Effect of Fungicide Seed Treatments, Cultivars, and Soils on Soybean Stand Establishment.

Seedling diseases are one of the major production problems for soybean. The primary control of soybean seedling diseases is by fungicide seed treatments but comparisons of seed treatments are difficult because stand responses are often erratic in the field. The objective of this study was to determine the efficacy of soybean fungicide seed treatments in naturally infested soils under controlled environments at three temperatures. Seed of three soybean cultivars were treated with six fungicide treatments or not treated and planted in two soil types collected from two fields in April, May, and June 2008 and 2009. Tests were conducted in growth chambers at 21°C (April planting), 25°C (May planting), or 28°C (June planting). Stands were determined when at least 25% of the seedlings reached the V4 growth stage. Seed treatments improved stands at all three temperatures, in both soils, and in both years. In general, the broad-spectrum fungicides trifloxystrobin + metalaxyl and mefenoxam + fludioxonil + azoxystrobin resulted in the highest stands. The selective fungicide treatments metalaxyl and pentachloronitrobenzene + carboxin were effective at all three temperatures, implying that Pythium spp. and Rhizoctonia solani, respectively, were involved in seedling disease and active over a range of temperatures. The efficacy of fungicides was not consistently associated with the cultivars used in this study.

Molecular characterization of a new Tospovirus infecting soybean.

A new, widespread disease was recently observed in soybean in the United States. The disease, named Soybean vein necrosis, is manifested by intraveinal chlorosis and necrosis, and has been found in almost all of the 50 fields visited over a period of 3 years in the midwest and midsouth part of the United States. A virus was isolated from symptomatic material, and detection protocols were developed. More than 150 symptomatic specimens collected from seven US States were tested, and all were found positive for the virus unlike 75 asymptomatic samples, revealing the absolute association between virus and disease. Protein pairwise comparisons coupled with phylogenetic analyses indicate that the virus is a new member of the genus Tospovirus.

Resistance of the Soybean Cultivar Archer to Pythium Damping-Off and Root Rot Caused by Several Pythium spp.

Archer, a maturity group I soybean cultivar with demonstrated flood tolerance and resistance to Pythium ultimum, was compared with Hutcheson, a widely planted maturity group V cultivar in Arkansas, for resistance to P. ultimum, P. irregulare, P. aphanidermatum, P. vexans, and group HS. Emergence and establishment assays demonstrated that Archer had greater emergence and fewer disease symptoms after 10 days for all pathogenic species of Pythium than did Hutcheson. Archer also demonstrated higher root weights and fewer disease symptoms compared with Hutcheson in assays conducted for 6 weeks. Similar results were found using two different seed lots of the cultivars, suggesting that the results were not due to seed quality differences. This study indicates that resistance in Archer to Pythium damping-off and root rot compared with Hutcheson is robust, with efficacy over a number of Pythium spp. and a range of plant developmental stages.

Soybean Response to Flooded Soil Conditions and the Association with Soilborne Plant Pathogenic Genera.

The role of soilborne pathogens in flood damage on soybeans, Glycine max, was examined using six genotypes representing a reputed range of flood tolerances. Genotypes were planted in single-row plots from 1996 to 1998 with flood treatments of no flood, flood at emergence (3-day duration), or flood at the fourth leaf node growth stage (7-day duration). Three or four days after removing each flood treatment, plant stands were estimated and 15 plants were collected from each plot, weighed, and rated for root discoloration. Roots were assayed for the presence of fungi and other filamentous eukaryotic organisms. Plant stands were reduced by flooding at emergence compared with the nonflooded control. Flooding at both growth stages caused significant increases in root discoloration compared with nonflooded soybeans. Plant weights were reduced in 2 of 3 years for flooding at emergence. Pythium was the only genus of filamentous organisms whose isolation frequency increased with flooding. Of the 60 Pythium isolates evaluated representing the different cultural groups based on appearance and growth rate on potato dextrose agar, cornmeal agar, and V8 agar, 47% were moderately to highly virulent on soybean. Pythium spp. isolated from soybean included the pathogenic species P. ultimum, P. aphanidermatum, P. irregulare, and P. vexans and Group HS. Nonpathogenic P. oligandrum also was isolated from soybean.

