Posted online April 23, 2001
Nontraditional products designated as biorational compounds have increasingly received attention as environmentally-friendly alternatives to traditional pesticides. In this study, microbial populations colonizing dogwood trees in different environments and plants treated with a biorational product Armicarb™ and with a traditional fungicide Basicop™ were compared. The predominant microorganisms in all environments were bacteria of the genera Xanthomonas and Pseudomonas, followed by fungi with a high incidence of Cladosporium spp.; colonies of yeasts were relatively few. Plants treated with Armicarb™ had a significantly larger microbial population than nontreated plants, or plants treated with the fungicide Basicop™. The composition of the microbial populations was also affected by the treatments. Bacterial populations were larger in treatments with Armicarb™ while Basicop™ reduced bacteria, yeast and fungi almost equally. These results provide preliminary data on the effect of biorational products on non-target microbial populations.
Relative resistance of two soybean cultivars to Pythium spp. G. D. BATES, C. S. Rothrock, and J. C. Rupe. Dept. Plant Pathology, University of Arkansas, Fayetteville, AR 72701. Publication no. P-2001-0002-SOA.
Previous research in Arkansas examining the response of soybean to temporary flooding indicated that the soybean cultivar Archer was more resistant to Pythium ultimum than the widely grown cultivar Hutcheson. To determine the response of Archer and Hutcheson to other Pythium spp. isolated from soybean roots, these cultivars were planted in soil infested with either P. aphanidermatum, P. ultimum, HS-group, P. irregulare, P. vexans, or P. oligandrum. P. oligandrum is a nonpathogenic species and P. vexans has not been reported previously as a soybean pathogen. Experiments were conducted at 20 C in pasteurized soil infested with sand-corn meal inoculum. After 10 days, Pythium damping-off and root rot were assessed using a disease index, stand count, percent seed rot and percent damping-off. While there was variation among isolates within a species, overall Archer was more resistant than Hutcheson to all pathogenic groups of Pythium, including P. vexans. Cultivars did not differ in their response to P. oligandrum.
Effects of calcium sources and residual fertility on peanut pod rot, seed quality, and yield. T. B. BRENNEMAN (1), G. J. Gascho (1), W. R. Guerke (2), and J. A. Baldwin (1). (1) Tifton Campus, University of Georgia, Tifton, GA 31793; (2) Tifton Seed Laboratory, Georgia Dept. of Agriculture, Tifton, GA 31793. Publication no. P-2001-0003-SOA.
The effects of calcitic lime (C) or dolomite (D) applied preplant or gypsum (G) applied at pegging were evaluated on Virugard peanut grown with strip tillage in alternating years with cotton from 1998-2000. The cotton received a standard fertility program or a standard program plus a double rate of K or Mg. Residual cotton fertility had no effect on peanut diseases or pod development. Pod rot was most severe in 1998 and both G and C had less pod rot than D. In 1999 and 2000 disease severity was low in all plots. In 2 of 3 years, C and G reduced the percentage of unfilled pods. Mean pod yields were 3053, 3454 and 3049 kg/ha in 1998, 1999 and 2000, respectively, and were not affected by calcium treatment. Germination of seed produced was evaluated in 1998 and 1999. G treated seed had the highest germination both years. Germination of C and D treated seed were significantly higher than the nontreated only in 1999. The calcium content of seed was evaluated in 1999 and was higher for G and C than for D or the nontreated seed.
Effect of ozonated air on postharvest fungal decay of blueberries. W. O. CLINE and B. K. Bloodworth. Dept. Plant Pathology, North Carolina State University, Raleigh, NC 27695. Publication no. P-2001-0004-SOA.
