004-0004-PCA.
Cucumbers are very susceptible to parasitism by the Southern root-knot nematode, Meloidogyne incognita. Severe infection reduces the root system and galled roots with a disorganized vascular system are impeded in water and nutrient uptake. Delaying nematode penetration during the highly sensitive seedling stage is often sufficient for establishment of a vigorous root system. Seed coating with the microbially derived nematicide abamectin at rates of ca. 7-20 g a.i./ha achieved good protection of cucumber seedlings grown in soil infested with M. incognita. Root length and plant height three weeks after seeding were significantly increased compared to non-treated control. Field trials with abamectin-coated cucumber seeds in M. incognita-infested soil resulted in yield increases up to 50 percent. This gain was mainly attributed to the increase in the number of fruits per plant. Seed coating with abamectin proved to be an effective tool to delay root-knot nematode damage and thereby significantly improve plant growth in M. incognita-infested soil.
Phytoplasma disease of watercress in Hawaii. W. B. BORTH (1), S. K. Fukuda (2), R. T. Hamasaki (1), and J. S. Hu (1). (1) Dept. of Plant and Environmental Protection Sciences; (2) Dept. of Tropical Plant and Soils Sciences, University of Hawaii, Honolulu, HI 96822. Publication no. P-2004-0005-PCA.
A yellows disease affecting watercress (Nasturtium officinale) in Hawaii has symptoms of reduced leaf size, leaf yellowing, and occasionally witches’ brooms. This disease is found on all the watercress farms on Oahu, but has not yet been found on other Hawaiian Islands. Watercress plants were tested for phytoplasma infection by polymerase chain reaction (PCR) assays using phytoplasma-specific primers. Amplicons of the expected sizes were produced from all symptomatic plants, but not from healthy plants raised from seed. Sequence analysis of the cloned PCR products indicate that the watercress is infected with a phytoplasma nearly identical to the severe strain of western aster yellows (SAY). Leafhoppers collected from affected watercress farms have been identified as Macrosteles sp. nr. severini, which had not been previously recorded in Hawaii. Six weed species, including Amaranth sp., Eclipta prostrata, Emilia sonchifolia, Plantago major, Myriophyllum aquaticum, and Sonchus oleraceus were also found to be hosts of this phytoplasma in Hawaii.
Control of Lophodermium twig blight and fruit rots of cranberry when fungicide timing was optimized for twig blight. P. R. BRISTOW and G. E. Windom. Dept. Plant Pathology, Washington State University, Puyallup, WA 98371. Publication no. P-2004-0006-PCA.
Ascospores of Lophodermium oxycocci infect leaves on new upright growth during the summer but the leaves do not blight until the following spring. The objective was to determine if fungicides timed to control twig blight also controlled fruit rots in the year the fungicides were applied. Blighted leaves were typically light tan with immature black ascocarps on the abaxial surface. Since 1997, uprights with dark reddish-brown leaves were observed among uprights with typical symptoms in the spring. Uprights with reddish-brown leaves were infected but only a few basal leaves showed typical symptoms. The first of 3 fungicide sprays for twig blight was based on the maturation of ascospores and repeat sprays were made every 14 days. Leaves on new upright growth were protected for approx. 6 wk (Jul - mid Aug). This timing usually coincided with the need to protect developing berries from infection by fruit rotting fungi. Control of twig blight by azoxystrobin and fenbuconazole equaled that of chlorothalonil and mancozeb. The incidence of fruit rots in the year of application was least in plots treated with azoxystrobin.
Infectious clones of Melon chlorotic leaf curl virus, a previously undescribed begomovirus species in the Squash leaf curl virus clade. J. K. BROWN and A. M. Idris. Dept. of Plant Sciences, University of Arizona, Tucson, AZ 85721. Publication no. P-2004-0007-PCA.
Melon chlorotic leaf curl virus (MCLCV) is a previously undescribed begomovirus of melon in Zacapa Valley, Guatemala. Symptoms were first observed in muskmelon and honeydew in spring, 2000 and disease incidence was 90-100%. Symptomatic melon exhibited leaf curling, shortened internodes, and spotting and cracking of fruit. Provisional identification was established by comparing the core Cp sequence with well characterized begomoviruses, which revealed <90% nucleotide (nt) identity, and suggested a new begomoviral species, herein MCLCV. A bipartite viral genome for MCLCV was cloned from DNA extracts of field-infected melon. Phylogenetic analysis for the DNA-A component revealed the closest MCLCV relatives were Squash yellow mild mottle virus (SYMMV) from Costa Rica and Squash leaf curl virus (SLCV) at 90 and 82% nt identity, respectively. The closest relatives of the MCLCV DNA-B component also were SYMMV and SLCV at 85 and 65% identity, respectively. Recombination analysis for MCLCV and its six closest relatives in the SLCV clade revealed no significant recombination.
Analysis of Squash leaf curl virus coat protein mutants reveals key amino acids involved in ingestion, acquisition, and transmission by the whitefly vector. J. K. BROWN (1), R. Caballero (1), K. Buckley (2), A. M. Idris (1), Z. Zhong (1), M. Hartitz (2), and D. M. Bisaro (2). (1) Dept. of Plant Sciences, Univ. of Arizona, Tucson, AZ 85721; (2) Dept. of Mol. Gen., Ohio State Univ., Columbus, OH 43210. Publication no. P-2004-0008-PCA.
Amino acid (AA) substitutions were introduced into the coat protein (CP) of Squash leaf curl virus (SLCV) to identify AA involved in whitefly (WF) Bemisa tabaci transmission. CP mutations were based on unique SLCV AA compared to the non-WF transmissible Abutilon mosaic virus. Mutants were tested for WF transmissibility. Plants were analyzed for presence of CP gene or CP by PCR + Southern and western analysis, respectively. WF transmission was considered evidence of wild type interactions between virions and the (i) gut membrane, (ii) WF chaperonin, and (iii) accessory salivary glands (ASG). Detection of viral DNA and CP in WF bodies and hemolymph indicated ingestion and gut passage of virions, respectively. Two non-transmissible mutants were defective for gut or ASG membrane interactions, respectively. WF ingestion of mutants and then of SLCV revealed that SLCV transmission was blocked, indicating mutants bind to but do not negotiate gut and ASG membrane receptors, respectively.
Pseudomonas syringae pv. alisalensis and Pseudomonas syringae pv. maculicola cause disease on crucifers used in cover crop mixtures. C. T. BULL (1), P. H. Goldman (1), R. F. Smith (2), and S. T. Koike (2). (1) USDA/ARS, Salinas, CA 93905; (2) Univ. Calif. Coop. Extension, Salinas, CA 93901. Publication no. P-2004-0009-PCA.
Pseudomonas syringae pv. alisalensis is a new pathogen causing bacterial leaf blight on crucifers in California. P. s. pv. maculicola is also an important pathogen of crucifers. We extended host range studies of these pathogens to crucifers used in cover crop mixtures because of the increased use of cover crops and the potential of diseased crucifers in cover crops to serve as inoculum sources for subsequent cash crops. In preliminary experiments, P. s. pv. alisalensis and P. s. pv. maculicola were sprayed to run-off on three-week-old crucifer seedlings. Plants were incubated for 48 hr in a mist chamber followed by four days in the greenhouse. Plants were considered to be hosts when leaf blight symptoms were present and the pathogen was isolated from symptomatic tissue. Varieties of Raphanus sativus, Brassica napus, and B. juncea were susceptible to P. s. pv. alisalensis, while only the first two species were susceptible to P. s. pv. maculicola. These data have implications for choosing cultivars in cover crop mixtures.