The Effect of Pythium ultimum and Soil Flooding on Two Soybean Cultivars.

The effect of flooding and Pythium ultimum on soybean, Glycine max, was determined in a series of greenhouse experiments using the cultivars Hutcheson and Archer. Seeds were planted into pasteurized soil either not infested or infested with sand-cornmeal inoculum of P. ultimum and either flooded at emergence for 2 days or at the four leaf node stage (V4) for 5 days. A nonflooded control was included in each experiment. Seeds placed directly into infested soil resulted in little or no stand for Hutcheson regardless of flood treatment, whereas stand was reduced for Archer only in the flooded infested soil treatment. Additional experiments were conducted by placing seed onto a 2- to 5-mm layer of pathogen-free soil on top of the infested soil. Flooding at emergence reduced plant height, growth stage, and top dry weight for Hutcheson and root fresh weight for both cultivars. Greater reductions for Hutcheson in root weight, and top dry weight in P. ultimum-infested soil in the soil layer experiments, also indicated that Hutcheson was more susceptible than Archer. Flooding alone decreased root weights, and infestation with P. ultimum reduced weights further resulting in an additive effect. This also was the case for plant height, growth stage, and top dry weight for Hutcheson for flooding at emergence. Root discoloration was greatly increased for both cultivars in infested soil flooded at emergence. Similar results were found when plants were flooded at V4; however, the effect was not as great as with flooding at emergence. These studies indicate that Pythium damping-off and root rot may account for a portion of the negative response of soybean to flooding. The results also indicate that Archer has some resistance to P. ultimum.

Races of Phytophthora sojae in Arkansas Soybean Fields and Their Effects on Commonly Grown Soybean Cultivars.

Isolates of Phytophthora sojae were collected during 1995 to 1998 from soil samples collected in 23 Arkansas soybean fields in 14 counties, and characterized by race. A total of seven races (races 2, 10, 14, 15, 24, 26, and 38) were found. Races 10, 24, and 15 were the most common and comprised 47, 22, and 9% of the 32 isolates, respectively. A single isolate each of races 2, 14, 26, and 38 also was found. Three of the isolates collected could not be characterized to race due to inconsistent results. In 1997 and 1998, a portion of a single soybean field at the University of Arkansas Southeast Research and Experiment Center near Rohwer, AR was surveyed intensively for P. sojae. The area was planted each year to the P. sojae-susceptible cv. Williams and both plants and soil were collected to assay for P. sojae from 16 and 28 plots (4.9 by 7.6 m) in 1997 and 1998, respectively. A total of 83 isolates were collected (11 from plants and 72 from soil), and found to represent 13 pathotypes, including 6 with virulence formulae that have not been described previously. Nine commercial soybean cultivars representing a range of reported resistance and tolerance to Phytophthora root and stem rot were screened for resistance to races 10, 15, and 26 of P. sojae using both hypocotyl injection and inoculum layer techniques. Cvs. Manokin, Hartz Variety 5545, and Riverside 499 were consistently resistant to all of the races using both inoculation methods. These results indicate that, although considerable pathogenic variability in P. sojae exists in soybean fields in Arkansas, cultivars with effective resistance are available to help growers manage Phytophthora root and stem rot.

Isolation and Identification of Fusarium solani f. sp. glycines from Soil on Modified Nash and Snyder's Medium.