Postharvest decay of blueberries results in visibly soft or moldy berries. When marketed in pint or half-pint clear plastic containers, defective berries are easily seen by the consumer at the time of retail purchase, greatly reducing sales. Some growers and marketers have installed small, portable ozone generators to modify air in cold storage rooms where berries are held prior to shipping, in hopes that this will reduce decay. This series of experiments was conducted to determine whether decay is in fact reduced. An ozone generator, monitor, and controller were installed on one of two matching 90 cu ft environmental chambers. In a series of twenty replicated tests over time, ozone levels varying from 0.30 to 0.45 ppm at temperatures ranging from 38 to 50° F were applied to blueberries in vented pint or half-pint retail containers. These were compared to non-ozonated fruit at identical temperatures and time durations. Storage times of 7 to 21 days were used, after which berries were sorted into good, soft, or moldy categories. No significant differences were observed between ozonated and non-ozonated berries. No phytotoxicity to berries was noted, however ozone treatments did produce cracks in older batches of plastic pint containers. In conclusion, ozone did not reduce decay, and may damage packaging.
Peanut leaf spot response to rates and application timings of BAS 500 fungicide. A. K. CULBREATH and T. B. Brenneman. Coastal Plain Expt. Stn., Tifton, GA 31793-0748. Publication no. P-2001-0005-SOA.
Field studies were conducted on Georgia Green peanut (Arachis hypogaea) cultivar at Tifton, GA during 1999-2000 for control of early leaf spot (Cercospora arachidicola). Treatments included a nontreated control, chlorothalonil, 1.26 kg/ha, three rates (0.112, 0.168, and 0.224 kg ai/ha) of BAS 500, and 0.224 kg/ha of tebuconazole. All fungicide treatments were applied on 14, 21, and 28 day schedules. Across both years, all rates of BAS 500 provided better leaf spot control than standard rates of chlorothalonil, and the two highest rates of BAS 500 provided better control than tebuconazole when compared within the respective spray schedules. Final Florida 1-10 scale (1 = no disease, 10 = killed by leaf spot) ratings for the three BAS 500 treatments applied on a 21-day schedule ranged from 3.3 to 3.6, and were lower (LSD = 1.3) than the severity ratings of 4.9 and 5.1 for the chlorothalonil and tebuconazole treatments, respectively, applied on a 14-day schedule. The two highest rates of BAS 500 applied on a 28 day schedule provided leaf spot control comparable to chlorothalonil applied on a 14 day schedule.
RT-PCR detection of Cowpea aphid-borne mosaic virus in peanut from Brazil. A. G. GILLASPIE, JR. (1), G. Pio-Ribeiro (2), G. P. Andrade (2), and H. R. Pappu (3). (1) USDA-ARS, Griffin, GA 30223; (2) Univ. Fed. Rural Pernambuco, Recife, Brazil; (3) Department of Plant Pathology, Univ. of Georgia, Coastal Plain Expt. Stn., Tifton, GA 31793. Publication no. P-2001-0006-SOA.
The Brazil strain of Cowpea aphid-borne mosaic virus (CABMV) has been reported as a severe pathogen in peanut. This virus is seedborne at less than 1% in peanut depending upon the cultivar. However, it is still a significant problem in the distribution of germ plasm to other countries. Detection of the virus among a large number of seed lots would strengthen quarantine programs. Utilizing 3' sequencing data (GenBank acc. # AF241233), primers have been designed from the coat protein gene for the detection of CABMV from seeds and leaves. Use of the forward primer 5'-CGCTCAAACCCATTGTAGAA-3' and the reverse primer 5'-TATTGCTTCCCTTGCTCTTTC-3' results in an RT-PCR product of 221 bp. The thick seed slices necessary for a reliable test mean that the workable sample size is 12-15 seeds so the test does not have a significant advantage over ELISA for testing large seed lots. However, there is an advantage for detecting the virus in quarantined seedlings from seeds imported from Brazil. One infected leaf among 99 leaves from healthy plants has greater sensitivity than with ELISA (one leaf among 9 healthy leaves). Thus, the RT-PCR method does offer improved detection.