Survey of USDA/ARS research on plant diseases in organic agricultural systems. C. T. BULL. USDA/ARS, Salinas, CA 93905. Publication no. P-2004-0010-PCA.
Although organic agriculture is the fastest growing sector of the agricultural economy, organic production far exceeds the proportion of public investment in research in these systems. The number of scientific articles published on organic agriculture is increasing, but there are few publications on plant disease research in organic production systems. The official research branch of the USDA, the Agricultural Research Service, has no formal organic research program and currently does not track research conducted on organic agriculture. A 2003 survey identified 188 scientists that were interested in organic agricultural research. Eighty eight of these scientists have conducted or are conducting research in certifiable organic production systems. Approximately 10% of these scientists are plant pathologists or are working on plant disease problems. The majority of these plant pathologists are working on biological control of plant pathogens. Additional research is needed in all aspects of plant disease management in these production systems.
The use of Melaleuca oil for crop disease control. J. M. CAOLO-TANSKI (1), L. E. Hanson (2), A. L. Hill (2), J. P. Hill (1), and H. F. Schwartz (1). (1) BSPM, Colorado State Univ., Ft. Collins, CO 80523; (2) USDA-ARS, Ft. Collins, CO 80526. Publication no. P-2004-0011-PCA.
Melaleuca alternifolia oil (Mo) has been used widely as an antiseptic. The anti-microbial activity of Mo was tested in vitro against seven fungal and two oomycetous plant pathogens. Different volumes of Mo were applied as equal aliquots on four 10 mm diameter filter paper disks set equidistant on 30 ml Potato Dextrose Agar plates, followed by centrally placing a 7 mm mycelial plug per plate. Growth of Pythium and Phytophthora was completely inhibited at 20 and 60 µl, respectively. Four fungi had no growth at 60 µl and all but one had no growth at 100 µl. Sugarbeet and potato were used to determine the efficacy of Mo for disease control in field plots. Leaves were treated for foliar pathogens and crowns (sugarbeet) for soil-borne pathogens. No significant differences were found (P > 0.05). In greenhouse tests, sunflower treated with Mo had significantly (P < 0.02) higher white mold ratings than the untreated check (perhaps due to phytotoxicity). Mo inhibited plant pathogen growth, however further research is needed to determine if Mo can reduce disease severity.
Effectiveness of controlled release applications of chlorine dioxide gas in killing pathogen inocula. G. A. CHASTAGNER and K. L. Riley. Washington State University, Puyallup, WA 98371. Publication no. P-2004-0012-PCA.
Chlorine dioxide is a biocide that is being used in place of formaldehyde to prevent the spread of Fusarium basal rot during the hot water treatment of daffodil bulbs. Due to recent advances in application technology, it may also be possible to use controlled release gas applications of chlorine dioxide for killing pathogen inocula on the surfaces of bulbs and other substrates. To determine chlorine dioxide’s effectiveness in killing pathogen inocula, a series of tests were conducted by exposing inocula of Alternaria alternata, Botrytis cinerea, Fusarium oxysporum f. sp. narcissi, Penicillium corymbiferum and Rhodococcus fascians on glass cover slips to various concentrations of gas generated from sachets (ICA TriNova, LLC). The exposure period was one hour at 20C. Following exposure, inocula were placed on media to assess their viability. Exposure of inocula to 5 ppm chlorine dioxide (the lowest rate tested) reduced viability by 79 to 100%. At 10 ppm, 92 to 100% of the inocula were killed. Tests are underway to determine the effects of chlorine dioxide gas on inocula viability on bulb surfaces and to determine if gas exposure affects bulb growth.
IR-4 fungicide study and registration in tropical crops. H. CHEN (1), M. Kawate (2), D. C. Thompson (1), V. R. Starner (1), and D. L. Kunkel (1). (1) IR-4 Project, Rutgers University, N. Brunswick, NJ 08902; (2) University of Hawaii, Honolulu, HI 96822. Publication no. P-2004-0013-PCA.
IR-4 conducts about 100 residue studies each year with about 30% in fungicides. So far, Hawaii has submitted 262 project clearance requests to IR-4, and 63 residue studies have been completed and uses registered including 16 fungicides or nematicides. IR-4 utilizes crop groups to obtain residue tolerances on a large number of crops. The crop-grouping scheme allows IR-4 to obtain many clearances for crops in designated groups with residue data from representative crops of the group. In 2002, IR-4 obtained 531 new food-use clearances. Among these clearances, 17 were for tropical crops. These 17 new uses in tropical crops were generated from only two residue studies. Lastly, to support registration of the vast number of uses and for prioritizing projects, efficacy data is needed. IR-4 is conducting limited efficacy studies on crops including tropical crops for the pests of high interest. We encourage researchers and commodity groups to communicate with us about fungicide efficacy data that identify useful fungicides or nematicides for control of plant diseases.
Purification and characterization of a proteinase from Trichoderma atroviride. M. CHENG, P. A. Gay, and J. H. McBeath. Plant Pathology and Biotechnology, University of Alaska Fairbanks, Fairbanks, AK 99775. Publication no. P-2004-0014-PCA.
A cold tolerant Trichoderma atroviride has been found to have strong biocontrol potential against Botrytis cinerea, Phytophthora infestans, P. capsici, Pythium spp, Sclerotinia sclerotiorum, Typhula spp, and Verticillium dahliae. However, mechanisms of mycoparasitisms have not been completely elucidated. Trichoderma proteinase may play a role in lyses of these pathogens by degrading fungal cell walls. To study the role of proteinase in mycoparasitism, autoclaved mycelia of Rhizoctonia solani, S. sclerotiorum, B. cinerea and P. capsici were used to induce T. atroviride to produce proteinase. Specific proteinase activity peaked in one to two days. Glucose inhibited proteinase production. One 18.8 kD proteinase was purified to electrophoretical homogeneity. The optimal pH of this proteinase was 9.0. Purified proteinase was found to inhibit conidial germination of B. cinerea by 80.2-86.7 %. We are determining the amino acid sequence of part of the proteinase in order to clone its biosynthetic gene.
Susceptibility of Hansen 536 peach:almond hybrid rootstock to bacterial canker in California stonefruit orchards. R. A. DUNCAN (1) and M. V. McKenry (2). (1) University of California Cooperative Extension, 3800 Cornucopia Way, Suite A, Modesto, CA 95358; (2) University of California Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648. Publication no. P-2004-0015-PCA.
