Loren J. Giesler

Evaluation of Stenotrophomonas maltophilia strain C3 for biocontrol of brown patch disease

Abstract Bacterial strains isolated from grass foliage were tested for inhibition of brown patch disease, caused by Rhizoctonia solani Kuhn, on detached blades and potted seedlings of tall fescue ( Festuca arundinacea Schreb. cv. Kentucky 31). Stenotrophomonas maltophilia strain C3 prevented growth of the fungus on leaf blades and reduced the severity of necrosis on seedlings. When strain C3 was tested on tall fescue cv. ‘Kentucky 31’ planted at three seeding densities, biological control was not achieved. When the strain was applied to field plots of six tall fescue cultivars, the severity of brown patch disease was reduced in one cultivar, but increased in another. A rifampicin-resistant mutant strain of C3 applied to four cultivars was recovered from all of the canopies at approximately 10 6 CFU/g foliage throughout the experiment. This population level is over 10-fold lower than that associated with effective antagonism under laboratory conditions.

Bean pod mottle virus: a threat to U.S. soybean production.

Bean pod mottle virus (BPMV) is widespread in the major soybean-growing areas in the southern and southeastern United States. A severe outbreak of BPMV in the north central and northern Great Plains states is currently causing serious concern to soybean growers and to the soybean industry in this region (30). BPMV is efficiently transmitted in nature, within and between soybean fields, by several species of leaf-feeding beetles. The deleterious effects of BPMV infection not only reduce yield but also reduce seed quality, as seeds from infected plants may be discolored. Furthermore, BPMV predisposes soybeans to Phomopsis spp. seed infection (85), a major cause of poor seed quality in soybean (78). The recent BPMV outbreak is linked to the warm winters of the past few years that have allowed the beetle vectors to overwinter and emerge in the spring in unprecedented numbers (Fig. 1).

The microclimate in tall fescue turf as affected by canopy density and its influence on brown patch disease

Microenvironment was monitored within tall fescue (Festuca arundinacea) experiment plots in which the severity of brown patch disease, caused by Rhizoctonia solani AG-1-IA, was found to correlate with canopy density. Canopy density was varied either by planting cultivars that produced different densities, or by planting a single cultivar at three seeding rates. Air and foliage temperatures within the canopy differed by only approximately 1°C between low- and high-density canopies during 1993 and 1994 in both studies. In a wet year, 1993, leaf wetness and relative humidity did not differ significantly between low- and high-density canopies. In 1994, which was more typical in regards to weather, a denser canopy promoted leaf wetness and high relative humidity in both studies. Leaf wetness duration averaged over 10 days was 0.8 h longer in the high-density cultivar Arriba than in the low-density cultivar Fawn. In addition, the period of relative humidity above 90% was 2.3 h longer in Arriba than in Fawn. Canopies of tall fescue with different plant densities were inoculated in the laboratory with R. solani and placed under uniformly high humidity and temperature. Hyphae grew between leaf blades separated by up to 8 mm. Interblade hyphal growth occurred more frequently in high-density canopies because of the closer proximity of leaf blades, and as a result, mycelia and necrosis spread more rapidly from inoculation sites in high-density canopies. It was concluded that microenvironmental conditions and the physical proximity of leaf blades in high-density turfs can be more favorable for brown patch disease.

Integrated management strategies for Phytophthora sojae combining host resistance and seed treatments

Phytophthora sojae has re-emerged as a serious soybean pathogen in the past decade. This may be due in part to changes in resistance levels in current cultivars, adoption of P. sojae populations to deployed Rps genes, and highly favorable environments in the past decade. This multilocation study evaluated the effect of seed treatments on the incidence and severity of Phytophthora root and stem rot on soybeans with different combinations of Rps genes and levels of partial resistance. The efficacy of the seed treatments was highly variable across locations. Seed treatments (metalaxyl and mefenoxam) provided protection and increased yields across cultivars in locations where rain or irrigation occurred shortly after planting (Ohio, South Dakota, and Ontario). However, there were no significant differences in stand or yield consistently across cultivars in Iowa, Nebraska, Wisconsin, or Ohio, where heavy precipitation did not occur until later growth stages. The environment, levels of inoculum, and pathogen complex may have played a role in the different responses to the seed treatments and to the different combinations of Rps genes and levels of partial resistance to P. sojae in the cultivars. Fields that are poorly drained and have P. sojae populations with complex pathotypes may benefit the most from seed treatments. Individual fields where producers may see the greatest benefit to utilizing these integrated management strategies will need to be identified.