Towards developing a diagnostic tool for powdery mildew pathogens of dogwood. M. MMBAGA (1), N. B. Klopfenstein (2), and M. Kim (3). (1) Tennessee State University, Nursery Crop Research Station, McMinnville, TN; (2) USDA Forest Service, RMRS, Moscow, ID; (3) Dept. of Forest Resources, University of Idaho, Moscow, ID. Publication no. P-2001-0007-SOA.
Two fungi, Phyllactinia guttata and Microsphaera pulchra have been associated with powdery mildew in dogwood (Cornus florida). The two pathogens are distinctly different at the teleomorph stage, but ascocarps form late in the season and are often not available for diagnosis. The internal transcribed spacer (ITS) regions of rDNA and the intervening 5.8S rRNA gene for P. guttata and M. pulchra, were amplified using universal primer pairs, ITS(1) and ITS(4), and standard PCR protocols. The PCR products from the two pathogens showed similar band size, but the DNA sequences were distinctly different. The DNA sequences were used to design specific primers for M. pulchra and P. guttata. A comparison of the DNA sequences with information available in the genbank showed a very close similarity between M. pulchra in U.S isolates with Japanese M. pulchra from C. kousa. The similarity between the U.S M. pulchra and Japanese M. pulchra raises a serious question as to whether this powdery mildew pathogen was introduced to the U.S.A.
Response of three peanut cultivars to tillage practices and application timing of fungicide regimes for early leaf spot. W. S. MONFORT, A. K. Culbreath, and T. B. Brenneman. Coastal Plain Expt. Stn., Tifton, GA 31793. Publication no. P-2001-0008-SOA.
Field experiments were conducted on Ga. Green (GG), C-99R and MDR-98 peanut (Arachis hypogaea) cultivars to determine the effects of tillage and fungicide timings on early leaf spot (Cercospora arachidicola). Fungicide treatments were: 1) no fungicide; 2) chlorothalonil (CHL) 1.26 kg/ha; 3) tebuconazole 0.23 kg/ha (sprays 3-6) and CHL 1.26 kg/ha (all other sprays); 4) azoxystrobin 0.33 kg/ha (sprays 3 and 5) and CHL 1.26 kg/ha (all other sprays), at 14-d intervals (7 sprays). Treatments 5-7 used the same fungicides as treatments 2-4, applied at 21-d intervals (4 sprays) with tebuconazole or azoxystrobin applied at sprays 2 and 3 in treatments 6 and 7 and CHL in all others. Treatments were evaluated within whole-plots of conventional (CONV) vs. strip-tillage (ST) treatments. Leaf spot severity was lower in ST plots than in CONV plots. Fla. 1-10 scale ratings in GG were 9.7, 4.8, 4.3, 3.6, 7.4, 7.3 and 6.2 (LSD = 1.2) for treatments 1-7, respectively in CONV plots, and 9.2, 3.3, 3.4, 2.7, 5.0, 4.3 and 3.3 (LSD = 0.8), respectively, in ST plots. Similar trends were observed on MDR-98 and C-99R, but at lower severity levels.
Diversity of Ralstonia solanacearum in the Southeastern United States. K. L. ONG, A. E. Robertson, B. A. Fortnum, and D. A. Kluepfel. Dept. of Plant Pathology & Physiology, Clemson University, Clemson, SC 29634. Publication no. P-2001-0009-SOA.
Ralstonia solanacearum, the causal agent of bacterial wilt, incurs major losses on tobacco in both North and South Carolina. In Georgia and Florida, bacterial wilt rarely occurs on tobacco but it is a common problem on tomatoes and peanuts. These observations prompted an examination of the diversity of R. solanacearum isolates within the Southeastern United States. All R. solanacearum isolates from tobacco and tomato in this study were identified as race 1, biovar 1. Antibiotic resistance phenotypes were assessed using 16 different antibiotics. Results of the antibiotic study suggest that the R. solanacearum strains tested were homogeneous. Gas chromatographic generated fatty acid retention time profiles of these strains were analyzed using multivariate analyses. Two-dimensional plots of the principal components generated from the fatty acid profiles revealed R. solanacearum strains clustered as a function of geographical origin. Rep-PCR, a DNA fingerprinting procedure, was used to generate genomic fingerprints. Similarity trees generated from the fingerprint data correlated with results from the fatty acid profiles revealing clusters based on geographical origin.
