Posted online May 2, 2007
Soybean cyst nematode reproduction related to tillage and rhizosphere microorganisms. I. ABDI (1), P. A. Donald (1), and D. D. Tyler (2). (1) USDA ARS, CGRPU, Jackson, TN 38301; (2) BESS, University of Tennessee, Jackson, TN 38301.
Differences in tillage may affect soybean cyst nematode, Heterodera glycines, (SCN) reproduction. Plots of no-tillage and tilled soybeans were established in 1979 in a randomized complete block design and individual plots were split in half in 2002 with conversion of tillage treatments to compare the long and short term differences of treatments. Research was initiated to determine if soil microorganisms were responsible for SCN reproduction differences. Data were collected on bacteria, fungi, protozoans, and SCN at planting and at harvest. The largest difference in SCN reproduction in 2006 was between the moldboard plow treatment established in 1979 and the portion of this plot which went no tillage in 2002. SCN reproduction increased 12 fold in the moldboard plowed area but only increased by 1.5 in the no till area. Over the growing season total bacteria increased 8 fold in the moldboard plow area but only 3 fold in the no till area. The interactions of the various organisms relative to cyst nematode populations were analyzed with regression and principal component analysis techniques.
Accounting for fungal spore inocula on detached leaves of strawberry for fungicide screening. M. ABRIL (1), K. J. Curry (1), and B. J. Smith (2). (1) The University of Southern Mississippi, Hattiesburg, MS 39406; (2) USDA-ARS Southern Horticultural Laboratory, Poplarville, MS 39470.
Uniform inoculation techniques would aid standardization of fungicide screening. Several factors were identified as causing variability in pipetting a precise spore concentration of inoculum on strawberry leaves. Subsequently techniques were devised in which inoculum was delivered to a glass slide in very small, precisely measured volume from which accurate counts could be taken and compared to the initial inoculum suspension concentration. A one-order magnitude reduction in spore number occurred between the concentration of the inoculum suspension and the number on the slide, similar to what was measured on leaves. The loss of spores in the inoculation process was traced back to an intermediary procedure. A large number of spores had adhered to the inside of the inoculating pipette, and would have to be accounted for to achieve precise inoculum loading.
Reduced environmental impact of fertilizers using PGPR. A. O. ADESEMOYE (1), J. W. Kloepper (1), and H. A. Torbert (2). (1) Dept. Ent & Plant Pathology, Auburn University, AL 36849; (2) USDA Soil Dynamics Laboratory, Auburn, AL 36849.
The use of fertilizers is becoming a threat to sustainability in agriculture. Inorganic fertilizer is linked to nitrate contamination of groundwater and phosphorus runoff. Even with organic fertilizers, such as poultry litter, high phosphorus bioavailability, nitrogen accumulation, and leaching may occur. This study was aimed at reducing fertilizer input and increasing nutrient uptake while maintaining yields, by adding plant growth-promoting rhizobacteria (PGPR) and/or mycorrhizae. Continuous corn on a 20-year rotation with wheat and soybean was treated with four inoculants (PGPR, mycorrhizae, PGPR plus mycorrhizae, and control) on two fertilizer types (organic and inorganic), and two tillage systems (conventional and no till) in a split-split-split plot design. Yield per plot, and nutrient content of silage and grain were monitored. In the greenhouse, tomato was grown with different rates of water-soluble N-P-K (20-10-20) with or without inoculants. Inoculants led to growth promotion in the greenhouse and field, increases in yield, and 30–60% increased nutrient uptake.
Effects of four commercial peanut seed treatments on plant stand, diseases, and pod yield. T. B. BRENNEMAN. Dept. Plant Pathology, University of Georgia, Tifton, GA 31794.