Bacterial canker (bc) of almond and stonefruit trees in California is caused by Pseudomonas syringae pv. syringae in association with orchards replanted into ring nematode (Mesocriconema xenoplax) infested soil. No bc resistant rootstock has been identified for commercial use. High vigor peach:almond hybrid rootstocks, such as Hansen 536, are becoming popular as growers try to achieve early, high yields. Six, commercial rootstocks were evaluated for performance in a bacterial canker site in a field trial with eight, four-tree replicates. The third generation peach orchard site was fumigated prior to planting with 448 kg/ha of 98%:2% methyl bromide:chloropicrin. By the fifth year after planting, 88% of trees on Hansen 536 died of bc, compared to 8% and 0% on rootstock standards Nemaguard and Lovell, respectively. Hansen 536 harbored high numbers of ring and root lesion (Pratylenchus vulnus) nematodes but low numbers of root-knot nematodes (Meloidogyne incognita).
Acquisition and aerial dissemination of Fusarium and Verticillium by adult shore flies. Z. A. EL-HAMALAWI and M. E. Stanghellini. Department of Plant Pathology, University of California, Riverside, CA 92521. Publication no. P-2004-0016-PCA.
Adult shore flies were demonstrated to function as aerial vectors for two vascular wilt pathogens: Verticillium dahliae and Fusarium oxysporum f. sp. basilici. Adult insects were attracted to sporulating cultures of these fungi, and conidia of both pathogens were acquired by adult insects internally by ingestion and externally by surface contamination. Large numbers of viable (95%) conidia were excreted (80,000 to 500,000 conidia per frass deposit, depending on the particular fungus) by adult insects. These insects were capable of transmitting the pathogens to healthy host plants, which subsequently became infected. Additionally, shore flies completed their entire life cycle (i.e., egg to egg) solely on a diet of each of these two fungal plant pathogens. Viable conidia of both pathogens persisted through pupation following ingestion by larvae. These results support and extend our conclusions from previous studies that adult shore flies may serve as an aerial vector for the introduction and spread of soilborne plant pathogens either into or within greenhouse facilities.
Pathogenicity of Phialophora sp. on grapevines in California. A. Eskalen, S. R. Latham, and W. D. Gubler. Dept. of Plant Pathology, Univ. of California, Davis, CA 95616. Publication no. P-2004-0017-PCA.
Phialophora sp. was isolated from grapevine roots and trunks with vascular discoloration. In the vineyards, affected plants exhibited a general physical stress condition. Diseased plants were found in eight geographic viticulture regions in California. Fungal identification was made using morphological characters and ITS sequence data. Species identification indicates potential of a new species. To prove pathogenicity, dormant grapevine cuttings were dipped in a spore suspension (10(^6) spore/ml). Cuttings were then planted in sterile soil mix and placed in a greenhouse. The extent of vascular discoloration was measured from the inoculated tip ten months later. Average extent of vascular streaking in Pinot Noir, Thompson Seedless, Chardonnay and Cabernet Sauvignon varieties were 8.4, 9.7, 7.6 and 11.2 cm, respectively. Pruning wounds of six-year-old Cabernet Sauvignon were inoculated in the field and averaged 9.2 cm of discoloration. The pathogen was reisolated in both experiments on potato dextrose agar amended with tetracycline (0.1g/L). This is the first report of Phialophora sp. occurring as a pathogen of grapevine in California.
Detection of Erysiphe (=Uncinula) necator with the polymerase chain reaction and species-specific primers. J. S. FALACY, H. Galloway, and G. G. Grove. Dept. Plant Pathology, Washington State University IAREC, Prosser, WA 99350-9687. Publication no. P-2004-0018-PCA.
A polymerase chain reaction (PCR) assay employing species-specific primers was developed to differentiate Erysiphe (=Uncinula) necator from other powdery mildews common to the Pacific Northwest, United States. Conidia, cleistothecia or mycelium were collected from grape leaves using a Burkard cyclone surface sampler and their DNA extracted. Primer pairs, Uncin144 and Uncin511, were developed by aligning internal transcribed spacer sequences (ITS) and choosing unique regions that distinguish E. necator from its closest powdery mildew relatives. Primers specifically amplified DNA products of E. necator, but not from powdery mildew species collected from 35 disparate hosts. The appearance of a single 367 base pair fragment by gel electrophoresis was considered evidence of successful detection. Amplification products were sequenced to verify the specificity of E. necator primers. This PCR-based test could enable the detection of E. necator in field samples within hours of collection.
Molecular safety assessment of transgenic papayas harboring the chimeric coat protein (CP) of Papaya ringspot virus. G. FERMIN, R. Keith, and D. Gonsalves. PBARC-PWA-ARS-USDA, 99 Aupuni St., Hilo, HI 96720. Publication no. P-2004-0019-PCA.
CP transgenic papayas resistant to Papaya ringspot virus (PRSV) have already been commercialized. To reach this stage and to comply with regulatory demands tests must be performed to assess the safety of the product, besides its ability to be resistant to the target virus. A hemizygous line (CP/-) was used to clone the transgenic insertion to establish the bordering genomic sequences, the order and arrangement of the transgenes or other changes to the plasmid used for transformation via bombardment. We also analyzed the potential emergence of new open reading frames, and we were able to reengineer the PRSV CP in a bacterial expression system for the purpose of conducting digestion analysis by simulated gastric and intestinal fluids. The cloned insertion revealed that only the testable transgenes in planta (CP, uidA and nptII) were inserted as a whole unit, that the site of insertion is non-coding DNA, that no other potential open reading frames were created upon insertion into the papaya genome, and that the CP expressed in planta is readily digested in simulated digestive fluids.
Preharvest applications of fungicides and a biocontrol agent for postharvest management of gray mold of strawberry fruit. H. Förster, A. O. Paulus, M. Vilchez, K. Prabakar (1), and J. E. ADASKAVEG. (1) TNAU, Coimbatore, India, and Dept. Plant Pathology, Univ. of California, Riverside, CA 92521. Publication no. P-2004-0020-PCA.
Gray mold (Botrytis cinerea) is a ubiquitous disease of strawberry fruit. With the loss of older protective fungicides, new alternatives are needed. In field trials, 2 or 3 preharvest sprays reduced postharvest gray mold from 86% in the check to 9, 24, and 27% by the fungicides fludioxonil/cyprodinil, fenhexamid, and boscalid/pyraclostrobin, respectively. In contrast, decay incidence with the biocontrol yeast Candida oleophila (Aspire) was 78%. Populations of the yeast on treated fruit decreased from 10 to 0 cfu/mm(^2) within 0 to 20 h after application. Similar results were obtained on laboratory-treated fruit, indicating that abiotic factors in the field did not cause the decline in population size. Furthermore, incubation of the biocontrol agent in water/fruit-disk solutions did not affect its viability. Thus, the yeast is not competitive in the phyllosphere or population levels are too low to effectively compete against B. cinerea. The new fungicides, each belonging to a new class, offer the best future management strategies.
Preliminary assessment of taxonomic diversity of Erysiphales (powdery mildews) in the Pacific Northwest. D. A. GLAWE. Dept. Plant Pathology, Puyallup Res. and Ext. Center, Washington State University, 7612 Pioneer Way E., Puyallup, WA 98371-4998. Publication no. P-2004-0021-PCA.