Control of Pythium root rot of float-grown tobacco seedlings with chitosan. B. H. OWNLEY and C. Y. Hamilton. Department of Entomology and Plant Pathology, The University of Tennessee, Knoxville, TN 37996. Publication no. P-2001-0010-SOA.
The objective of this study was to evaluate chitosan for control of Pythium root rot of float-grown tobacco. Treatments were applied to float water 24 h before inoculation with P. myriotylum, 3 wk after seeding. In the first test, treatments were 400 and 500 mg/l chitosan, 0.008 and 0.012 ml/l biosurfactant, and the fungicides metalaxyl (0.002 ml/l) and etridiazole (0.15 g/l). Controls were inoculated, uninoculated, and uninoculated with 400 mg/l chitosan. To prevent chitosan precipitation, the pH of the float water of chitosan treatments was maintained at 5.6 to 5.8. Disease rating was lower for the higher chitosan rate; however, ratings from both rates were lower than the inoculated control. Shoot and root weights of chitosan and fungicide treatments were greater than the inoculated control. The biosurfactant treatments were ineffective for control of Pythium. In a second test, the biosurfactant treatments were replaced with inoculated chitosan at 400 mg/l without float water pH adjustment, and an inoculated, untreated control with float water pH adjusted (5.6 - 5.8). The disease ratings of chitosan-treated plants (both rates, with or without pH adjustment) were lower than the inoculated control with pH adjustment. Shoot weights of plants treated with chitosan or the fungicides were greater than the inoculated controls.
F500: A new broad-spectrum fungicide for control of peanut diseases. H. E. PORTILLO (1), R. R. Evans (1), J. S. Barnes (2), and R. E. Gold (2). (1) BASF Corporation, Agricultural Research Station, Greenville, MS 38701; (2) BASF Corporation, Research Triangle Park, NC 27709. Publication no. P-2001-0011-SOA.
F500 is a new broad-spectrum strobilurin fungicide being developed in field crops by BASF in North America under the trade name Headline™. In research trials conducted from 1996-2000, F500 provided excellent control of early leaf spot (Cercospora arachidicola) and late leaf spot (Cercosporidium personatum) at rates between 100-168 g ai/ha. F500 applied at 14-day intervals provided comparable or better control of early and late leaf spot than all registered peanut fungicides. F500 applied at 21-day intervals was more effective on early and late leaf spot than all disease management programs tested, thus providing the opportunity for reduced number of sprays without affecting control. Disease management programs that included F500 resulted in better leaf spot control than programs without F500. F500 applied at 14-d intervals at rates between 150-281 g ai/ha also provided control of southern stem rot (Sclerotium rolfsii). Fungicide programs with F500 alone or in alternations with other peanut fungicides registered for control of southern stem rot provided similar control as the currently used fungicide programs, in most cases adding the benefit of extended leaf spot control.
F500: A new broad-spectrum fungicide for control of key turfgrass diseases. H. E. PORTILLO (1), R. R. Evans (1), J. S. Barnes (2), R. E. Gold (2), and D. E. Millhouse (3). (1) BASF Corporation, Agricultural Research Station, Greenville, MS 38701; (2) BASF Corporation, Research Triangle Park, NC 27709; (3) BASF Corporation, Agricultural Research Station, Dinuba, CA 93618. Publication no. P-2001-0012-SOA.
