J. C. Rupe

Sudden death syndrome of soybean

 Sudden death syndrome (SDS) is one of the most important diseases of soybean in North and South America.  Soybeans are at greater risk for SDS when planted into cool, wet soils, when soybean cyst nematode (SCN) is present, and when summer rains cause saturated soils.  Managing SDS can involve several practices such as relieving soil compaction, delaying planting, and selecting soybean products with a good disease tolerance package.

Effect of crop rotation on soil population densities of Fusarium solani and Heterodera glycines and on the development of sudden death syndrome of soybean

Abstract The effect of crop rotation on the development of sudden death syndrome (SDS) of soybean and on the soil population densities of Fusarium solani and the soybean cyst nematode ( Heterodera glycines ) was observed for four years in a field naturally infested with both pathogens. In 1989 all plots were planted to a soybean cultivar susceptible to both pathogens (cv. Lee 74). In 1990 and 1991, the plots were planted to one of the following crops: soybean cv. Lee 74, soybean cv. Hartz 6130 (resistant to both pathogens), sorghum, fescue, or wheat (not double-cropped). In 1992 all plots were planted to soybean cv. Lee 74. The parameters measured to determine rotation effects were severity of SDS over the season, the soil population densities of both pathogens, soybean yields and soybean seed weights. Rotation to any crop except soybean significantly reduced the H. glycines population densities in 1990 and 1991. In 1991, the H. glycines population densities began to rise in the nematode-resistant soybean treatment, but were lower than those in the susceptible soybean treatment. Averaged over all years, the F. solani population densities were significantly lower when sorghum or wheat were grown than when soybean or fescue were grown. Considering the whole season, the F. solani population densities were generally the highest at harvest. Negative correlations between SDS and yield were significant, as were positive correlations between SDS and population densities of F. solani . In 1992, SDS was very low in all treatments, but yields were highest in plots planted to non-soybean crops and lowest in plots planted to the susceptible soybean.

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.

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

Modified Nash and Snyders 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.

Vertical and temporal distribution of Fusarium solani and Heterodera glycines in fields with sudden death syndrome of soybean

Abstract Sudden death syndrome (SDS) is an important soilborne disease of soybean caused by the fungus Fusarium solani . This disease is also associated with the soybean cyst nematode (SCN), Heterodera glycines . To quantify the vertical and temporal distribution of these pathogens in the soil profile, a study was conducted at the University of Arkansas Pine Tree (PTS) and Cotton Branch Stations (CBS) in 1991. Soil samples were taken at six depths (0 to 7, 7 to 15, 15 to 30, 30 to 50, 50 to 75 and 75 to 100 cm) at four growth stages (planting, R2, R6 and harvest) and numbers of F. solani , SCN and total fungi were determined. Soil physical and chemical characteristics were measured at planting and soybean root densities were measured at R2 and R6. Disease development was followed weekly in August and September. Soil numbers of F. solani were greatest in the top 15 cm of soil throughout the season. SCN egg numbers also were greatest in this zone until harvest. At harvest, numbers significantly increased at depths of at least 50 cm. The greatest root densities were in the 0- to 15-cm zone. At both locations, the 0- to 15-cm zone was over a plow pan under which the pH decreased from 6.4 to 7.0 above the pan to 3.8 to 4.5 below the pan. Total fungal numbers were greatest in the upper soil layers at planting but declined until there were no significant differences in numbers of total fungi with depth from R2 to harvest. SDS developed sooner and was more severe at PTS than at CBS. PTS had greater numbers of F. solani but supported lower numbers of SCN at the end of the season than CBS.

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Oomycete Species Associated with Soybean Seedlings in North America—Part I: Identification and Pathogenicity Characterization

Oomycete pathogens are commonly associated with soybean root rot and have been estimated to reduce soybean yields in the United States by 1.5 million tons on an annual basis. Limited information exists regarding the frequency and diversity of oomycete species across the major soybean-producing regions in North America. A survey was conducted across 11 major soybean-producing states in the United States and the province of Ontario, Canada. In 2011, 2,378 oomycete cultures were isolated from soybean seedling roots on a semiselective medium (CMA-PARPB) and were identified by sequencing of the internal transcribed spacer region of rDNA. Sequence results distinguished a total of 51 Pythium spp., three Phytophthora spp., three Phytopythium spp., and one Aphanomyces sp. in 2011, with Pythium sylvaticum (16%) and P. oopapillum (13%) being the most prevalent. In 2012, the survey was repeated, but, due to drought conditions across the sampling area, fewer total isolates (n = 1,038) were collected. Additionally, in 2012, a second semiselective medium (V8-RPBH) was included, which increased the Phytophthora spp. isolated from 0.7 to 7% of the total isolates. In 2012, 54 Pythium spp., seven Phytophthora spp., six Phytopythium spp., and one Pythiogeton sp. were recovered, with P. sylvaticum (14%) and P. heterothallicum (12%) being recovered most frequently. Pathogenicity and virulence were evaluated with representative isolates of each of the 84 species on soybean cv. Sloan. A seed-rot assay identified 13 and 11 pathogenic species, respectively, at 13 and 20°C. A seedling-root assay conducted at 20°C identified 43 species as pathogenic, having a significantly detrimental effect on the seedling roots as compared with the noninoculated control. A total of 15 species were pathogenic in both the seed and seedling assays. This study provides a comprehensive characterization of oomycete species present in soybean seedling roots in the major production areas in the United States and Ontario, Canada and provides a basis for disease management and breeding programs.

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.

Mating-Type Distribution and Genetic Diversity of Cercospora sojina Populations on Soybean from Arkansas: Evidence for Potential Sexual Reproduction

Cercospora sojina causes frogeye leaf spot of soybean, which can cause serious economic losses in the United States. In this study, 132 C. sojina isolates were collected from six fields (from two counties, Cross and Crawford) in Arkansas. To determine mating type, a multiplex polymerase chain reaction assay was developed with primers specific for C. sojina. Of the 132 isolates, 68 isolates had the MAT1-1-1 idiomorph and 64 isolates had the MAT1-2 idiomorph; no isolates possessed both idiomorphs. Both mating types were present in a variety of spatial scales, including separate lesions on individual leaves. Clone-corrected data from eight microsatellites indicated that mating-type loci were present in approximately equal proportions in all populations analyzed, which suggests that Arkansas populations of C. sojina are undergoing cryptic sexual reproduction. All six populations evaluated had high genotypic diversity of 26 to 79%. In addition, among strains isolated from a single leaf, multiple and distinct haplotypes were associated with both mating types, supporting the hypothesis that sexual reproduction occurs within the populations. Most populations showed significant gametic disequilibrium but levels of disequilibrium were relatively low, particularly in populations from Crawford County. A low differentiation index (GST) was observed for all simple-sequence repeat markers across all populations. Furthermore, the value of G statistics between populations suggests that significant genetic exchange exists among the populations. Taken together, these results demonstrate that C. sojina populations from Arkansas are genetically diverse and most likely undergoing sexual reproduction.

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.