In 2002, work began on a taxonomic revision of Erysiphales in the Pacific Northwest (AK, BC, MT, ID, OR, and WA). 550 specimens on 56 host families were collected. Collections included 100 new host records for the region from 47 host families. Families with 3 or more new host records included (in decreasing order of new records): Asteraceae, Lamiaceae, Berberidaceae, Scrophulariaceae, Boraginaceae, Brassicaceae, Cucurbitaceae, Fabaceae, Fagaceae, Papaveraceae, and Rosaceae. Comparison of names of Erysiphales in Shaw’s Host-Fungus Index for the Pacific Northwest and Farr et al.’s Fungi on Plants and Plant Products of the United States with recent work by U. Braun and others emphasizes that newer taxonomic systems tend to use narrower species concepts, and significantly different nomenclature, than older systems. It appears that taxonomic diversity of Erysiphales in the region has been greatly underestimated.
Pathogenicity of 14 isolates of Phoma sclerotioides pathogenic to alfalfa. F. A. Gray (1), C. R. Hollingsworth (2), C. J. Reedy (1), L. B. Powers (3), D. E. Legg (1), and R. W. Groose (1). (1) Univ. of Wyo.; (2) Univ. of Minn., Crookston; (3) Ohio State Univ. Publication no. P-2004-0022-PCA.
Fourteen isolates of Phoma sclerotioides were compared for pathogenicity in producing root rot in alfalfa. Thirteen isolates were obtained from diseased alfalfa plants collected in Wyoming and one was from Canada (ATCC #56515). Barley grain inocula of each of the 14 isolates were placed adjacent to the upper root of 4-6 month old potted alfalfa plants. Following inoculation, plants were placed outside the greenhouse for a winter exposure period. In the spring, plants were given a Disease Severity Rating (DSR) for Brown root rot on a scale of 1-5 (1 = healthy, 5 = severe root rot, plant dead). Surviving plants were placed outside for a second winter and again rated for Brown root rot the following spring. The entire experiment was repeated once. Final DSRs for experiments 1 and 2 were 4.83 and 4.53, while final mortality was 91% and 59.5%. Although all 14 isolates were pathogenic to alfalfa, some appeared to be more pathogenic than others. WY isolate #2, being used in our breeding program, ranked 5th with a DSR of 4.17. This isolate is available from the American Type Culture Collection, #MYA-295.
Phylogenetic analyses of Xylella fastidiosa strains isolated from ornamental hosts. R. HERNANDEZ-MARTINEZ (1), C. K. Dumenyo (1), H. Azad (1), H. S. Costa (2), F. P. Wong (1), and D. A. Cooksey (1). (1) Dept. Plant Pathology; (2) Dept. Entomology, University of California, Riverside, CA 92521. Publication no. P-2004-0023-PCA.
Xylella fastidiosa (Xf) strains cause several economically important plant diseases including Pierce’s disease (PD) on grapevines, almond leaf scorch and oleander leaf scorch (OLS). We have isolated, for the first time, Xf from liquidambar, olive, and ornamental plum trees. In this study, we evaluated the phylogenetic relationships of these new isolates with strains from other hosts through analyses of 125 PCR fragments amplified with specific Xf primers and DNA sequences of the 16S-23S rRNA spacer region. Both analyses produce similar results with very little variations. In both cases, almond isolates are distributed in several clusters, the ornamental plum isolate is close to the almond strain Dixon and the olive seems related to some other almond strains and is not clustered with the PD or OLS strains which are causing devastation in California. Finally, the liquidambar isolate forms a separate group in the PCR analysis, and is clustered with the phony peach isolate in the 16S-23S analysis.
Field trials to evaluate alternatives to pre-plant soil fumigation in Idaho forest nurseries. R. L. JAMES (1) and J. K. Stone (2). (1) USDA For. Serv., Forest Health Protection, Coeur d’Alene, ID 83814; (2) Dept. of Botany and Plant Pathology, Oregon State Univ., Corvallis, OR 97331. Publication no. P-2004-0024-PCA.
Field trials were conducted to evaluate pre-plant soil fumigation alternatives in two USDA Forest Service bare root forest nurseries in Idaho. In one nursery (CDA), bark compost and sewage sludge amendments, fallow with periodic cultivation and pine needle mulch were compared with dazomet fumigation in production of Douglas-fir seedlings. In the other nursery (LP), mushroom compost and sawdust amendments, and fallowing with cultivation were compared with methyl bromide/chloropicrin fumigation in production of ponderosa and lodgepole pine seedlings. Density and diameter of 2-0 seedlings were the best measure of seedling quality. At CDA, addition of bark or sewage sludge compost improved seedling density in one trial. At LP, bare fallow with cultivation and sawdust amendments resulted in seedlings of comparable quality to fumigation. Our results indicated that modification of cultural practices may reduce need for pre-plant soil fumigation in forest nurseries. Non-fumigation alternatives must be specifically designed for each nursery.
Aerial photography used for spatial pattern analysis of late blight infection in irrigated potato circles. D. A. JOHNSON (1), J. R. Alldredge (2), and P. B. Hamm (3). (1) Dept. of Plant Pathology, Washington State University, Pullman, WA 99164-6430; (2) Dept. of Statistics, Washington State University; (3) Oregon State University, Hermiston, OR. Publication no. P-2004-0025-PCA.
Spatial and temporal dynamics of late blight were investigated from color, infrared aerial photographs of five commercial potato fields in the Columbia Basin. Aerial photographs were taken at 6 to 21 day intervals, scanned and pixels, representing 1 m(^2) in the field, were used in the analysis. Variograms indicated the existence of autocorrelation among infected plants in four directions; the range of influence increased as disease incidence increased except at the highest levels of disease. A field where initial inoculum likely originated from infected seed tubers exhibited less initial aggregation than the other fields, perhaps due to a different source of primary inoculum. Aerial photography coupled with spatial analyses of late blight infected plants was an effective technique to quantitatively assess disease patterns in relatively large fields and was useful in quantifying an intensification of aggregation during the epidemic process on a large scale.
Epiphytic growth of Erwinia amylovora on flowers that are nonhosts of fire blight. K. B. JOHNSON, T. L. Sawyer, and V. O. Stockwell. Dept. Botany & Plant Pathology, Oregon State University, Corvallis, OR 97330-2902. Publication no. P-2004-0026-PCA.
The epiphytic ecology of the fire blight bacterium, Erwinia amylovora, is not understood completely. As part of studies to enhance this understanding, we evaluated the potential for flowers of nonhosts of fire blight to support epiphytic growth of E. amylovora. Floral bouquets of common nectar and pollen sources (Acer, Amelanchier, Brassica, Cytisus, Populus, Prunus, Rubus, Salix, Taraxacum, Trifolium and Symphoricarpus) were inoculated with E. amylovora and incubated at 15°C for 96 h. Nonhosts of the rose family as well as Acer, Cytisus, Populus and Salix supported high epiphytic populations of E. amylovora. Brassica, Taraxacum and Trifolium, however, were relatively poor supporters of epiphytic growth. Because vectors of E. amylovora, principally bees, visit many kinds of flowers in landscape areas between orchards, our data indicate in that flowers of nonhosts are potential inoculum reservoirs at certain times of the season.
A filtration and colony blot immunoassay for Clavibacter michiganensis subsp. michiganensis in tomato seeds. W. S. KANESHIRO and A. M. Alvarez. Dept. Plant and Environmental Protection Sciences, University of Hawaii, Honolulu, HI 96822. Publication no. P-2004-0027-PCA.