F500 is a new broad-spectrum strobilurin fungicide being developed in turfgrass by BASF in North America under the trade name Insignia™. In research trials conducted from 1996-2000, F500 provided effective control of all major turfgrass diseases, including brown patch (Rhizoctonia solani), Pythium blight (Pythium aphanidermatum), and gray leaf spot (Pyricularia grisea), and suppression of dollar spot (Sclerotinia homeocarpa). F500 applied at 14-day intervals provided excellent control of brown patch and gray leaf spot at rates between 250-300 g ai/ha. Spray intervals for control of these two diseases may be extended up to 21-28 days at spray volumes as low as 400 l/ha. Control level of brown patch and gray leaf spot was comparable to or better than that of leading standards such as azoxystrobin, iprodione, propiconazole, chlorothalonil, and others. F500 applied at 14-day intervals provided effective control of Pythium blight at rates between 300-560 g ai/ha, being comparable to metalaxyl, mefenoxam, and azoxystrobin. Applied preventatively at 300-560 g ai/ha and 14-day intervals, F500 provided good suppression of dollar spot. F500 has proven to be safe on all common turfgrass species.
A comparison of disease assessment methods for southern stem rot of peanut. S. L. RIDEOUT (1), T. B. Brenneman (1), and K. L. Stevenson (2). (1) Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton, GA, 31793; (2) Department of Plant Pathology, University of Georgia, Athens, GA 30602. Publication no. P-2001-0013-SOA.
Traditionally, disease assessment of southern stem rot of peanut (Sclerotium rolfsii) has been evaluated using disease incidence ratings reflecting percentage of infected row feet. During 1999 and 2000, several alternate disease assessment methods were evaluated in five fungicide trials both at digging and during the growing season. Pearson correlation coefficients were calculated for disease ratings and yields for all trials. At digging, total disease severity per plot (r = -0.31 to -0.63), average severity per disease focus (r = -0.37 to -0.53), total length of row with symptoms (r = -0.34 to -0.61), average length per disease focus (r = -0.33 to -0.62), and disease intensity (r = -0.27 to -0.61) were significantly correlated to yield (P < 0.05) in all five trials. Disease incidence was significantly correlated with yield in four of the five trials (r = -0.44 to -0.59). Similar trends were noted in the mid-season ratings, where correlation coefficients were usually higher for disease severity and intensity ratings than for disease incidence. Disease incidence ratings took 2.33 min per plot (15.2 m of row), whereas more detailed evaluations took from 2.75 to 3.01 min per plot.
Strategic application of fungicides for control of brown patch on creeping bentgrass. J. K. ROYALS II and S. B. Martin. Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634. Publication no. P-2001-0014-SOA.
Creeping bentgrass (Agrostis stolonifera L.) is susceptible to brown patch (Rhizoctonia spp.) in summer months. Fungicides are used to control brown patch, but their strategic use in a changing epidemic has not been studied. In this study, preventive and curative fungicide programs were evaluated. Preventive treatments were made on a 14-day schedule, with low label rates of each product. Curative fungicides were chlorothalonil or azoxystrobin, at high label rates. Programs were replicated 4 times in RCB design within four cultivars, Penn A-1, Crenshaw, L-93, and Penncross, in a Latin square design. Plots were rated visually for disease severity and programs were compared using area under disease progress curves. Most preventive programs were more efficacious than curative programs. Programs with azoxystrobin or chlorothalonil plus fosetyl-Al performed well during periods of high disease pressure. Programs with thiophanate methyl failed to control R. zeae. R. solani AG 2,2 IIIB was present throughout the season. In 1999 and 2000, disease severity on Crenshaw exceeded that on other cultivars. Disease severity among other cultivars differed in 1999 and 2000. During periods of high disease pressure, fungicides effective against both R. solani and R. zeae needed to be used.
Effect of inoculum type of Rhizoctonia solani and cultivar on damping-off of tomato. D. SETH and B. H. Ownley. Dept. of Entomology and Plant Pathology, The University of Tennessee, Knoxville, TN 37996. Publication no. P-2001-0015-SOA.