Georgia Green peanut (Arachis hypogaea L.) seed (about 80% germination) were planted with no treatment, Vitavax PC, Dynasty PD, Trilex Optimum, or Trilex Star, each at 250 g per 100 kg seed, in one test in 2005 and two tests in 2006. Plant stands ranged from 2.6 to 7.9 plants/m with nontreated seed. All treatments increased plant stands compared to the nontreated control (6.6 to 10.5 plants/m), and were similar except Vitavax PC which had slightly fewer plants in one test. Tomato spotted wilt (TSWV) was severe in all tests, especially with nontreated seed (44–74% incidence). All treatments reduced TSWV compared to the nontreated control except Trilex Optimum in one test; otherwise all treatments were similar. Plots with nontreated seed yielded an average 2655 kg/ha. All treatments significantly increased yield compared to the nontreated in at least two of three tests with no differences among the seed treatments. The average yields were 3674, 3905, 3957 and 3762 for the Trilex Optimum, Trilex Star, Dynasty PD, and Vitavax PC, respectively. All four treatments effectively increased plant stands, reduced TSWV, and increased yield.
Effects of microbial inoculants on soil microbial activity, bacterial populations, and diseases suppressiveness. M. BURKETT-CADENA (1), N. Kokalis-Burelle (2), K. Lawrence (1), and J. W. Kloepper (1). (1) Dept. of Entomology and Plant Pathology, Auburn University, AL 36849; (2) USDA-ARS, Ft. Pierce, FL 34945.
Among the various management options for root knot nematodes, are inducing soil suppressiveness by microorganisms. Induction of soil suppressiveness by commercially-available microbial inoculants (Bioyield and FZB42) and the relation to microbial activity and population size were studied. Tomato seedlings were first inoculated with microbial inoculants and then challenged with Meloidogyne incognita race 3 eggs. Results showed significant reductions in numbers of nematode eggs per gram of root, numbers of juveniles per ml, and numbers of galls in Bioyield and FZB42 treatments. Additionally, increases in bacterial population size were detected by direct plate counting, although there was no correlation between microbial activity detected by Fluorescein diacetate hydrolysis and population size. These results indicated that selected microbial inoculants colonize the root system, establish stable populations, and thereby can be used to induce suppressiveness to soilborne pathogens.
Evaluation of seed treatments, cultural practices, and fungicide sprays to control ashy stem blight of snap bean. C. H. CANADAY. Dept. of Entomology and Plant Pathology, University of Tennessee, WTREC, Jackson, TN 38301.
Ashy stem blight of snap bean, caused by Macrophomina phaseolina, occurs as both a seedling blight and as a stem blight in west Tennessee. In seedling blight, a hypocotyl rot is usually observed that appears to originate at the cotylendonary node. In 2006 eight seed treatments and two herbicide treatments were evaluated in a factorial field experiment for control of the seedling blight phase. No single treatment provided complete control. Postemergence losses were reduced by azoxystrobin applied either as a seed treatment or as an in-furrow spray compared to standard seed treatments. Switching from a preplant incorporated application of the herbicide S-metolachlor to a preemergence application improved seedling stands by 25%. In a separate test, a postemergence spray of boscalid reduced stand losses to ashy stem blight by 7.5%. A combination seed treatment (mefenoxam + thiamethoxam + streptomycin sulfate) plus an in-furrow spray of azoxystrobin had the least postemergence damping-off and the highest yield.
Effects of a SAR inducer, cultivar, and plant growth-promoting rhizobacteria (PGPR) on foliar and fruit diseases of tomato and on marketable yields. C. H. CANADAY. Dept. of Entomology and Plant Pathology, University of Tennessee, WTREC, Jackson, TN 38301.
Both early blight (Alternaria solani) and Septoria leaf spot (Septoria spp.) are common diseases of staked tomato in west Tennessee. In 2006 an inducer (acibenzolar-S-methyl) of systemic activated resistance (SAR) was evaluated in a field experiment for effects on diseases and yield of six tomato cultivars grown with and without PGPR. Plots received either full rate fungicide sprays or fungicides at 1/3 rate + SAR inducer (fung/3+SAR). Cultivars varied in their response. Fung/3+SAR reduced the severity of foliar diseases on BHN640 and Mountain Crest two and three months after transplanting, respectively, but had no effect on Amelia, Florida 47, Mountain Fresh Plus, or Mountain Spring. Fruit rot incidence (Alternaria rot + buckeye rot [Phytophthora nicotianae var. parasitica]) increased with fung/3+SAR. Marketable yield of BHN640 was increased with fung/3+SAR. Marketable yields of the other cultivars were unaffected by spray treatment. Use of PGPR had no effect on the severity of foliar diseases, fruit rot incidence, or marketable yield.