Bacterial canker, caused by the seedborne pathogen, Clavibacter michiganensis subsp. michiganensis (Cmm), is a serious tomato disease that occurs worldwide. Reliable seed assays are essential for detection and identification of Cmm in seed lots. A procedure consisting of a three-unit filtration system to capture bacteria from seed extract, followed by a membrane incubation period and a colony blot immunoassay was developed to test 50 ml seed extract volumes for Cmm infestation. Cmm was recovered when as few as 10 CFU per sample (0.2 CFU per ml) were initially added to the system. The CMM1 antibody used in the immunoassay differentiated Cmm from bacterial saprophytes even when Cmm-to-saprophyte ratios reached 1:3500. When compared with a standard spread plating assay using two semiselective media, the filtration and immunoassay technique identified 6 of 6 infested seed samples, while the plate assay identified 4 of 6 samples. This technique improves detection sensitivity over existing seed assays, allows accurate Cmm identification, and can reduce assay time by as much as 3 weeks.
Alginate gene expression by Pseudomonas syringae pv. tomato DC3000 in planta. R. C. KEITH (1), L. M. Keith (1), G. Hernández-Guzmán (2), S. R. Uppalapati (2), and C. L. Bender (2). (1) PBARC, USDA-ARS, Hilo, HI 96720; (2) Dept. Plant Pathology, Oklahoma State University, Stillwater, OK 74078. Publication no. P-2004-0028-PCA.
P. syringae produces the exopolysaccharide alginate. In this study, an algD::uidA transcriptional fusion (pDCalgDP) was constructed to monitor alginate gene expression in plants inoculated with P. syringae pv. tomato DC3000. When susceptible collard plants were spray-inoculated with DC3000(pDCalgDP), algD was activated 72 hours post-inoculation (hpi) and was associated with the development of water-soaked lesions. In susceptible tomato cv. Rio Grande-PtoS, algD activity was lower than in collard and was not associated with watersoaking. The expression of algD was also monitored in tomato cv. Rio Grande-PtoR, which is resistant to the pathogen.12 hpi, micro-HR was observed in Rio Grande-PtoR spray-inoculated with P. syringae pv. tomato DC3000(pDCalgDP). Results indicate that algD is expressed in both susceptible and resistant host plants. This study demonstrates alginate gene expression in planta, indicating that alginate gene expression occurs in both compatible and incompatible plant host-pathogen interactions.
Identification of a SCAR marker linked with resistance to Beet curly top virus in snap bean. R. C. LARSEN and P. N. Miklas. USDA-ARS, Prosser, WA 99350. Publication no. P-2004-0029-PCA.
Beet curly top virus (BCTV) is a persistent disease problem for bean production in the Pacific Northwest and other arid regions where the virus and leafhopper vector, Circulifer tenellus, are present. Breeding for resistance has been difficult because field screening usually results in sporadic infection across and within test plots. To circumvent the lengthy screening process required for identification of genetic resistance, a DNA marker tightly linked with a major BCTV resistance gene was developed with application for marker-assisted selection (MAS). Separate DNAs bulked from eight curly top-resistant recombinant inbred-lines (RILS) and eight curly top-susceptible RILs were screened with 750 random decamer RAPD primers. A 1550 bp RAPD marker that selectively amplified products present in the resistant bulk DNAs was converted to a SCAR marker to facilitate high throughput MAS of the Ctv-1 resistance gene across breeding programs. The SCAR was located 1.1 cM from Ctv-1 in a population of 96 RILs. A survey of 70 cultivars and breeding lines revealed that this marker has broad application for MAS of Ctv-1 in snap bean.
Initial evaluation of the relative resistance of lettuce cultivars to Fusarium wilt. M. E. MATHERON (1), J. D. McCreight (2), B. R. Tickes (3), and M. Porchas (1). (1)Yuma Agricultural Center, University of Arizona, Yuma, AZ 85364; (2) USDA, ARS, U.S. Agricultural Research Station, Salinas, CA 93905; (3) University of Arizona Cooperative Extension, Yuma, AZ 85364. Publication no. P-2004-0030-PCA.
Fusarium wilt of lettuce was observed for the first time in six different fields in western Arizona in the 2001-2002 growing season. Eleven additional sites were revealed in 2002-2003. A cultivar evaluation trial was conducted in a field known to contain the wilt pathogen, Fusarium oxysporum f. sp. lactucae, to determine the relative resistance of lettuce cultivars grown in the desert to Fusarium wilt. The 127 tested cultivars were grouped into three different planting dates: Sep 7, Oct 17 and Dec 6, 2002. In general, Fusarium wilt in the first, second or third planting was very severe, moderate or very mild, respectively. Disease severity decreased with declining soil temperatures, which ranged from 18 to 30, 12 to 23, and 9 to 18°C, during the first, second and third planting, respectively. Among lettuce types tested, head lettuce was usually most susceptible, whereas romaine was most tolerant. This field contained race 1 of the pathogen.
Isolation of a tymovirus from ornamental plants in southern California. D. M. Mathews, J. A. Heick, and J. A. DODDS. Dept. Plant Pathology, Univ. of California, Riverside, CA 92521. Publication no. P-2004-0031-PCA.
Continual removal of cuttings from ornamental nursery plants for propagation material creates difficulties for indexing in mother plants. Three ornamental hosts, Verbena, Diascia, and Lobelia, were all suspected to be virus infected. Systemic symptoms were obtained when each was inoculated onto Nicotiana clevelandii. DsRNAs were found associated with each spp.: 3 segments, ca. 3,000-11,000 nt (Verbena); 6 segments, ca. 300-11,000 nt (Diascia); and 4 segments, ca. 1,200-4,000 nt (Lobelia). The dsRNA titer was highest from the Diascia plants so our virus purifications focused on this plant initially. A SDGC profile with 2 peaks similar to that expected for a tymovirus was found. The first peak was empty capsids and the other peak yielded a nucleoprotein with a ssRNA of approx. 6,500 nt and a coat protein of 20 kD. Intact virions moved toward the anode in agarose gel electrophoresis. Purified virus caused the same mosaic symptoms on N. clevelandii seen from the original host tissue and the same dsRNAs were recovered. These analyses all point to the presence of a member of the tymovirus group.
Moncut™: A flutolanil-based systemic fungicide for control of Rhizoctonia disease in potato. G. L. MELCHIOR. Gowan Co., Walla Walla, WA 99362. Publication no. P-2004-0032-PCA.
Moncut is the trade name for formulations containing flutolanil, a fungicidal compound belonging to the benzanilide class of chemistry. Spectrum of activity of flutolanil is specific to the Basidiomycete class of fungi, and especially to Rhizoctonia solani. Mode of action is inhibition of succinate dehydrogenase complex II, a critical enzyme complex for respiration. Yield and grade losses to Rhizoctonia infection have been increasing in major potato production areas. Small plot and commercial scale studies were initiated in 1998 in potatoes in the Pacific Northwest to evaluate in-furrow applications of Moncut at planting to control soil-borne Rhizoctonia. In-furrow placement of flutolanil (half-life = 40 - 60 days) is critical for long-term uptake and protection within the rhizosphere of the developing potato crop. Research trials show significant and consistent high level reduction in Rhizoctonia stem and stolon canker at active ingredient rates of between 0.5 - 0.75 pounds per acre, resulting in significantly improved yields and tuber grade. In over 85% of commercial scale applications, Moncut increased yield and tuber grade, and provided a positive return on investment.