The main objective of this study was to evaluate 10 tomato cultivars for their susceptibility to damping-off caused by R. solani. The cultivars included Carolina Gold, Celebrity, Hybrid 882, Mountain Belle, Mountain Delight, Mountain Fresh, Mountain Pride, Mountain Spring, Mountain Supreme, and Sun Pride. The second objective was to compare two forms of R. solani inoculum for ability to cause disease on tomato seedlings. Inoculum of R. solani was produced on either rice or cornmeal sand and was added to potting soil at 1 percent 0.5-1.0-mm sieved rice, and 4 percent cornmeal sand. Percent surviving seedlings was determined at 21 days after planting. Disease severity of surviving plants was recorded also. There was no difference in percent surviving seedlings for the two types of inoculum. However, for surviving plants, disease was more severe with cornmeal sand inoculum. There was a significant interaction between inoculum type and cultivar for percent surviving seedlings. With cornmeal sand inoculum, survival was greatest for Carolina Gold and least for Mountain Spring, Celebrity, and Sun Pride. With rice inoculum, seedling survival was greatest Hybrid 882 and Mountain Supreme, and least for Mountain Belle, Celebrity, and Mountain Spring.
Phytophthora disease of citrus associated with root weevils in Texas. M. SKARIA and J. V. French. TAMU-K Citrus Center, 312 N. International Blvd., Weslaco, TX 78596. Publication no. P-2001-0016-SOA.
Since August 2000, we have been trying to identify the cause(s) of the rapid decline and death of orange trees in several citrus orchards in the Lower Rio Grande Valley of Texas. Affected trees first showed leaf wilt, yellowing and defoliation, followed by tree death in 4-5 weeks. Trees were removed and the roots washed with a handgun sprayer. The roots showed extensive insect feeding injury (channeling), together with severe Phytophthora root rot. The channels varied from 1.25 to more than 30 cm in length, and up to 1.25 cm wide. White larvae subsequently identified as the blue-green citrus root weevil, Pachnaeus opalus, and the sugarcane root stalk borer weevil, Diaprepes abbreviatus were identified in the soil. To date, one adult of D. abbreviatus has been captured in a ‘Tedders trap’ placed in the orchard. Soil and root analysis confirmed the presence of the Phytophthora fungus. This weevil(s)-Phytophthora complex is the first report from Texas citrus. It exists in several locations and in one orchard, 4.7% of the 1584 trees surveyed, were either dead or declining.
Multiplex PCR for an identification and diagnostic system of Meloidogyne species. D. D. SUI (1), D. A. Kluepfel (1), B. A. Fortnum (2), and S. A. Lewis (1). (1) Dept. Plant Pathology & Physiology, Clemson University, Clemson, SC 29634-0377; (2) Dept. Plant Pathology, Clemson University, Pee Dee REC, Rte. 1, Box 531, Florence, SC 29501-9603. Publication no. P-2001-0017-SOA.
Species identification in root-knot nematodes, Meloidogyne spp., has been a critical component of integrated pest management and in basic nematology research. Differentiation of the four major species, M. incognita, M. arenaria, M. javanica, and M. hapla, plus M. chitwoodi, requires extensive expertise in perineal and isozyme patterns. Here we report a simple, reliable technique for identification and a diagnostic system for these species. The PCR amplicons on agarose gel for identification of these species were 420, 820, 1120, 340, and 1020 base pairs, respectively. For the first four species we were able to design a one-step octaplex PCR diagnostic system that distinguishes them from each other by a single band at 1350, 1050, 420, and 700 base pairs, respectively. This is the first report that identification and diagnosis of these root-knot nematode species can be achieved in one-step PCR without the need of post-PCR restriction digestion. These species-specific PCR primers have potential to be optimized into a multiplex PCR diagnostic kit.