A tospovirus in the sweetpotato virus complex. C. A. CLARK and M. W. Hoy. Dept. Plant Pathology & Crop Physiology, Louisiana State Univ. AgCenter, Baton Rouge, 70803-1720.
Viruses identified in sweetpotato in the US: SPFMV, SPVG, and IVMV, do not reproduce the common symptoms or reductions in yield observed in the field. Titer of these potyviruses is dramatically lower in artificially inoculated plants than in ‘naturally’ infected plants. We hypothesize that additional unknown viruses enhance the effects of the common potyviruses. Transmissions made directly or indirectly from sweetpotato or Ipomoea leucantha from near sweetpotatoes to I. nil cv. Scarlet O’Hara (SOH) caused unique necrotic lesions and localized wilting in SOH and did not give reactions in ELISA to antisera of known sweetpotato viruses. These plants reacted positively with four different antisera to Tomato spotted wilt virus (TSWV). Bands of expected size were obtained from amplifications of RNA extracts from several isolates using the AgDia (Elkhart, IN) PCR test for Tospovirus group. One product was cloned, sequenced and had 94% identity to GenBank accession AY070218 (TSWV RNA dependent RNA polymerase). No symptoms were induced in virus-tested or SPFMV-infected Beauregard or Porto Rico plants when grafted with TSWV.
Rate and intervals of hydrogen dioxide applications to control Puccinia hemerocallidis on daylily. W. E. COPES. USDA/ARS Southern Horticultural Laboratory, Poplarville, MS 39470.
Hydrogen dioxide (H(2)O(2)) is a disinfestant used to kill fungal spores on plant surfaces. Many trials have shown label rates at weekly applications are not effective. In an in vitro study, H(2)O(2) rates were sprayed on excised sections of daylily leaves with rust pustules. Percent urediniospore germination decreased with increasing H(2)O(2) concentration until 100% mortality was achieved at 11.9 part per hundred (pph) H(2)O(2). In a outdoor study, one label rate (0.27 pph H(2)O(2)) and two high rates (5.4 and 10.8 pph H(2)O(2)) were applied 1 and 2 times per week on healthy daylily plants. Only the highest H(2)O(2) rate, which damaged plants, provided rust control equal to the fungicide treatment (azoxystrobin and chlorothalonil rotation). In a greenhouse study, rates labeled for plants (0.20, 0.27 pph H(2)O(2)) and production surfaces (0.34 pph H(2)O(2)) were applied 2, 3, and 5 times per week on healthy daylily plants. Five applications per week at 0.27 pph H(2)O(2) did not damage plants and provided rust control equal to the fungicides when rust incidence of control plants was 59%. When incidence was 89%, rust control was not equal to the fungicides.
Mustard cover crop for management of tobacco black shank. A. S. CSINOS, L. L. Hickman, and L. Mullis. Dept. Plant Pathology, University of Georgia, Tifton, GA 31793.
The race structure of Phytophthora parasitica var. nicotianae (Ppn), the causal agent of tobacco black shank, has shifted from race 0 to race 1 with the introduction of the Ph gene into tobacco cultivars for race 0 resistance. In 2004 a test was initiated to evaluate the use of ‘Florida Broadleaf’ mustard (M) green manure to manage Ppn, by planting in the fall and incorporating the mustard crop just prior to planting the following spring. In 2005 the highest yields were 2965 kg/ha for Peanut (P)-Rye(R)-Tobacco (T) (6% disease) with mefenoxam compared to 2723 kg/ha for P-M-T (3% disease) with mefenoxam. However, T-R-T untreated yielded 516 kg/ha (83% disease) compared to T-M-T untreated which yielded 1116 kg/ha (63% disease). In 2006, yields for plots with a wheat cover crop with and without mefenoxam were 2371 kg/ha (30% disease) and 768 kg/ha (85% disease), respectively. Yields for plots with the mustard cover crop, with and without mefenoxam, were 2918 kg/ha (16% disease) and 1578 kg/ha (68% disease), respectively. Yields were all significantly different from each other, and the mefenoxam treated plots were different from the nontreated for % disease in both crop scenarios.