Iris yellow spot virus in onion seed and bulb crops. S. K. MOHAN (1) and J. W. Moyer (2). (1) Parma Research & Extension Center, University of Idaho, Parma, ID 83660; (2) Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695. Publication no. P-2004-0033-PCA.
A disease affecting onion seed crops causing dry, necrotic lesions, mainly on the scapes (flower stalks), was first observed in 1989 in Idaho and Oregon, and later in Arizona and California. The putative causal agent was identified as a Tospovirus, which was subsequently characterized as Iris Yellow Spot Virus (IYSV). The disease caused up to 90% loss of seed yield in severe cases. During the 2001 and 2002 growing seasons, onion bulb crops in the Treasure Valley region of Idaho and Oregon were observed with foliar symptoms of oval to lenticular, dry, chlorotic or necrotic lesions, that sometimes caused premature drying of the tops and reduction in bulb size. No fungus or bacterium was found associated with the lesions. Bioassay on Nicotiana benthamiana and serological tests found the symptomatic leaves to be infected with IYSV. This virus disease of onions, if widespread and severe, may potentially be a serious production problem for onion seed and bulb crops.
Phytophthora in recirculating cultural systems: The influence of different irrigation regimes on disease development. C. J. NIELSEN, M. E. Stanghellini, and D. M. Ferrin. Dept. Plant Pathology, Univ. of California, Riverside, CA 92521. Publication no. P-2004-0034-PCA.
Zoospores of Phytophthora are commonly the primary inoculum responsible for spread in recycled irrigation water. We examined the influence of irrigation duration on inoculum production and disease progression. A second study examined the influence of irrigation timing (day or night), as it has been shown that some Oomycetes have a cyclic pattern of sporangia and zoospore production. In the duration study, disease onset occurred 2 wks earlier (P = 0.005) and spread throughout the system about 3 times faster (P = 0.070) with two, 30-min irrigations than with two, 5-min irrigations per day. In the timing study, disease onset occurred about 3 wks earlier (P = 0.004) and spread throughout the system about 7 times faster (P = 0.056), when irrigated at night rather than during the day. Preliminary results regarding inoculum densities over a 30-min irrigation cycle showed a pattern in which high numbers of zoospores are released early in the cycle, then taper off, and begin to increase again after 20 min. This information may be useful in the development of an integrated disease management program.
A new strain of Mucor isolated from guava fruit in Hawaii. K. A. NISHIJIMA (1), H. T. Chan, Jr. (1,2), and W. T. Nishijima (3). (1) USDA-ARS-PBARC, Hilo, HI 96720; (2) Retired; (3) Univ. of Hawaii-Manoa, Coop. Ext. Ser., Hilo, HI 96720. Publication no. P-2004-0035-PCA.
Guava puree is one of the major products in the tropical fruit beverage industry in Hawaii. In 1997, concern over high mold counts (>10(^3) cfu/g) in fresh guava puree from Kauai led to an investigation into the source and identification of the contaminant. A gray-colored Mucor strain (GMS), isolated consistently from puree samples and the surface of fruit in the field, was identified as M. hiemalis f. hiemalis Wehmer (CABI, England) or M. hiemalis f. luteus (Linnemann) Schipper (CBS, Netherlands). In 1999 surveys using a fruit core bioassay technique, GMS was isolated from 11-25% and 33-44% of guava fruit from the islands of Kauai and Hawaii, respectively. Inoculation, cultural, and thermal studies indicated that GMS is not pathogenic to artificially wounded guava fruit (Beaumont B-30) and differs morphologically and physiologically from the Mucor rot pathogen, yellow-colored M. hiemalis (YM). We conclude that GMS is an endophyte of guava fruit that inhabits exocarp and mesocarp tissue without producing decay symptoms and infested fruit are difficult to cull out during harvesting and processing.
Evaluation of fungicides for control of rapid blight of Poa trivialis. M. W. OLSEN and D. M. Bigelow. Dept. of Plant Pathology, The University of Arizona, Tucson, AZ 85721. Publication no. P-2004-0036-PCA.
Rapid blight is a disease of cool season turf grasses that has occurred on several golf courses in Arizona over the past five years. It is now known to be caused by a Labyrinthula sp., an organism characterized by fusiform vegetative cells and gliding motility. Early symptoms of the disease include patches of sunken turf with a water-soaked appearance. Infected turf turns yellow and dies. A trial was conducted to evaluate efficacy of selected fungicides for rapid blight control at a golf course in central Arizona with a previous disease history. Plots were established in October 2002 on a putting green planted with hybrid bermuda grass and over-seeded with rough bluegrass, Poa trivialis. Disease symptoms appeared on the P. trivialis within three days after the first mowing. Treatments including trifloxystrobin, pyraclostrobin, mancozeb, myclobutanil and a polyol soil surfactant were applied to 2.8 m(^2) plots arranged in a randomized complete block design with eight replications. Treatment with mancozeb combined with trifloxystrobin that included a pre-plant application of mancozeb or of post-plant application of pyraclostrobin gave the best disease control.
Production of transgenic pineapple plants with a coat protein gene of PMWaV-2. E. A. PEREZ (1), M. J. Melzer (1), D. M. Sether (1), C. Nagai (2), W. B. Borth (1), and J. S. Hu (1). (1) Department of Plant and Environmental Protection Sciences, University of Hawaii, Honolulu, Hawaii 96822; (2) Hawaii Agricultural Research Center, Aiea, Hawaii 96701. Publication no. P-2004-0037-PCA.
The symptoms of mealybug wilt of pineapple develop in plants infected with Pineapple mealybug wilt associated virus - 2 (PMWaV-2) and infested with mealybugs (Dysmicoccus spp.). The coat protein (CP) gene of PMWaV-2 was constructed as an inverted repeat with the PMWaV-2 heat shock protein (HSP70) as a linker in pCAMBIA 1300 vector. This construct was introduced into protocorm-like bodies (plbs) of pineapple using particle bombardment. Transformants were selected under stepwise antibiotic concentrations of 16, 35, and 50 mg/L and regenerated through organogenesis. Linker HSP sequences were amplified from 29 in vitro selected plants after the third regeneration cycle. Approximately 10 percent of the plants regenerated from these lines were resistant to PMWaV-2 when challenged with viruliferous mealybugs, as determined by tissue blot immunoassay with antisera to PMWaV-2 coat protein. Further characterization of these putatively resistant plants is ongoing.
Fusarium wilt of lettuce in California. J. C. PETERSEN and T. R. Gordon. Dept. Plant Pathology, University of California, Davis, CA 95616. Publication no. P-2004-0038-PCA.