A PCR-based assay for mating type determination in Magnaporthe grisea. L. P. TREDWAY (1), S. F. Covert (2), S. E. Gold (1), K. L. Stevenson (1), and L. L. Burpee (1). (1) Dept. Plant Pathology, University of Georgia, Athens, GA 30602; (2) School of Forest Resources, University of Georgia, Athens, GA 30602. Publication no. P-2001-0018-SOA.
Magnaporthe grisea is a major pathogen of grasses, including several of economic importance. The genetic structure of M. grisea populations has important implications in disease management, and the distribution of mating types influences population structure. Our objective was to develop a PCR-based assay for determining mating type in M. grisea. Specific PCR primers were designed for each mating type idiomorph. The primer pairs specific to Mat1-1 and Mat1-2 mating types yielded products of 552 bp and 390 bp, respectively. The primers retained their specificity in multiplex PCR reactions, and the products were easily differentiated in a 1% agarose gel. The assay was tested on 40 M. grisea isolates of known mating type from several hosts. PCR products of the expected size were observed in all cases. Southern analysis was used to verify that the PCR products were homologous to the mating type idiomorphs. The assay was used to determine mating type distribution in Georgia populations of M. grisea. Preliminary results will be presented.
Evaluation of disease resistance of transgenic and conventional soybean [Glycine max (L.) Merrill] varieties and further determination of the associated pod and seed mycoflora in Mississippi. D. A. VILLARROEL and R. E. Baird. Dept. of Entomology and Plant Pathology, Mississippi State University, Mississippi State, MS 39762. Publication no. P-2001-0019-SOA.
During 1999 and 2000, experiments were conducted in soybean at two locations (Stoneville and Starkville, 33° 26 minutes N latitude) in Mississippi. Two transgenics, with resistant reaction to the herbicide glyphosate, and two conventional soybean varieties, from early and late maturity group V, were used in the study. The objectives were to identify fungi associated with soybean plants and to determine if the mycobiota identified in the first objective differs between transgenic and conventional varieties at reproductive stages R6 (full bean) and R8 (harvest maturity). Pod samples were collected at those two reproductive stages. Pod and seed tissues were plated onto petri dishes containing potato dextrose agar. After 14 days of incubation at 24-26°C, the samples were evaluated for fungal growth. Data will be presented comparing frequency of isolations from pod and seed tissues at R6 and R8, and between varieties.
The importance of histopathology in conifer diseases when Koch’s postulates cannot be fulfilled. C. WALKINSHAW (1), W. J. Otrosina (2), and N. Hess (1). (1) USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360; (2) 320 Green Street, Athens, GA 30602. Publication no. P-2001-0020-SOA.
Decline of conifers before maturity occurs in adult trees of most tree species. Fulfilling Koch’s postulates in affected sites is prevented by difficulties in controlling abiotic and biological variables. In contrast, adopting consistent examination schedules similar to the two lateral root sampling method used in annosum root rot studies would permit comparison of cellular damage among different species. Our objective is to define damage in cells of loblolly pine roots that are involved in an expanding decline occurring in the Talladega National Forest. Roots were fixed, embedded, sectioned, stained, and observed with a light microscope. Observations of cambial abnormalities and secondary cellular product accumulation were useful variables for analyses. Also, damage to specific root tissues such as resin canals may help to differentiate declines within pine species. As variation is defined for tree cell and tissue traits, management actions taken to mitigate decline disease syndromes can be evaluated accurately using these histological criteria.
Botrytis cinerea isolates from Louisiana strawberry farms resistant to benzimidazole and dicarboximide fungicides. D. E. WEDGE (1), K. J. Curry (2), J. E. Boudreaux (3), and B. J. Smith (4). (1) USDA-ARS, University, MS 38677; (2) Univ. of Southern Mississippi, Hattiesburg, MS 39406; (3) Louisiana State Univ., Baton Rouge, LA 70894; (4) USDA-ARS, Poplarville, MS 39470. Publication no. P-2001-0021-SOA.