Evaluation of resistance to Cylindrocladium parasiticum in peanut in naturally infested or inoculated fields, and in the greenhouse. W. B. DONG (1), T. B. Brenneman (1), C. C. Holbrook (2), and A. K. Culbreath (1). (1) Dept. of Plant Pathology, University of Georgia, Tifton, GA 31793; (2) USDA-ARS, Coastal Plain Exp. Stn. Tifton, GA 31793.
Our objective was to evaluate six runner peanut (Arachis hypogaea) genotypes with varying resistance to Cylindrocladium black rot (CBR) in a naturally infested field, an inoculated field, and in greenhouse trials. The genotypes GA-02C and Georganic had low plant mortalities, whereas C-99R and DP-1 had high mortalities in infested fields in 2005 and 2006. Plant mortalities in GA-01R were moderate in both years, but were inconsistent in C34-24-85. GA-02C and Georganic also showed partial resistance to CBR in greenhouse tests. Field inoculations of GA-02C and Georganic had little pod rot from CBR, and Georganic also had relatively lower pod loss. Plant mortality in the infested field test was correlated with disease and yield in the inoculated test, but neither were correlated with disease ratings for greenhouse tests. The variability of disease levels was highest in the infested field. Peanut genotypes are most reliably screened in field tests, but greenhouse evaluations may help identify and characterize components of resistance.
Management of Asian soybean rust in Georgia with fungicides. R. C. KEMERAIT (1), L. E. Sconyers (1), and P. H. Jost (2). (1) Dept. of Plant Pathology, The University of Georgia, Tifton, GA 31794; (2) Dow Agrosciences, Statesboro, GA 30458.
Trials were conducted on soybean in 2006 to assess efficacy of fungicides for control of Asian soybean rust (ASR). Fungicides were applied in a spray volume ranging from 140 to 163 L per ha at 276 kPa beginning at early reproductive growth stages. Incidence (% symptomatic leaves), leaf severity (1-8 Bayer scale), % defoliation, and yield were collected from each plot. In the Tift Co. trial, leaf severity at final rating ranged from 0.0 (2 applications of pyraclostrobin + tebuconazole) to 5.9 (~25%) for the untreated control. In a Colquitt Co. trial, leaf severity at time of final rating ranged from 0.0 (azoxystrobin followed by either tetraconazole or flutriafol, and two applications of tetraconzazole) to 5.9 (~25%) for the untreated control. In trials conducted in Decatur Co., leaf severity at final rating ranged from 1.6 (<5%) for myclobutanil + pyraclostrobin to 7.5 (67.5%) for the untreated control and 1.8 (<5%) for tebuconazole + pyraclostrobin to 7.7 (67.5%) for the untreated control. Yield data was being collected at time of abstract submission.
Fern distortion syndrome: A newly described disease of Leatherleaf fern. J. W. KLOEPPER (1), H. A. Mills (2), F. Saborío (3), and E. Bustamante (4). (1) Dept. Entomology and Plant Pathology, Auburn University, Auburn, AL 36849; (2) MMI Labs, Athens, GA 30607; (3) Centro de Investigaciones Agronómicas, Universidad de Costa Rica; (4) San José Costa Rica.
Leatherleaf fern (Rumohra adiantiformis) is valued for use in floriculture because of its highly symmetrical pyramidal shape of fronds. Here we present a new description of a syndrome, termed fern distortion syndrome (FDS) which is present in Florida and Costa Rica. The main above-ground symptom of FDS is twisting and distortions of fronds. Bronze and chlorotic streaks are sometimes present on frond in addition to distortions. In advanced cases, fronds become thickened, new frond growth ceases or slows dramatically, and uneven sporulation is apparent on the underside of fronds. Symptoms of FDS below ground are reduced diameter of rhizomes, reduced overall root mass, and distorted growth of root hairs. Using a rating system, we found that the incidence of FDS in commercial ferneries in Costa Rica was typically over 60%. Vegetative propagation by cuttings of rhizomes from FDS-affected plants typically results in daughter fern plants that develop FDS symptoms.
