February 15-17, 2004 - Tulsa, Oklahoma
(Joint with the Southern Association of Agricultural Scientists (SAAS))
Posted online May 3, 2004
Development of standardized techniques for screening natural product fungicides. M. ABRIL (1), K. J. Curry (1), D. E. Wedge (2), and B. J. Smith (3). (1) University of Southern Mississippi, Hattiesburg, MS; (2) USDA-ARS, University, MS; (3) USDA-ARS, Poplarville, MS. Publication no. P-2004-0035-SOA.
We developed two standardized procedures that improve the reliability of in vitro fungicide testing using strains of Colletotrichum and Botrytis. They required different incubation temperatures, but all were grown under cool-white fluorescent lights with a 12 h photoperiod. Fungi were grown on Roswell Park Memorial Institute medium 1640. We developed a protocol for standardizing spore germination times. A concentration of 10(^5) conidia/ml, harvested after 10 to 14 days, was optimal. Higher concentrations inhibited germination. We washed conidia three times in sterile water, thus significantly reducing germination time. We developed a leaf-clearing technique to assess reproducible germination conditions using detached strawberry leaves. Small leaf pieces were placed overnight in a solution of 1:2 absolute ethanol:glacial acetic acid, stained with 0.05% acid fuchsin for two minutes, and water rinsed. The leaves were clear, viable conidia were bright red, and nonviable conidia were clear.
Dimethomorph and phosphites for control of Phytophthora root rot in ornamentals and Fraser fir. D. M. BENSON (1), J. R. Sidebottom (2), and C. Y. Warfield (1). North Carolina State University, (1) Dept. of Plant Pathology; (2) Dept. of Forestry, Raleigh, NC 27695. Publication no. P-2004-0036-SOA.
Phytophthora root rot caused by P. cinnamomi is a serious disease of ornamentals and Fraser fir grown for Christmas tree production in NC. Successful management of the disease relies on the use of fungicides to prevent root infection. The efficacy of drenches of dimethomorph (12.8 oz/100gal, Stature-DM) and sprays of the phosphites, Biophos (64 and 128 fl oz/100gal) and Vital (32 and 64 fl oz/100 gal) were evaluated in season-long experiments in 2002 and 2003. Rhododendron, azalea, or Fraser fir plants in pinebark-based mix were inoculated in June of each year using rice grains colonized by P. cinnamomi and irrigated daily. The dimethomorph drench was begun at the time of inoculation and the phosphites 5 days prior to inoculation. Root rot symptoms developed by mid-July in unprotected plants. In 2002, dimethomorph and both phosphites were effective on all crops compared to the control when applied every 14 (rhododendron) or 30 days (azalea and fir) from June to September. Disease was more severe in 2003 due to wet weather, and only dimethomorph was effective on all three crops.
Effect of delayed initiation and early-termination of fungicide programs on pecan scab. T. B. BRENNEMAN. Department of Plant Pathology, Univ. of Georgia, Tifton, GA 31794. Publication no. P-2004-0037-SOA.
Pecan (cv. Desirable) trees were sprayed in 2003 with a full-season program of nine fungicide applications, or with an abbreviated program of one, two, or three less sprays left off either the start or end of the season. A mix of Orbit 45WP (0.28 kg/ha) and Super Tin 80WP (0.35 kg/ha) was used for all applications. Frequent rains resulted in a severe scab epidemic with nontreated nuts having 100% incidence and 88% severity by July 31. Spray programs 1-9, 2-9, 3-9 and 4-9 had 40%, 58%, 69%, and 82% severity, respectively, of nut scab at harvest, whereas programs 1-8, 1-7, and 1-6 sprays had 58%, 85%, and 91% severity, respectively (LSD = 10.0). All delayed sprays also resulted in more severe leaf scab. Since the nuts did not start forming until about the fourth spray, the prepollination sprays (1-3) apparently served to reduce inoculum either directly or indirectly via reducing leaf scab. Delayed sprays resulted in greater nut drop than full season or early-terminated programs, especially at mid season, as well as more defoliation at harvest. Late season sprays, even after shell hardening at spray 8, also reduced final nut scab severity.
Development of effective strategies for control of spring dead spot in bermudagrass. E. L. BUTLER and L. P. Tredway. Dept. Plant Pathology, North Carolina State Univ., Raleigh, NC 27695. Publication no. P-2004-0038-SOA.