Modified Nash and Snyder's medium (MNSM) has been used to study soil populations of the fungus Fusarium solani f. sp. glycines, the causal agent of sudden death syndrome (SDS) of soybean. However, no studies have been conducted to confirm the accuracy of this technique in enumerating the SDS pathogen. To determine what portion of the colonies enumerated on MNSM are the SDS pathogen, 282 isolates resembling F. solani f. sp. glycines were collected from soil of two fields with a history of SDS in Arkansas using a soil dilution method with MNSM. The colony morphology of these isolates was compared on MNSM and on potato dextrose agar (PDA). Of these isolates, 112 were tested for pathogenicity on soybean in the greenhouse. In addition to fungi that produced colonies typical of F. solani f. sp. glycines on MNSM, 5 isolates each of four common colony types on MNSM were collected from each field for a total of 40 isolates. These isolates were compared to F. solani f. sp. glycines for colony morphology on PDA and for pathogenicity. Isolates that had colonies on MNSM that resembled F. solani f. sp. glycines had colony morphologies on PDA similar to this pathogen and produced SDS-like foliar symptoms in greenhouse inoculations of soybean. There was a significant quadratic relationship between foliar symptom severity and plant fresh weight. None of the other isolates tested resembled F. solani f. sp. glycines on either MNSM or PDA or produced SDS-foliar symptoms in greenhouse inoculations. These non-SDS isolates were F. solani, F. merismoides, a Pythium sp., and a Paecilomyces sp. All of these isolates produced some root rot, but significantly less than the F. solani f. sp. glycines reference isolate. The results of this study established that soil populations of F. solani f. sp. glycines can be accurately enumerated using MNSM. It should be noted that, on both MNSM and PDA, F. solani f. sp. glycines can resemble some isolates of F. solani f. sp. phaseoli, so pathogenicity tests of random isolates may be required to confirm the forma specialis especially in fields that have a history of both Glycines max and Phaseolus vulgaris.

Effect of Irrigation and Soil Water Stress on Densities of Macrophomina phaseolina in Soil and Roots of Two Soybean Cultivars.

The effects of irrigation and soil water stress on Macrophomina phaseolina microsclerotial (MS) densities in the soil and roots of soybean were studied in 1988, 1989, and 1990. Soybean cvs. Davis and Lloyd received irrigation until flowering (TAR2), after flowering (IAR2), full season (FSI), or not at all (NI). Soil water matric potentials at 15- and 30-cm depths were recorded throughout the growing season and used to schedule irrigation. Soil MS densities were determined at the beginning of each season. Root MS densities were determined periodically throughout the growing season. Microsclerotia were present in the roots of irrigated as well as nonirrigated soybean within 6 weeks after planting. By vegetative growth stage V13, these densities reached relatively stable levels in the NI and FSI treatments (2.23 to 2.35 and 1.35 to 1.63 log [microsclerotia per gram of dry root], respectively) through reproductive growth stage R6. After R6, irrigation was discontinued and root densities of microsclerotia increased in all treatments. Initiation (IAR2) or termination (TAR2) of irrigation at R2 resulted in significant changes in root MS densities, with densities reaching levels intermediate between those of FSI and NI treatments. Year to year differences in root colonization reflected differences in soil moisture due to rainfall. The rate of root colonization in response to soil moisture stress decreased with plant age. Root colonization was significantly greater in Davis than Lloyd at R5 and R8. This was reflected in a trend toward higher soil densities of M. phaseolina at planting in plots planted with Davis than in plots planted with Lloyd. Although no charcoal rot symptoms in the plant were observed in this study, these results indicated that water management can limit, but not prevent, colonization of soybean by M. phaseolina, that cultivars differ in colonization, and that these differences may affect soil densities of the fungus.

Effect of Chloride and Soybean Cultivar on Yield and the Development of Sudden Death Syndrome, Soybean Cyst Nematode, and Southern Blight.