Evaluation of commercial watermelon rootstocks for tolerance to Phytophthora blight. C. S. KOUSIK (1) and Richard Hassell (2). (1) USDA, ARS, U.S. Vegetable Laboratory, Charleston, SC; (2) Clemson University, CREC, Charleston, SC.
Phytophthora blight and fruit rot caused by Phytophthora capsici is becoming an important and emerging disease of watermelons (Citrullus lanatus). In recent years the practice of grafting seedless watermelons (triploids) onto rootstocks belonging to other Cucurbitaceae genera is also gaining importance. We evaluated five week old plants of commercial rootstocks for tolerance to Phytophthora blight by inoculating them with a zoospore suspension (10,000 zoospores/plant) consisting of a mixture of seven isolates of P. capsici in the greenhouse. Commercial rootstocks called Macis and Emphasis were tolerant to Phytophthora blight compared to RS-1330, PST04-109W and Shintosa-Camel. Similarly triploids grafted on Emphasis appeared to be tolerant compared to the susceptible cultivar black diamond. As a part of our new program, we will be conducting further studies in a P. capsici infected field to test the effectiveness of rootstocks and grafts and also screen plant introductions of various cucurbit genera to identify resistant rootstocks.
Accessing fungal pathosystems with new tools. S. M. MAREK. Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74074.
The cytology of and molecular mechanisms employed by fungi during pathogenesis of plant hosts can be studied using numerous approaches. Agrobacterium-mediated transformation of fungi to express the reporter transgenes GFP and DsRed has permitted the cellular interactions of novel pathosystems to be defined and elucidated. Using Medicago truncatula as a model host, transformants of the foliar necrotrophs, Phoma medicaginis and Leptosphaerulina trifolii, were found to trigger localized epidermal cell death upon penetration. Intracellular hyphae appeared quiescent but then accelerated leaf senescence to facilitate colonization. A library of over one thousand T-DNA-tagged P. medicaginis transformants has been constructed, from which morphogenic and hypovirulent mutants have been isolated. Transformants of Botrytis cinerea and Ophiosphaerella herpotricha are also being used for cytological studies. In a separate collaborative project, we have begun sequencing the genome and transcriptome of the Phymatotrichum (Texas) root rot fungus, Phymatotrichopsis omnivora, a soilborne Pezizomycete highly destructive to many crops in the southwestern US.
Phytophthora cinnamomi isolate identification for development of a greenhouse screening technique for identifying resistant blueberry (Vaccinium sp.) germplasm. M. MILLER-BUTLER (1), K. J. Curry (1), and B. J. Smith (2). (1) University of Southern Mississippi, Hattiesburg, MS 39406; (2) USDA-ARS Southern Horticultural Laboratory, Poplarville, MS 39470.
Blueberry production (Vaccinium sp.) in Mississippi, occupies over 2000 acres. Phytophthora cinnamomi causes severe root rot on many woody plants including blueberry. Soil samples were collected from symptomatic blueberry plants at farms in Mississippi. Isolates were recovered using a floating leaf technique and PAR(PH)-V8 media. Identification of isolates was confirmed to genus with a commercial ELISA test (Agdia). Susceptible seedlings were inoculated with all of the recovered isolates, including a P. cinnamomi isolate. Virulent isolates will be identified to species and used to develop a greenhouse technique for large scale screening of blueberry germplasm for resistance to Phytophthora root rot.
Distribution of Iris yellow spot virus in onion leaves. C. Nischwitz (1), S. Mullis (1), R. Torrance (2), D. Langston Jr. (3), A. Sparks (3), and R. Gitaitis (1). (1) Dept. of Plant Pathology, Univ. Georgia, Tifton, GA 31793; (2) UGA-CES, Reidsville, GA 30453; (3) UGA-CES, Tifton, GA 31793.