The objective of this research is to develop effective strategies for control of spring dead spot of bermudagrass. Four fungicides were evaluated using several application methods and timing regimes. Method studies at two sites included five application methods (surface application in 0.1, 0.2, or 0.4 L H(2)O/m(^2), in 0.1 L H(2)O/m(^2) watered-in with 6 mm irrigation, or high pressure soil injection) of four fungicides (azoxystrobin, fenarimol, myclobutanil, or propiconazole). At Site 1, all fungicides significantly reduced initial disease incidence (Y(i)), but all fungicides except myclobutanil significantly reduced recovery rate (r). At Site 2, fenarimol and propiconazole reduced Y(i), but did not affect r. Surface applications (0.2 or 0.4 L) or watered-in applications tended to reduce Y(i), but did not affect r. Application timing studies also were conducted at two sites. Twelve timing regimes and four fungicides (as above) were used. No differences were detected among timing regimes in either location, but treatments applied on monthly intervals from Aug-Nov, Sept-Nov, or Oct-Nov tended to reduce Y(i).
Increases in the incidence of snap bean seedling diseases associated with preplant incorporation of S-metolachlor. C. H. CANADAY (1) and J. E. Wyatt (2). (1) Dept. of Entomology and Plant Pathology; (2) Dept. of Plant Sciences, The University of Tennessee, Knoxville, TN 37901. Publication no. P-2004-0039-SOA.
The effects of herbicide application method on snap bean seedling diseases, plant growth, and yield were investigated in two fields naturally infested with Rhizoctonia solani and Macrophomina phaseolina. Seedling disease incidence was significantly higher with preplant incorporation (PPI) of S-metolachlor at 1.6 kg/ha than with a preemergence application (PRE) at the same rate. Snap bean seedling vigor, plant stands, and plant heights were all significantly greater with PRE than with PPI application of S-metolachlor. In a field with moderate seedling disease pressure, yields with PRE were 30% greater than with PPI. In a field with severe seedling disease pressure, yields with PRE were 60% greater than with PPI. A simple change in the application method of S-metolachlor may decrease snap bean seedling disease incidence and greatly increase grower profits.
Integrated disease management of early leaf spot of peanut. E. G. CANTONWINE and A. K. Culbreath. Univ. of Georgia, Dept. of Plant Pathology, Tifton, GA 31793. Publication no. P-2004-0040-SOA.
Early leaf spot caused by Cercospora arachidicola is one of the most important diseases of peanut (Arachis hypogaea) worldwide. Crop rotation, reduced tillage, and moderately resistant cultivars can suppress leaf spot epidemics independently. Experiments were carried out to assess the impact of these practices when combined into an integrated disease management system. Peanut genotypes that vary in their resistance to C. arachidicola, Georgia Green, C-99R, Hull, DP-1, GA-01R, C-11-2-39, C-28-305 and C-24-34, were grown under conventional and strip-tillage in a field under a 1 year peanut-cotton rotation. The cultivar Georgia Green was grown under both tillage situations in a non-rotated field. Under crop rotation, strip-tillage caused a delay in the onset of the epidemic that resulted in greater suppression for the least resistant cultivars. The suppressive effect of host resistance was greater under conventional tillage. Up to 65 percent less defoliation occurred when strip-tillage and host resistance were used alone, and 90 percent when combined. Epidemics were not suppressed by strip-tillage in non-rotated fields.
Integrated management of bacterial spot of pepper in Oklahoma. J. P. DAMICONE and M. A. Trent. Dept. of Ento. and Plant Path., Okla. State Univ., Stillwater, OK 74078. Publication no. P-2004-0041-SOA.
The responses of susceptible and resistant (Bs2) bell pepper cultivars to bactericide and plant defense activators used to control bacterial spot were assessed over two years. Peppers, grown in a field with a history of bacterial spot caused by races of Xanthomonas campestris pv. vesicatoria incompatible with Bs2, received weekly applications of copper (Cu), Cu+maneb, acibenzolar-s-methyl (ASM), and harpin. Disease incidence was moderate (50%) and defoliation was low (<10%) in control plots of the susceptible cultivar each year. Cultivar effects on disease and yield were inconsistent. Disease incidence was similar for the two cultivars in 1999, but was 50% lower for the resistant cultivar in 2000. Similarly, yields of the two cultivars did not differ in 1999, but were 73% higher for the resistant cultivar in 2000. Cultivars had similar disease and yield responses to treatments each year. All treatments except harpin, reduced disease and increased yield compared to the control. Cu and Cu+maneb reduced disease incidence, and increased yields twofold or more. ASM provided nearly complete disease control, but yields were reduced compared to Cu and Cu+maneb.