Fusarium wilt of lettuce, caused by Fusarium oxysporum f. sp. lactucum was discovered in the San Joaquin Valley of California in 1990, where it has been regarded as only a minor problem. More recently, Fusarium wilt has become a serious problem in production areas near Yuma, Arizona, and in 2002, the disease was confirmed to occur in the nation’s largest lettuce growing region in coastal California. Fusarium wilt diseases are traditionally managed through genetic resistance and/or crop rotation to reduce soil-borne inoculum. Our research is designed to develop information needed to exploit both management options. To this end, we are screening 47 currently grown lettuce cultivars so that growers can select those least susceptible to the disease in areas where Fusarium wilt is a problem. Preliminary results show that root dip inoculations with 10(^5) spores/ml differentiate between susceptible and resistant cultivars, Lighthouse and Salinas, respectively. We are also developing methods for detection of the lettuce wilt pathogen in soil, in order to monitor its survival in soil in the presence of non-susceptible crops grown in rotation with lettuce.
Systematics of the causal agent of Eutypa dieback of grapevine in California. P. E. ROLSHAUSEN (1), F. Trouillas (1), N. Mahoney (2), R. Molyneux (2), and W. D. Gubler (1). (1) Dept. Plant Pathology, UC Davis, CA 95616; (2) USDA-ARS, WRRC, Albany, CA 94710. Publication no. P-2004-0039-PCA.
Eutypa dieback, caused by Eutypa armeniacae, was first reported on apricot. However, the morphological characteristics of this species were identical to the Eutypa lata previously described in the literature. These species have been regarded as synonymous by some but have also been separated based on pathogenic properties by others. A recent report supported the separation of the species based on molecular analysis of several Eutypa isolates. However, further analysis that included some of the isolates published in this previous report along with the collection maintained at UC Davis, led to a different conclusion. The species identified as Eutypa lata in the previous report was identified in this study as Diatrype sp. based on nucleotide sequence analysis, morphological description of the mitosporic stage and secondary metabolite profile. Therefore, given these results, the two species, Eutypa armeniacae and Eutypa lata, still cannot be separated.
Differentiation of Ustilago scitaminea races in Hawaii. S. SCHENCK and R. Ming. Hawaii Agriculture Research Center, Aiea, HI 96701. Publication no. P-2004-0040-PCA.
The most severe sugarcane disease in Hawaii is smut, caused by Ustilago scitaminea. In 2001, up to 20 percent of stools in some Maui fields of cultivar H78-7750, previously classified as completely smut-resistant, were observed to have smut sporulating structures, called “whips”. A test using DNA markers confirmed the cultivar identity. Smut whips collected on Maui and on Oahu were used to inoculate disease-free H78-7750. After six months, the plots inoculated with the new Maui isolate produced whips while those with Oahu smut had none. Subsequently, a field trial was installed testing ten commercial cultivars for susceptibility to the new Maui smut isolate. These preliminary results indicated that a new race of U. scitaminea had appeared with a different range of host susceptibility ratings than the old race. A study was undertaken to determine genetic variation among Hawaiian isolates of U. scitaminea using amplified fragment length polymorphism (AFLP) markers.
Effect of fumigation depth on root rot of melon caused by Monosporascus cannonballus. M. E. Stanghellini, D. M. FERRIN, and K. C. Radewald. Department of Plant Pathology, University of California, Riverside, CA 92521. Publication no. P-2004-0041-PCA.
A field trial was conducted to compare the efficacy of chloropicrin (249 kg/ha) applied through drip tape placed 20 or 40 cm below the soil surface on root rot of cantaloupe caused by Monosporascus cannonballus. All plots were irrigated through drip tape at the 20-cm depth. Regardless of application depth, fumigation reduced the percentages of roots with lesions and perithecia compared to the nonfumigated controls (P < 0.05). Root rot severity (rating scale of 0-4 based on the percentage of the root system covered with lesions) was less than the nonfumigated controls only for the 20-cm depth (P < 0.05). Fumigation at 20 or 40 cm increased the number of marketable fruit by 34.6 and 36.9%, respectively, compared to the nonfumigated controls (P < 0.05). The number and location of lesions on root systems of plants from fumigated and nonfumigated soil were recorded for each of four, 10-cm increments distal to the root-stem interface. Fumigation at the 20- or 40-cm depth reduced the percentage of roots with lesions, the number of lesions per root system or the number of lesions per individual root within each distance increment.
New weed hosts of potato viruses and their impact on potato virus epidemiology. P. E. THOMAS and K. Richards. USDA, ARS, 24106 N. Bunn Road, Prosser, WA 99350. Publication no. P-2004-0042-PCA.
The common, aphid-transmitted potato viruses have few weedy hosts. Three nightshade species, Hairy Nightshade (Solanum sarachoides), Black Nightshade (Solanum nigrum) and Cut Leaf Nightshade (Solanum trifolium) are predominant weeds of potato that are difficult to control in the crop with herbicides. Until now, their susceptibility to potato viruses was largely unknown. Our studies show that all three species host potato viruses M and X, both the standard and tuber necrosis variants of the O and N strains of potato virus Y, and potato leafroll virus (PLRV). Hairy and Black, but not Cut Leaf Nightshade also host potato virus A and are the first known weedy hosts of this virus in the US. Furthermore, we found that Hairy Nightshade and, to a lesser extent, Black Nightshade are much better hosts than potato of the Green Peach Aphid (Myzus persicae), a major vector of all of these viruses. In addition, infected Hairy Nightshade plants accumulate much higher concentrations of PLRV and are much better sources of PLRV for transmission than is potato. These weeds serve as oversummering hosts, and they markedly impact potato virus dissemination in potatoes.
Lack of virus strain specificity of replicase gene mediated resistance to potato leafroll virus in potato. P. E. THOMAS (1) and W. K. Kaniewski (2). (1) USDA, ARS, 24106 N Bunn Rd., Prosser, WA 99350; (2) Monsanto Co., 700 Chesterfield Village Pkw, St. Louis, MO 63198. Publication no. P-2004-0043-PCA.
High levels of resistance to potato leafroll virus (PLRV) were selected among potato (cv. Russet Burbank) lines transformed with unmodified, full-length constructs of the PLRV replicase gene. The virus isolate source of the replicase gene was used to select for resistance. The resistance was effective against PLRV genotypes that occur naturally in the Columbia Basin of Northwest US. The resistance of 12 selected lines was tested against 19 PLRV isolates collected from throughout the US. Although some lines were more resistant than others and some virus isolates somewhat more virulent than others, all lines were resistant to all virus isolates. Foliage infection rarely occurred in inoculated plants in the current season, but a few tubers from asymptomatic plants were infected. The resistance of the potato lines was tested against specific virus isolates obtained from infected tubers of resistant lines. The resistant lines were not more susceptible to these isolates. Thus, replicase gene mediated resistance to PLRV appears to be nonspecific for virus strain.
Production of Puccinia thlaspeos “woad” strain inoculum using traditional farming equipment. S. V. THOMSON and B. R. Kropp. Dept. of Biology, Utah State University, Logan, UT 84322-5305. Publication no. P-2004-0044-PCA.