Iris yellow spot virus (IYSV) has been detected in Georgia onions since 2003. Correlations between symptomatic onion leaves and lab assays (DAS-ELISA and RT-PCR) are high in seedlings but are low in more mature onion leaves. With seedling samples, the majority of the leaf or the entire leaf is tested, however with more mature leaves, only a small fraction (about 2.54 cm) of the leaf is tested. To determine the distribution of IYSV in mature onion leaves we screened 90 onion plants for IYSV. An area near symptomatic lesions or signs of heavy thrips feeding was selected for processing. 51 leaves testing positive in the initial screen were used for further analysis. Individual leaves were cut into 2.54 cm segments from tip to base and each segment was tested with DAS-ELISA. The results indicate that IYSV distribution within the leaf was not uniform and the virus may infect onion leaves locally but not systemically. These results show that more segments per leaf sample need to be tested in order to determine the presence or absence of IYSV in mature onion samples.
The influence of exogenous nutrients on the abundance of yeasts on the phylloplane of tall fescue. S. NIX-STOHR, L. L. Burpee, and J. W. Buck. Dept. of Plant Pathology, University of Georgia, Griffin, GA 30223.
Four experiments were conducted to assess the effect of foliar applications of various nutrient solutions on the phylloplane yeast community of tall fescue (Fescue arundinacea Schreb.). The objective was to understand the effect of exogenous nutrients on phylloplane yeast populations with a long term goal of increasing leaf carrying capacity for use in biocontrol. Significant positive linear relationships were observed between the number of yeast cfu and applications of yeast extract and sucrose plus yeast extract. Foliar applications of sucrose alone had no significant effect on yeast abundance, indicating that phylloplane yeasts of tall fescue are not limited by the amount or availability of carbohydrates. In the fourth experiment, tryptone and yeast extract, both with abundant amino acids, significantly increased the yeast population while yeast nitrogen base and ammonium sulfate had no affect on yeast abundance. These results suggest that organic nitrogen stimulates yeast community abundance on tall fescue while carbohydrates, inorganic nitrogen and non-nitrogenous nutrients have little positive effect.
Changes in rhizosphere bacterial communities of Leatherleaf fern affected by fern distortion syndrome and treated with the systemic fungicide, Benlate. C. RAMÍREZ, M. Liles, and J. W. Kloepper. Auburn University, Auburn, AL 36849.
We are testing the hypothesis that FDS (fern distortion syndrome) of Leatherleaf fern is associated with changes in bacterial communities in the rhizosphere and inside plants that result from treatment with the systemic fungicide, Benlate. Here we report results of molecular analyses of rhizosphere samples from a) field-collected fern with and without FDS symptoms and b) greenhouse tests applying Benlate. From the field plants, no significant differences were detected in total bacterial community as studied by DGGE of amplicons obtained with eubacterial primers. Likewise, amplification by nested PCR was inconsistent or did not show different DGGE profiles for bacterial groups such as alpha-Proteobacteria, beta-Proteobacteria, gamma-fusobacteria, Actinobacteria, Verrucomicrobia, and Acidobacteria. However, significant differences were detected in the DGGE profile of Pseudomonas and Bacillus specific-PCR products. A 132% increase of Pseudomonas-specific amplicons was obtained from fern with FDS symptoms. In the greenhouse experiment, DGGE analyses showed a profound effect of Benlate treatment on Pseudomonas population, with some changes in total bacteria and bacilli. Overall, our results support a role for Pseudomonas spp. in FDS and response of plants to Benlate.
Management of whitefly populations for the control of watermelon vine decline in Florida. P. D. ROBERTS (1), P. A. Stansly (2), S. A. Adkins (3), C. S. Kousik (4), and B. Bruton (5). (1) University of Florida, Department of Plant Pathology, SWFREC, Immokalee, FL; (2) University of Florida, Department of Entomology & Nematology, Immokalee, FL; (3) United States Department of Agriculture-ARS, USHRL, Fort Pierce, FL; (4) US Vegetable Lab, USDA, Charleston, SC; (5) United States Department of Agriculture-ARS, Lane, Oklahoma.