The expression of polygalacturonase-inhibiting protein activity as a function of cantaloupe fruit maturation and fruit tissue origin. W. W. FISH. USDA-ARS-SCARL, Lane, OK 74555. Publication no. P-2004-0042-SOA.
Netted cantaloupe (Cucumis melo var. cantalupensis cv. Magnum 45) were harvested from five to thirty-five days postanthesis. The fruit of each age group were divided into exocarp, outer mesocarp, mid mesocarp, inner mesocarp, placenta, and seed tissues. Each tissue was extracted and assayed for polygalacturonase-inhibiting protein (PGIP) activity against polygalacturonases (PGs) from three fungal pathogens of cantaloupe fruit. The PGIP activity of all tissues except placenta was high from the flower stage through the first week of fruit development but decreased markedly between five and ten days postanthesis. PGIP activity in placental tissue against Phomopsis cucurbitae PG remained high and nearly constant throughout fruit development. However, PGIP activity in placental tissue versus Fusarium solani PG decreased during fruit development to about twenty-five percent of its original level in the five day-old fruit. This differential change in PGIP activity toward the two PGs suggests that different forms of the inhibitor are expressed between the early and late stages of cantaloupe fruit development.
Influence of new breeding lines and tillage on peanut rust (Puccinia arachidis). S. K. GREMILLION (1), A. K. Culbreath (1), J. W. Todd (1), R. Pitman (2), and T.A. Kucharek (3). (1) Univ. of Georgia, Coastal Plain Expt. Stn., Tifton, GA; (2) USDA-ARS, Georgia Expt. Stn., Griffin, GA; (3) Univ. of Florida, Gainesville, FL. Publication no. P-2004-0043-SOA.
The goal of this research was to define the level of resistance of new peanut breeding lines to peanut rust (Puccinia arachidis) and to determine if tillage practices affect disease severity. The first field test included the breeding lines RP-01, RP-08, RP-14 and RP-20, and the cultivars Bayo Grande (BG), Georgia Green (GG), Florida MDR-98 and C-99R. Tillage practices included conventional and strip-tilled soils. A second field test included the aforementioned genotypes with the extra breeding lines, RP-15, RP-19 and RP-22, and excluding GG and C-99R. Results showed that tillage was not an influential factor in rust development (P = 0.24). When compared to the cultivars BG, GG, MDR-98 and C-99R in the first test, the new breeding lines possessed similar peanut rust severity; therefore, improved resistance to peanut rust was not indicated. However, in the second test with higher disease pressure, the new breeding lines and BG had significantly less disease than that of MDR-98 (P = 0.04).
Remote detection and differentiation of Wheat streak mosiac virus and nitrogen deficiency. D. JONES, K. Steddom, and C. M. Rush. Texas Agric. Exp. Stn., Bushland, TX 79012. Publication no. P-2004-0044-SOA.
Wheat streak mosiac virus (WSMV) and nitrogen deficiency both cause chlorosis in hard red winter wheat (Triticum aestivum L.). We investigated remote sensing techniques for detecting and differentiating these stresses. Digital images and reflectance data were collected from a factorial combination of wheat plots with and without nitrogen deficiency or WSMV. Image pixels were classified as green, yellow, or other and the percentage of each class was calculated using image analysis software. Normalized difference vegetative indices (NDVI) and other vegetative indices were calculated with reflectance data. Image data showed disease and nitrogen had a significant effect only on the green class, but healthy and chlorotic wheat could be distinguished using the percentage of green pixels in the image. The reflectance data showed disease and nitrogen significantly lowered NDVI. However, our analysis could not differentiate between chlorosis caused by WSMV and nitrogen deficiency. Stress that causes yellowing can be detected with these techniques but it is not yet evident if we will be able to differentiate WSMV and nitrogen deficiency using these techniques alone.
Recovery of Tilletia indica teliospores from regulated fields in Texas. H. W. MAPLES, J. M. Stein, and C. M. Rush. Texas Agric. Exp. Stn., Bushland, TX 79012. Publication no. P-2004-0045-SOA.