Puccinia thlaspeos “woad” strain is a systemic rust used for biological control of Isatis tinctoria (Dyer’s woad) and registered with the Environmental Protection Agency under the name Woad Warrior. It is an obligate parasite and inoculum can only be produced on living plants. Isatis tinctoria was planted in March 2001 and inoculated by spraying with Puccinia thlaspeos teliospores on 17 May and 1 June 2001. Symptoms were apparent on some plants on 20 October 2001 but 83% showed systemic infections by 3 April 2002. The rust-infected plants were harvested 24 May by cutting with a Milholland 1495 swather with crimping to condition plants for rapid drying. Plants were left in the windrow to dry until 28 May and baled using a Heston 4950 inline baler. Bales of infected woad were ground on 30 May using a Gehl chopper with a 1/16-inch screen. The number of basidiospores produced from farmed inoculum was 80% of the number of basidiospores produced from hand-harvested leaves from rust-infected wild woad plants. Traditional farming methods and equipment can be used to produce large quantities of Puccinia thlaspeos “woad” strain inoculum.
New findings on the distribution and host range of Eutypa lata in California. F. P. TROUILLAS, P. E. Rolshausen, and W. D. Gubler. Dept. of Plant Pathology, Univ. of California, Davis, CA 95616. Publication no. P-2004-0045-PCA.
The fungus Eutypa lata causes a destructive disease of grapevine and apricot known as Eutypa dieback. New infections are solely dependent on airborne ascospores releases during periods of rainfall. However, little is known regarding the distribution of the sexual stage of E. lata in California. The high incidences of Eutypa dieback in some vineyards have suggested the existence nearby of major sources of inoculum. During the years 2000 to 2002, we conducted surveys for the presence of the perfect stage of E. lata. Surveys were extended to fruit orchards, native and ornamental flora growing in the vicinity of diseased vineyards. Our surveys have identified several new hosts of this pathogen in California, e.g. Malus, Pyrus, Acer and Salix. These species contained mature stroma and perithecia. Overall, the willow species appeared as a significant source for E. lata ascospore production. Perithecia were also found commonly on grapevine, cherry and apricot trees. Inoculum was found in the counties of Mendocino, Napa, Yolo, Solano, Sonoma, San Joaquin, San Benito, El Dorado, Merced, Contra Costa and Stanislauss.
The relationship of Phytophthora sp. to the cause of taro pocket rot in Hawaii. J. Y. UCHIDA, C. Y. Kadooka, and M. Aragaki. Dept. of Plant and Environmental Protection Sciences, University of Hawaii, Honolulu, HI 96822. Publication no. P-2004-0046-PCA.
Numerous cavities in mature taro corms have been a continuing, major problem for taro growers in Hawaii and other Pacific islands. The etiology has been an enigma with several hypotheses offered by different researchers including insect or bird damage, fertilizer or herbicide phytotoxicity and arthropod activity. A microbial etiology was pursued and many possible pathogens were isolated from the cavities of hundreds of sample corms. However, none of the fungi reproduced the disease on healthy taro corm. The taro corm takes a year to mature and while one to five cavities may accumulate before harvest, early stages of the disease at any time are difficult to discern. Isolations from these infrequent earliest stage yielded a slow growing Phytophthora sp. Inoculations of healthy taro with this new Phytophthora resulted in the reproduction of pocket rots, which has been repeated three times. Based on these tests, one of the probable causes of pocket rots is this new Phytophthora. Efforts to fulfill Koch’s postulates are progressing and three isolates of the new Phytophthora are being characterized.
New disease of greenhouse sweet pepper fruits in British Columbia, Canada. R. S. UTKHEDE and S. Mathur. Agriculture and Agri-Food Canada, P.O. Box 1000, Agassiz, British Columbia, Canada V0M 1A0. Publication no. P-2004-0047-PCA.
Recently, a fruit rot of orange sweet peppers (Capsicum annum L.) was observed in commercial greenhouses in British Columbia on cultivar Sympathy MZ. Estimated losses are 10 - 40% of fruits. The disease appeared as discolored soft patches or necrotic spots mostly at the calyx end and sometimes anywhere on the mature fruit. Seeds and surrounding area inside the fruits were covered with fungal growth and orange pink spore masses. Fungal isolations revealed that the disease is caused by Fusarium subglutinans (Wollenweber & Reinking) Nelson et al. Pathogenicity of this fungus was confirmed on fruits of sweet pepper cv. Sympathy MZ. Flowers inoculated with F. subglutinans at different stages developed more disease compared to fruit inoculations. Higher inoculum concentrations of F. subglutinans resulted in higher disease incidence as compared to lower concentrations. None of the seeds from infected fruits germinated. This pathogen does not infect other greenhouse crops such as tomatoes, cucumbers or lettuce. This appears to be the first reported occurrence of fruit rot caused by F. subglutinans on greenhouse sweet peppers in Canada.
T/DGGE differentiation of virulent Clavibacter michiganensis subsp. michiganensis (Cmm) from nonvirulent Clavibacter-like saprophytes. B. G. VINE, W. S. Kaneshiro, and A. M. Alvarez. Dept. of Plant and Environmental Protection Sciences, Univ. of Hawaii, Honolulu, HI 96822. Publication no. P-2004-0048-PCA.
Cmm is a pathogen of tomato and pepper. It is usually transmitted in contaminated seed lots, which also contain many culturable saprophytes. The purpose of this work was to separate Cmm from the saprophytes that are phenotypically similar to Cmm. Genomic DNA isolated from both saprophytes and Cmm-nonvirulent, -hypovirulent, and -virulent strains, was used to amplify the ribosomal DNA (rDNA) by PCR. The amplicons were compared by T/DGGE, which can differentiate mixed bacteria by their rDNA sequences due to their different requirements for strand melting. For example, a gene with a slightly higher G:C content will migrate farther through a gradient of temperature or denaturant than will a rDNA sequnce with a lower G:C content; and mixed populations show multiple rDNA bands per lane. Pathogenic Cmm can be identified in mixed populations of bacteria by loading individual lanes with rDNA from known Cmm, adjacent to lanes containing the mixed rDNAs. In addition to other testing methods, T/DGGE can be useful as an additional confirmation of Cmm-free seed lots.
Resistance of Monilinia fructicola from stone fruit to thiophanate methyl, iprodione and tebuconazole. M. A. YOSHIMURA, Y. Luo, Z. Ma, and T. J. Michailides. Dept. Plant Pathology, University of California Davis, Kearney Agricultural Center, Parlier, CA 93648. Publication no. P-2004-0049-PCA.
A survey of sensitivities of M. fructicola isolates from stone fruit in California detected resistance to thiophanate methyl (TM), but not to iprodione and tebuconazole. EC(50) values (micrograms/ml) of 152 isolates to iprodione ranged from 0.01 to 0.65, and to tebuconazole from 0.004 to 0.061. EC(50) values of TM low-resistant (LR) isolates were between 2 and 30 and high-resistant (HR) isolates were greater than 30. Of isolates collected in 1992-98 and 2002, 39 of 52 and 19 of 100 were LR while 1 and 3 were HR, respectively. Laboratory inoculation of nectarine blossoms did not reveal any differences in pathogenicity among TM-sensitive (S), LR and HR isolate groups. Use of TM at full and half dosages controlled S but not LR and HR isolates. Reisolation from blighted blossoms inoculated with an equal mixture of spores from the three resistance groups showed equal frequencies of isolates from the untreated control. Both TM treatments had significantly higher frequencies of LR and HR than S isolates and the full dosage treatment had a higher frequency of HR than LR isolates.
Get ALL the Latest Updates for CHANGING LANDSCAPES OF PLANT PATHOLOGY. Follow APS!