Field studies were designed to confirm that a new ipomovirus, Squash vein yellowing virus (SqVYV), causes watermelon vine decline (WVD) in Florida and is transmitted by the silverleaf whitefly Bemisia tabaci (Biotype “B”). Two field studies were conducted in 2006 at SWFREC, Immokalee, FL, in the spring and fall growing seasons. Watermelon plants mechanically inoculated with SqVYV in the field at different growth stages developed WVD symptoms. Watermelon plants exposed to virus-infected squash plants developed symptoms typical of WVD and produced fruit with typical rot symptoms of WVD. Watermelon plants grown in screened cages that prevented whitefly infestation did not develop symptoms of WVD including fruit rot. Insecticide applications of imidachloprid at transplanting and subsequent foliage applications of pymetrozine did not prevent WVD on treated plants in the spring trial. However, in the fall trial, the rate of spread, severity of WVD, and the number of whiteflies on plants treated with these insecticides decreased compared to the untreated plants. Experiments targeting management of WVD by insecticidal control of whiteflies and integrated with cultural methods will be explored.
Sentinel plot monitoring and comparison of Asian soybean rust development in Georgia in 2005 and 2006. L. E. SCONYERS (1), R. C. Kemerait, Jr. (1), D. V. Phillips (2), and P. H. Jost (3). Dept. of Plant Pathology, University of Georgia, (1) Tifton, GA 31793, (2) Griffin, GA 30223; (3) Dow AgroSciences, Statesboro, GA 30458.
In 2005 and 2006, kudzu and soybean sentinel plots were monitored for Asian soybean rust Phakopsora pachyrhizi (SBR). Weekly, SBR severity (low, moderate or heavy), incidence (number of leaflets infected of 100 collected), and growth stage were recorded for soybean sentinels. SBR was first detected on 22 Apr 2005 and 30 Jan 2006 on kudzu. In 2006, SBR developed in 5 of 10 kudzu plots and 8 of 14 soybean plots, while 1 of 5 kudzu and 14 of 17 soybean plots developed rust in 2005. Based upon sentinel plots and samples submitted from grower fields in Georgia, it appears that SBR was moving north at an estimated 97 km and 80 km per week in 2005 and 2006, respectively. By 31 Oct, SBR was confirmed in 15 counties in 2006 and 31 counties in 2005. SBR development in 2006 may have been slowed due in part to a calm hurricane season and a severe drought that resulted in a –12 to –56-cm deviation from the 20-year rainfall average. Sentinel plots will need to continue to be monitored in order to understand the epidemiology of this disease.
Management of target spot on burley tobacco with azoxystrobin. K. W. SEEBOLD (1) and G. K. Palmer (2). (1) University of Kentucky, Plant Pathology Dept. and (2) University of Kentucky, Dept. of Plant and Soil Sciences, Lexington, KY 40546.
Target spot (TS), caused by Rhizoctonia solani AG-3, has become a serious constraint to the production of burley tobacco in many parts of Kentucky. Varieties with resistance to TS are not available, and control of the disease with mancozeb (the only labeled option prior to registration of azoxystrobin) is poor. Trials were conducted 2005 and 2006 to evaluate the effects of application rate and frequency on TS. Azoxystrobin was applied one, two, or three times (14–21 day schedule) to tobacco at rates of 8 or 12 fl oz/A beginning 3–6 weeks after transplanting. Generally, azoxystrobin reduced leaf spotting due to TS compared to untreated plots by 75–85% for the 8 and 12 fl oz/A rates, respectively. One or two applications of azoxystrobin were generally as effective against TS as three applications of the fungicide. Effects on yield were variable by location and year.
Cultural practices and chemical treatments affect Phytophthora root rot severity of blueberries grown in South Mississippi. B. J. SMITH. USDA-ARS, Small Fruit Research, P.O. Box 287, Poplarville, MS 39470.
Phytophthora root rot of blueberries is most severe when plants are grown in wet soils with poor drainage. Symptoms include small, chlorotic leaves, lack of new growth, and root necrosis. Two five year studies conducted in Mississippi evaluated the effect of drainage (subsoil, tile, none) and fungicide (metalaxyl, none) treatments on Phytophthora root rot severity of mature, infected rabbiteye blueberries (Study 1) and the effect of drainage, fungicide, and bed height (raised, flat) treatments on disease severity of young rabbiteye and southern highbush blueberries grown in infested soil (Study 2). Significant improvements in Study 1 were due to subsoil treatment and twice a year treatments with the fungicide metalaxyl. In Study 2 there were no significant main effects due to bed height, drainage, or fungicide treatment in the height, size, or percentage of living plants, but there was a significant interaction between bed height and fungicide treatment. Rabbiteye plants grown on raised beds treated with metalaxyl were taller and had a greater percentage of living plants than those grown on untreated flat beds.