Tilletia indica, the causal agent of Karnal bunt (KB) of wheat (Triticum aestivum) and other small grains, was first detected in Central Texas in 1997. However, the survival and spatial distribution of teliospores in naturally infested field soils is unknown. In 2002, soil samples were collected in a grid pattern from nine regulated fields in Central Texas and six in North Texas. These fields were found to be positive for KB in 1997, 2001, or both years. Three fields, negative for KB, were included as controls. As a preliminary step to determining spore viability and distribution, soil was taken from each grid point and pooled to represent the entire field. Teliospores were extracted by sieving and sucrose centrifugation and then teliospores were enumerated for each pooled soil sample. Next, soil type was determined using a sedimentation method. The maximum number of spores recovered from a single field was 307 teliospores/kg of soil, no teliospores were found in two fields and the mean number was 80 teliospores/kg of soil. The soil type for the fields ranged from sand to clay loam. No significant correlation was found between the number of teliospores, soil type, or the year the field tested positive.
Using real-time PCR to quantify resistance in sugar beets to Beet necrotic yellow vein virus. K. L. MAXSON-STEIN and C. M. Rush. Texas Agricultural Experiment Station, Amarillo, TX 79106. Publication no. P-2004-0046-SOA.
Beet necrotic yellow vein virus resistance in commercial sugar beet cultivars is largely dependent on the Rz gene and its selection can be inaccurate and time-consuming. It was hypothesized that real-time PCR could quantify differences in viral infection between resistant and susceptible cultivars. Seedlings of seven sugar beet cultivars were inoculated with BNYVV and repotted. Viral infection of leaves and roots was quantified over time with real-time PCR and compared among cultivars. Additionally, resistance of a susceptible and resistant cultivar to a standard isolate of BNYVV and a new, aggressive isolate was compared. Viral infection increased over time and was higher in roots than leaves. During the second week post-inoculation, infection by standard BNYVV was significantly higher in susceptible vs. resistant cultivars; however, infections by the aggressive isolate were not significantly different between a susceptible and resistant cultivar. This study indicates that real-time PCR may be useful in quantifying resistance to BNYVV beyond that based on the Rz gene, and supports the identification of a new, aggressive BNYVV strain.
Detection and quantification of Beet necrotic yellow vein virus in soil with real-time quantitative PCR. K. L. MAXSON-STEIN, J. M. Stein, and C. M. Rush. Texas Agricultural Experiment Station, Amarillo, TX 79106. Publication no. P-2004-0047-SOA.
Beet necrotic yellow vein virus (BNYVV) causes Rhizomania in sugar beets and is vectored by the plasmodiophorid, Polymyxa betae, an obligate parasite. Traditionally, detection of BNYVV in soil requires a bioassay in which P. betae is baited with sugar beet roots, and BNYVV is then detected by ELISA or conventional RT-PCR methods. Inoculum density can be estimated using the Most-Probable-Number technique which is time-consuming and inaccurate. P. betae has been detected in soil using real-time quantitative PCR, which suggests this method could also be used to detect and quantify BNYVV in soil. Total RNA was isolated from soil infested with viruliferous P. betae and was reverse transcribed to cDNA. Primers and probes designed for the coat protein gene of BNYVV were used to detect and quantify BNYVV using real-time PCR. Detection from different soil types will be discussed. To the authors’ knowledge, this is the first report of the detection of a soil-borne fungal-vectored virus directly from soil. Results indicate this method may be useful in determining inoculum density in field soils used for sugar beet production.
The effects of soil moisture on soil electrical conductivity measurements for estimating soil texture in the field. W. S. MONFORT (1) and T. L. Kirkpatrick (2). (1) Dept. Plant Pathology, University of Arkansas, Fayetteville, AR 72745; (2) Dept. Plant Pathology, University of Arkansas, Hope, AR 71801. Publication no. P-2004-0048-SOA.
Soil texture is strongly associated with the movement, survival, and reproduction of plant parasitic nematodes. Field experiments were conducted on the Southwest Research and Extension Center in Hope, Arkansas in 2003 to determine the effects of soil moisture on the output of a Veris 3100 field soil electrical conductivity meter. Treatments were: 1) no irrigation; 2) application of 0.127 cm irrigation; and 3) application of 2.54 cm irrigation. Electrical conductivity (EC) was recorded with the Veris 3100 three times in sequence for each treatment. EC readings were higher with the addition of 2.54 cm of irrigation than the no irrigation treatment. EC readings were 10.19, 9.96, and 11.75 (LSD = 1.56) for no irrigation, 0.127 cm irrigation, and 2.54 cm irrigation, respectively. Replication effects on the EC readings were observed with lower EC values in the first replication than the others.
Evaluation of fungicide combinations for reduction of Fusarium infection in caladium tubers. R. SINGH and R. J. McGovern. University of Florida, Plant Pathology Department and Plant Medicine Program, Gainesville, FL 32611. Publication no. P-2004-0049-SOA.