Yields of irrigated soybean in Arkansas are threatened by two problems: chloride toxicity and sudden death syndrome (SDS). Soybeans are sensitive to chloride, which accumulates in the upper soil profile when water with high salt content is used for irrigation. Sudden death syndrome is a soilborne disease often associated with well-irrigated fields. Even though these problems both affect irrigated soybeans, there are no reports on the effect of chloride toxicity on SDS. To determine if there is an effect of chloride toxicity on SDS, a test was established at the Cotton Branch Station, Marianna, AR, in 1995 and 1996. Four cultivars were selected that were either susceptible to SDS (S) or resistant to SDS (R) and either translocated chloride to the leaves (includer, I) or confined chloride in the roots (excluder, E). The cultivars were Hartz 6686 (SE), Terra Vig 6653 (SI), Hartz 6200 (RE), and Asgrow 6297 (RI). Soil chloride concentrations were adjusted by the addition of KCl. Before planting, KCl was applied to achieve the recommended concentration of K over the entire field. At V4, chloride treatments were applied by either adding no additional KCl (low Cl) or adding 1,120 kg of KCl per ha (moderate Cl) or 2,240 kg of KCl per ha (high Cl). Soil samples were taken within the center two rows of each plot at planting, flowering (R2), and harvest and assayed for populations of Fusarium solani and Heterodera glycines. Soil chloride concentrations were determined at R2, and leaf chloride levels were determined at R3. Weekly disease ratings were made on SDS and converted to area under the disease progress curve (AUDPC). Plant lodging and the incidence of southern blight (Sclerotium rolfsii) were determined during mid-reproductive growth. Leaf chloride concentrations were influenced by both chloride treatment and cultivar: elevated concentrations occurred with the includer cultivars in the moderate and the high Cl treatments. Soil concentrations of chloride reflected the chloride treatments in 1995, but not 1996. Soil populations of F. solani did not respond consistently to either chloride treatment or cultivar; however, H. glycines egg densities increased with increased soil chloride treatments in Hartz 6686 (SE) and Terra Vig 6653 (SI) at R2, but not at harvest. Increased soil chloride treatments increased SDS in both years with Hartz 6686 (SE), but did not affect this disease in the other cultivars. Higher soil chloride treatments decreased yield in all cultivars except Hartz 6200 (RE) in 1996. Although Terra Vig 6653 (SI) did not develop severe levels of SDS foliar symptoms, it did have increased lodging and significant increases in southern blight with moderate and high soil chloride treatments. These results indicate that growers with fields that have both elevated concentrations of soil chloride and SDS should select SDS-resistant excluder cultivars to minimize yield losses due to both problems.

Effect of Soybean Growth Stage at the Time of Inoculation with Diaporthe phaseolorum var. meridionales on Stem Canker Development and Yield.

Infections at early stages of plant development followed by a long incubation period before symptoms appear during reproductive stages is characteristic of the disease cycle of stem canker of soybean, caused by Diaporthe phaseolorum var. meridionales. To determine the effect of plant growth stage at the time of infection on symptom development and yield components, soybean plants (cv. Walters) were grown in microplots and inoculated with a suspension of ascospores (106/ml) at the V1, V4, V6, V10, or R2 growth stage. Noninoculated plants served as controls. Development of foliar symptoms was quantified weekly after flowering (R2), and yields and seed weights were determined. The experiment was conducted twice, in 1992 and 1994. Foliar symptoms of stem canker developed for all treatments except the noninoculated control. Foliar symptoms appeared at the R2 to R5 growth stage and reached 90 to 100% incidence for most treatments. Disease development was delayed for the R2 inoculation, suggesting that a minimum incubation period of 34 to 41 days is necessary before foliar symptoms develop. Disease (area under the disease progress curve) was greater and yield, seed number, and seed weight were less in 1994 than in 1992. Quadratic equations significantly related these variables to time of inoculation and reached maximum or minimum values between 40.9 and 43.3 days after planting. This corresponded to the V6 growth stage. Yield and seed number were affected by disease development during pod formation (R2 to R5) in both years, while yield and seed weight were affected by disease development during seed formation (R5 to R6) in 1992.