Infection of longleaf pine seedlings with Cronartium quercuum f. sp. fusiforme. C. H. WALKINSHAW. U.S. Forest Service, Pineville, LA.
Field plantings of longleaf pines appeared to possess a high level of resistance to fusiform rust. However, when infection did occur, galls caused severe damage to saplings. In this study histological methods and tissue reactions were used to describe fusiform rust development in longleaf pine seedlings. The objective was to define stem resistance in longleaf pine by measuring changes in anatomy of inoculated seedlings. A total of 188 inoculated seedlings were sectioned, stained, and evaluated for traits including gall formation, percent with fungal hyphae in the cambium, and presence of reaction zones, tannin cells, haustoria, and enlarged cortical cells. Six months after inoculation, the average percent of galled seedlings was 44. The percent of stems with fungus in the cambium was 39. The variation within the measurements of the traits was low Reactions of bark, cambium, cortex and pith resembled those previously reported for loblolly and slash pines. One exception was a high incidence of multiple stem production. Fusiform rust in longleaf pines was increased when nearby loblolly and slash pine were heavily infected.
Screening peanut germplasm for resistance to Cylindrocladium parasiticum. J. N. WILSON (1), T. A. WHEELER (2), and M. C. Black (3). (1) Department of Plant and Soil Science, Texas Tech University, Lubbock TX 79409; (2) Texas Agriculture Research and Extension Center, Lubbock, TX 79403; (3) Texas Agriculture Research and Extension Center, Uvalde, TX 78801.
Soil borne diseases have been a significant concern in West Texas peanut production. Cylindrocladium parasiticum is an important peanut pathogen in the southeastern U.S. and was first confirmed in Texas in 2004. Experiments were conducted to refine a screening technique for C. parasiticum using peanut germplasm with known resistance levels. Peanuts were screened using two container sizes (66 and 164 ml), three inoculum densities (0, 15, and 25 microsclerotia/g soil), and were sampled at three different times. Root rot ratings, fresh root and shoot weights, and percentage visible taproot and secondary root necrosis were taken at the three sampling intervals. Results indicated that inoculum density was the most dominant factor affecting the relationship between root rot ratings for susceptible and resistant genotypes. The greatest difference between resistant and susceptible genotypes occurred at an inoculum density of 25 microsclerotia/g soil. Spanish hi-oleic and runner peanut germplasm developed by Texas A&M are being screened for resistance to C. parasiticum.
Evaluation of Rhizoctonia diseases on peanut cultivars and advanced breeding lines in West Texas. J. E. WOODWARD (1) and T. A. Baughman (2). (1) Dept. of Plant Pathology and Microbiology, Texas A&M University, Lubbock, TX 79403; (2) Dept. of Soil and Crop Sciences, Texas A&M University, Vernon, TX 76384.
Diseases caused by Rhizoctonia solani commonly occur in peanut (Arachis hypogaea L.) fields throughout West Texas. During the 2006 season, different assessment methods were evaluated in cultivar trials conducted near Wellington and Vernon, TX. Pod rot was the predominant disease at both locations, and pod rot severity was determined for each plot after plants were inverted. Limb rot was enumerated by counting the number of lesions on twenty branches from each plot. At Wellington, limb rot incidence ranged from 0.6 to 1.9 lesions per limb for the cultivars Georgia-02C and ANorden, respectively. Pod rot was greatest for the breeding lines TROL07 and TROL02, and lowest for the cultivars Georgia-03L and AndruII. Flavorrunner 458, the commercial standard, exhibited intermediate levels of limb rot and pod rot. Similar trends were observed at the Vernon site. These results indicate that varying levels of susceptibility to Rhizoctonia diseases are present in peanut cultivars and breeding lines grown in West Texas.