Fungicides were applied to tubers of caladium ‘Florida Cardinal’ as a hot water soak (122 F/30 min). Fungicides tested included: chlorothalonil (Daconil Ultrex 82.5 WDG), fludioxonil (Medallion 50 WP), azoxystrobin (Heritage 50 G), triflumizole (Terraguard 50 W), combinations of chlorothalonil with either fludioxonil, azoxystrobin, or triflumizole, and a combination of chlorothalonil, fludioxonil, and azoxystrobin. Treatments and non-treated control were replicated three times using 20 or 10 tubers in experiments one and two, respectively. Following treatment, a core was removed from each tuber and cultured on Komada’s medium. After 9 days at 76 F, infection severity (0-4) and incidence (%) of Fusarium spp. were determined. All fungicide treatments significantly reduced infection severity and incidence compared to the control and hot water. Infection severity ratings ranged from 0.76-0.83 and 1.04-1.42 for fungicide combinations and individual fungicides, respectively, compared to 2.99 for the control and 2.45 for hot water. Similar trends were seen in reduction of Fusarium incidence.
Fungicide management of fruit diseases of strawberry in Louisiana and Mississippi. B. J. SMITH (1), R. J. Constantin (2), J. P. Quebedeaux (2), and D. E. Wedge (3). (1) USDA-ARS, Poplarville, MS 39470; (2) Louisiana State University, Hammond Research Station, Hammond, LA; (3) USDA-ARS, University, MS. Publication no. P-2004-0050-SOA.
Fungicide studies were conducted at Hammond, LA and Poplarville, MS during the 2002 and 2003 fruiting seasons. Ten (MS) or 13 (LA) fungicide treatments were applied at 7–10 day intervals to the strawberry cultivars, Camarosa (LA), Chandler (MS) and Pelican (MS). Berry diseases were identified and counted at harvest. The amount of fruit diseases, particularly anthracnose and gray mold, was low. Stem-end rot caused by Gnomonia comari was prevalent and was controlled by the combination fungicides, Elevate + Captan, Pristine, and Switch. Gray mold incidence was reduced by Scala, Elevate + Captan, Pristine, Switch, boscalid, and Elevate. In at least two of the four trials, the number of berries with diseases symptoms was lowest on plots treated with Pristine, Switch, Cabrio, Captevate, Captan, and Quadris.
Regulation of coronatine in Pseudomonas syringae pv. tomato DC3000. A. SREEDHARAN and C. L. Bender. Department of Entomology and Plant Pathology, Stillwater, OK 74078. Publication no. P-2004-0051-SOA.
Coronatine (COR) is produced by various pathovars of P. syringae, including P. syringae pv. glycinea PG4180 and P. syringae pv. tomato DC3000. COR is composed of coronafacic acid (CFA) and coronamic acid (CMA), which function as intermediates in COR biosynthesis. COR production in PG4180 is controlled by a two-component regulatory system consisting of corS (environmental sensor), corR and corP (response regulators). Both PG4180 and DC3000 synthesize COR; however, COR production is differentially regulated in these two strains. In this study, corR and corS mutants of DC3000 were defective in COR, CFA and CMA production, in multiple experiments. Furthermore, studies were designed to investigate whether cross-talk exists between the different systems that are required for pathogenicity and virulence. For example, hrpL encodes an alternate sigma factor required for the expression of various transcripts in the hrp gene cluster, which encodes the type III secretion system (TTSS). A hrpL mutant of DC3000 was defective in COR production, suggesting that mutations in the TTSS may have regulatory effects on the production of virulence factors like COR.
Comparing disease assessments methods for late season foliar diseases of wheat. K. STEDDOM (1), M. McMullen (2), B. Schatz (3), and C. M. Rush (1). (1) Texas Ag. Exp. Sta., Bushland, TX 79012; (2) Dept. of Plant Path., NDSU, Fargo, ND 58105; (3) NDSU, Carrington, ND 58421. Publication no. P-2004-0052-SOA.
Visual and radiometric disease assessments were made at the soft dough stage on spring wheat fungicide plots at Fargo and Carrington, North Dakota. Ten fully expanded flag leaves were collected from each plot, scanned with a flat bed scanner, and quantified for necrosis with software. Visual assessments had lower precision (higher CV’s) but higher treatment F statistics than other methods. Necrosis, as measured by image analysis, and yield were significant at both sites with higher precision and lower treatment F statistics than visual assessments. Radiometric methods were only significant at Carrington, where disease pressure was higher and senescence was less pronounced. Visual estimates had less precision at intermediate disease levels, where the human eye is less sensitive. We conclude that visual estimates can be augmented with image analysis to quantify late season foliar diseases of wheat. However, at this late growth stage there is insufficient leaf area on plants in any plots for a radiometer to be effective.