Fungicidal sprays are widely used for the control of Botrytis fruit rot (gray mold), one of the most destructive diseases of strawberry. Since B. cinerea has become resistant to many antifungal compounds, new approaches for control are necessary. A standardized 96-well microtiter assay was used to evaluate 6 isolates of B. cinerea collected from different Louisiana strawberry farms for sensitivity to benomyl, azoxystrobin, captan, chlorothalonil, cyprodinil, dodine, fosetyl-Al, iprodione, metalaxyl, thiobendazole, thiram, vinclozolin, butrizol, PCNB, quinomethionate, and thiophanate methyl. Each fungus was challenged in a dose-response format with each test compound. Four isolates were determined to be benomyl resistant and two showed intermediate sensitivity. Two isolates were sensitive to the dicarboximide fungicides (vinclozolin, iprodione), three had intermediate sensitivity, and one was resistant. Butrizol, cyprodinil, and azoxystrobin provided nearly 100% growth inhibition for all 6 Botrytis isolates. These results suggest that Louisiana gray mold appears to be benzimidazole resistant and are becoming dicarboximide resistant. Butrizol, azoxystrobin, and cyprodinil should be considered for future disease control of Botrytis fruit rot of strawberry.
Temporal and spatial distributions of fungi in maturing pearl millet stalks. J. P. WILSON. USDA-ARS Crop Genetics & Breeding Research Unit, Tifton, GA 31793. Publication no. P-2001-0022-SOA.
Fungi associated with internal colonization and rotting of stalks were evaluated from pearl millet HGM 100 planted on two dates in 1996 and 1997, and randomly sampled at panicle emergence, stigma emergence, milk, and hard dough stages. Thirty fungal genera or species were isolated from nodes of split, surface-sterilized stalks. Isolations of the most common fungi were affected by growth stage and node position. Fusarium moniliforme was most prevalent at all stages (isolated from 24% of nodes), and isolation increased with advancing growth stage. Fusarium semitectum (11%) was predominantly isolated from lower nodes at panicle and stigma emergence, but was more uniformly distributed within plants at milk and dough stage. Bipolaris setariae (9%) was infrequently isolated during early growth stages, but increased greatly at dough stage. Isolation of Curvularia spp. (7%) was greatest at lower nodes, and was not affected by growth stage. Isolations of Alternaria spp. (7%) and Rhizopus spp. (6%) were greatest at milk stage. Fungi were frequently isolated from asymptomatic nodes of less mature stalks, and node discoloration increased with plant maturation and grain fill.
Sources of inoculum for bacterial leaf spot diseases of leafy crucifers in Oklahoma. Y. F. ZHAO, J. P. Damicone, and C. L. Bender. Entomol. & Plant Pathol. Dept., Oklahoma State Univ., Stillwater, OK 74078. Publication no. P-2001-0023-SOA.
Xanthomonas campestris pv. campestris (XCC), pv. armoraciae (XCA), and Pseudomonas syringae pv. maculicola (PSM) are bacterial pathogens that cause leaf spot diseases on leafy crucifers in Oklahoma. In the pathogen survival studies, turnip and collards were inoculated with rifampicin-resistant strains of XCA and PSM and were either buried under the soil or left on the soil surface in the Fall of 1998 and 1999. Both pathogens were recovered from plant debris buried under the soil for only two months in 1998-1999 and less than a month in 1999-2000, but not from the soil after the debris decayed. However, both pathogens were recovered from plant debris left on the soil surface for up to 5 months. During a survey in 1999-2000, XCC was isolated from Shepherd's purse, volunteer turnip and kale plants. XCC was also detected in 6 of 51 commercial seedlots of leafy crucifers. Plant debris on the soil surface, but not under the soil appears to be an important source of inoculum. Weeds and seeds may play an important role in the epidemiology of XCC. However, weeds and seeds were not demonstrated to be inoculum sources for XCA and PSM in this study.