The use of real-time PCR to quantify the number of Tilletia indica teliospores. J. M. STEIN, K. L. Maxson-Stein, and C. M. Rush. Texas Agricultural Experiment Station, Amarillo, TX 79105. Publication no. P-2004-0053-SOA.
Karnal (partial) bunt of wheat, caused by the fungus Tilletia indica, is a USDA-APHIS quarantined organism. Little is known about the spatial distribution of T. indica teliospores in soil and the standard extraction process is tedious; requiring microscopic examination of every sample. We examined the use of real-time PCR to detect and quantify teliospore number in soil samples. Primer pairs and probes specific to T. indica, T. tritici, and Triticum aestivum were designed using published Internal Transcribed Spacer (ITS) sequences. After being spiked with 0, 10, 100, 1000, or 10,000 T. indica teliospores, DNA was extracted from i) complete soil and ii) the sucrose-pellet and float partitions following the standard sieve-extraction protocol. Teliospore number from each extraction was quantified with real-time PCR using the primers and probes. We believe that real-time PCR has potential to greatly simplify detection and quantification of T. indica teliospores in soil. Use of real-time PCR in screening for resistance to T. indica will also be discussed.
The effect of variety rotation on races of Phytophthora parasitica var. nicotianae. M. J. SULLIVAN, T. A. Melton, and H. D. Shew. Dept. of Plant Pathology, NC State University, Raleigh, NC 27695. Publication no. P-2004-0054-SOA.
Race 1 of the tobacco black shank pathogen has become more predominant in NC due to the deployment of varieties with single-gene (complete) resistance to race 0 of the pathogen. Variety rotation studies were conducted over a 4 yr period to assess how different levels and types of host resistance affect pathogen race structure. In a field infested with a mixture of races, K 346 (high level of partial resistance) was most effective in decreasing prevalence of race 1 and exhibited lowest disease incidence. Use of NC 71, with complete resistance to race O, resulted in intermediate levels of disease, but race 1 became predominant. The variety K 326 (low level of partial resistance) had the highest level of disease and race 0 was dominant. In a field where no race 1 was detected initially, NC 71 was most effective in reducing disease incidence, but race 1 was recovered after only one season. By the fourth growing season, race 1 was recovered from five of the eight treatments that included NC 71. A rotation of complete resistance and a high level of partial resistance provided the best disease control and minimized race shifts in the pathogen population.
The suitability of tall fescue selections containing novel strains of Neotiphodium coenophialum as hosts for plant-parasitic nematodes. A. C. THOMAS (1), T. L. Kirkpatrick (2), and C. P. West (3). (1) SAU, Magnolia, AR 71753; (2) SWREC, Hope, AR 71801; (3) UofA, Fayetteville, AR 72701. Publication no. P-2004-0055-SOA.
Neotiphodium coenophialum, is an endophyte of tall fescue that produces alkaloids that are toxic to ruminant animals. Certain of the alkaloids are, however, considered to enhance the resistance of infected plants to pests. Fescue containing different novel strains of N. coenophialum were studied in established pastures for their association with plant-parasitic nematodes. Spiral, lesion and a root-knot species tentatively identified as Meloidogyne incognita, were consistently found associated with all the fescue selections. Ring, lance, dagger, and stunt nematodes were also found at low levels. Populations of root-knot and lesion were higher in association with fescue containing the strain HM09 and the endemic strain, while spiral was lower in association with plants containing the endemic strain. In greenhouse tests, fescue containing strain HM09 supported higher Meloidogyne marylandi reproduction than fescue containing the other strains, while lowest nematode reproduction was found on the fescue with the endemic strain.
Effect of bird cherry-oat aphids and barley yellow dwarf virus on winter wheat. T. B. Trent (1), R. M. HUNGER (2), K. Giles (2), T. Royer (2), and M. Payton (3). (1) Heifer International, Little Rock, AR; (2) Department of Entomology and Plant Pathology; (3) Department of Statistics, Oklahoma State University, Stillwater, OK 74078. Publication no. P-2004-0056-SOA.
The bird cherry-oat (BCO) aphid (Rhopalosiphum padi):barley yellow dwarf virus (BYDV) complex can significantly damage wheat, but damage caused by aviruliferous (AVIR) BCO aphids compared to viruliferous (VIR) BCO aphids has not been clearly differentiated. Hence, the objectives of this research were to quantify the effect of AVIR and VIR BCO aphids on root and shoot growth of wheat seedlings grown hydroponically, and determine the effect on tillering, height, fertile head production, and the number and weight of seed when wheat seedlings growing in soil were infested with AVIR and VIR BCO aphids. Results indicated that both AVIR and VIR BCO aphids significantly reduced the measured parameters compared to non-infested controls, and that VIR BCO aphids caused the greatest reduction. Hence, these results emphasize the importance to control seedling infestation of wheat by BCO aphids.
Effect of seed size on uniformity of growth of hydroponically grown wheat seedlings. T. B. Trent (1), R. M. HUNGER (2), B. Olson (2), and M. Payton (3). (1) Heifer International, Little Rock, AR; (2) Department of Entomology and Plant Pathology; (3) Department of Statistics, Oklahoma State Univ., Stillwater, OK 74078. Publication no. P-2004-0057-SOA.
Growing wheat seedlings hydroponically facilitates measuring the effects of the aphid:barley yellow dwarf virus (BYDV) complex on shoot and root growth. However, minimizing variation between seedlings would maximize consistency of results. In this study, the effect of seed size on seedling root and shoot growth was determined. Three seed sizes of the hard red winter wheat cultivar Karl 92 were used; large seed (48-43 mg/seed), medium seed (38-33 mg/seed), and small seed (28-23 mg/seed). A seed from each size was placed in an individual hydroponic pouch, with three pouches (one for each seed size) randomly placed in each aluminum rack. Sixteen racks (replications) arranged in a randomized complete block were used. Growth of roots and shoots (total length) was determined 10 days after planting by scanning and the use of Rootedge scanning software. Results showed that medium-sized seed, which was the most abundant, resulted in seedlings with more uniform shoot and root length than large or small seed.
Genetic variability of beet soilborne mosaic virus within a field. E. VILLANUEVA, F. Workneh, and C. M. Rush. Plant Pathology, Texas Agricultural Experiment Station, Bushland, TX 79012. Publication no. P-2004-0058-SOA.
Beet soilborne mosaic virus (BSBMV) and Beet necrotic yellow vein virus (BNYVV) are vectored by Polymyxa betae and have similar genetic structure and organization. The genetic sequence of BNYVV is highly conserved; in contrast, preliminary studies of various BSBMV isolates have revealed considerable variability in the RNA 2 read-through open reading frame (ORF) and the RNA 3 29 kDa ORF. To determine the degree of genetic variability among BSBMV isolates within a single sugar beet field, soil was grid-sampled from a field in Colorado, bio-assayed in the green house, and tested for virus presence by DAS-ELISA. cDNA, synthesized from RNA isolated from positive samples, was amplified with primers designed for RNA 1, RNA 2, RNA 3, and RNA 4. Genetic variability was observed only in the PCR products from primers designed for the RNA 3 29 kDa protein. At least 8 different banding patterns were observed, suggesting that the genome of BSBMV is much more variable than that of BNYVV and the greatest degree of variability is detected in the RNA 3. The spatial distribution of the genotypes within the field will be discussed.
Potential management approaches for the sting nematode in bermudagrass sod production. N. R. WALKER (1), H. Zhang (2), and D. L. Martin (3). (1) Dept. Entomology & Plant Pathology; (2) Dept. Plant & Soil Science; (3) Dept. Horticulture & Landscape Architecture, Oklahoma State University, Stillwater, OK 74078. Publication no. P-2004-0059-SOA.
The sting nematode (Belonolaimus longicaudatus) is a destructive pest of bermudagrass grown for sod. Non-pesticide based management approaches were evaluated in an existing sod field containing a high population of the nematode. For each of two years, treatments consisted of plots planted to a green manure of oilseed rape (Brassica napus), or tilled once in late winter and spring with a rotary hoe, or amended once with poultry litter (2,950 kg/ha). Control treatments were plots continuously planted to bermudagrass or fumigated with Dazomet. In the first year, none of the treatments reduced sting nematode populations in comparison to continuous bermudagrass. In the second year, plots treated with oilseed rape, tillage, and the fumigant contained lower sting nematode populations than those with continuous bermudagrass. This reduction was present two months following incorporation of the rape, fumigant or the second tillage. Sod quality in both years was increased only by the soil fumigant treatment.
Growth of loblolly pines measured 14 years after screening for fusiform rust resistance. C. H. WALKINSHAW. USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360. Publication no. P-2004-0060-SOA.
Assessing rust resistance and growth of loblolly pine indicates that many progeny selected for resistance to fusiform rust exhibit reduced growth after 14 years in the field on a site with low rust incidence. Fifty different crosses of loblolly pines with rust resistance (Pinus taeda L.) were planted in Louisiana to evaluate diameter growth. Selection of crosses for outplanting was done by screening nine-month-old rust-inoculated seedlings for symptoms without galls. Crosses with lowest incidence of fusiform rust after 9 months had the smallest diameters after 14 years. Diameters ranged from 2.1 to 11.9 inches with a mean of 8.2 inches for the 50 crosses. Large diameters occurred in the susceptible open pollinated check. If crosses that are highly resistant to fusiform rust are planted where rust incidence is low, such as on this site, a significant reduction in wood production can result. Crosses from parent 209 in this study are excellent examples of resistant crosses with reduced diameter growth in the test plantation. Discretion in family selection should be used in planning for optimum wood harvest.
Foregut morphology of Anasa tristis, the vector of the causal agent of cucurbit yellow vine disease, Serratia marcescens. A. C. WAYADANDE (1), J. Fletcher (1), B. Bruton (2), S. Pair (2), and F. Mitchell (3). (1) Dept. of Entomology and Plant Pathology, Oklahoma State University, Stillwater, OK 74078; (2) USDA-ARS, SCARL, Lane, OK 74555; (3) Texas Agricultural Experiment Station, Stephenville, TX 76401. Publication no. P-2004-0061-SOA.
Cucurbit yellow vine disease is caused by a phloem-associated bacterium, Serratia marcescens. A competent field vector is the squash bug, Anasa tristis. The objective of this study was to describe the foregut morphology of A. tristis as a precursor to conducting foregut adhesion studies with S. marcescens. Examination by light and electron microscopy revealed that the cibarium (sucking pump) is tubular; the walls of the hypopharynx are formed from invaginations of the anteclypeal wall. Unlike leafhoppers and aphids, there was no discernable precibarial valve. Papillae were scattered on the hypopharyngeal wall; these likely function as gustatory chemosensilla. Foregut structure is relevant to understanding retention site of S. marcescens.
Effects of full and reduced fungicide programs on foliar and soilborne diseases of peanut (Arachis hypogaea). J. E. WOODWARD, T. B. Brenneman, and R. C. Kemerait. Dept. Plant Pathology, Univ. of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793. Publication no. P-2004-0062-SOA.
Most Georgia peanut producers use seven fungicide sprays per season to manage foliar and soilborne diseases. In 2003, full and reduced fungicide programs were evaluated in three fields having low, moderate, or high disease risk according to the Georgia Risk Index for Leaf Spot and Soilborne Diseases of Peanut. Final leaf spot ratings for reduced programs were higher (P < 0.0001) than the full program at two of the three locations and an increase (P < 0.0001) in stem rot incidence was observed in a reduced program in the moderate risk trial. No difference (P < 0.05) in yield was found for full or reduced programs at any of the locations. Average yields for soilborne programs were higher than foliar programs by 653, 794 and 155 kg/ha, respectively for the high, moderate and low risk fields. Disease and yield data from a soilborne-based AU-Pnut advisory model showed similar results as the calendar-based programs for soilborne diseases. These results suggest that reduced fungicide programs may be used to adequately manage both leaf spot and stem rot in fields with low or moderate risk.
Relationships between weather factors and sorghum ergot severity in the Texas Panhandle. F. WORKNEH and C. M. Rush. Texas Agricultural Experiment Station, Bushland, TX 79012. Publication no. P-2004-0063-SOA.
Sorghum ergot caused by the fungus Claviceps africana was reported in the United States for the first time in 1997. Since then, there have been consistent yearly reports of the disease in the Texas Panhandle with periodic incidence reaching epidemic levels. Sorghum ergot is strictly a disease of unfertilized flowers. Thus, male-sterile lines are at risk of infection in the absence of pollen. Experiments were conducted to determine the effects of weather factors on severity of ergot. A male-sterile line was planted in succession such that flowers would be continually available for inoculation throughout variable ranges of weather conditions during the season. The flowers were inoculated with the spore suspension periodically from the beginning of August through the middle of October, and disease severity was recorded as a percentage of infected florets. Weather data were collected from the weather station located in the experimental field. Temperature and relative humidity significantly correlated (r = 0.74, P = 0.0093, and r = 0.89, P = 0.0003, respectively) with ergot severity. Implications of the findings for model development will be discussed.