Phytophthora root and stem rot of soybean.

Phytophthora sojae is a soil borne pathogen that in the past has caused very large economic losses. During the late 1970s, 300,000 soybean acres (approximately 10% of total soybean production in Ohio) were lost due to P. sojae. This disease has since been effectively managed predominately through the incorporation of singlegene mediated resistance but quantitative or partial resistance has been used as well. In fact, today, we can repeat 100% loss by planting soybean cultivars that were popular during earlier epidemics. Without high levels of resistance to this pathogen, many soybean acres would be lost each year to this disease. Phytophthora doesn’t forget and it doesn’t go away! Disciplines Agricultural Science | Agriculture | Plant Pathology Comments This is an article from The Plant Health Instructor (2007): 1, doi:10.1094/PHI-I-2007-0830-07. Posted with permission. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. Authors Anne E. Dorrance, Dennis Mills, Alison E. Robertson, Martin A. Draper, Loren Giesler, and Albert Tenuta This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/plantpath_pubs/86 Phytophthora root and stem rot of soybean www.apsnet.org /edcenter/intropp/lessons/fungi/Oomycetes/Pages/PhytophthoraSojae.aspx Dorrance, A.E., D. Mills, A.E. Robertson, M.A. Draper, L. Giesler, and A.Tenuta, 2007. Phytophthora root and stem rot of soybean. The Plant Health Instructor. DOI: 10.1094/PHI-I-2007-0830-07 Reviewed 2012. DISEASE: Phytophthora root and stem rot of soybean PATHOGEN: Phytophthora sojae HOSTS: Soybean: Glycine max, the cultivated soybean, is the primary economic host. Glycine soja, the wild progenitor of soybean, is also susceptible Authors Anne E. Dorrance and Dennis Mills, The Ohio State University Alison E. Robertson, Iowa State University Martin A. Draper, USDA-CSREES, formerly @ South Dakota State University Loren Giesler, University of Nebraska-Lincoln Albert Tenuta, Ontario Ministry of Agriculture, Canada

A site-specific sensor for measuring leaf wetness duration within turfgrass canopies

Leaf wetness is a key factor in the initiation and development of many plant diseases. Measurement of leaf wetness duration in turfgrass requires sensors that do not disrupt the canopy environment and are representative of leaf surfaces. Therefore, a small (5 mm head width) sensor was developed that can be inserted into grass blades with only minor modification of the canopy and can capture the spatial variability of leaf wetness. The sensor detected wetness onset and depletion in a tall fescue turf canopy in the field with an average error of less than 20 min when compared with a tactile method. The sensor indicated leaf wetness over 1 h before moisture was visible and detected wetness depletion 1 h after moisture was no longer visible, suggesting that the sensor is sensitive to microscopic water films. When the sensor was compared with a leaf latent heat flux. model in tall fescue canopies, no significant differences between the two methods were found in detecting wetting and drying episodes and in measuring leaf wetness durations. However, the sensor was sufficiently sensitive to record 1 h later wetness depletion (P < 0.01) and 1 h longer periods of leaf wetness (P < 0.01) in a high plant density canopy as compared with a low-density canopy, whereas no significant differences in moisture conditions between the two canopies was found using the leaf latent heat flux model.

From select agent to an established pathogen: The response to Phakopsora pachyrhizi (soybean rust) in North America

The pathogen causing soybean rust, Phakopsora pachyrhizi, was first described in Japan in 1902. The disease was important in the Eastern Hemisphere for many decades before the fungus was reported in Hawaii in 1994, which was followed by reports from countries in Africa and South America. In 2004, P. pachyrhizi was confirmed in Louisiana, making it the first report in the continental United States. Based on yield losses from countries in Asia, Africa, and South America, it was clear that this pathogen could have a major economic impact on the yield of 30 million ha of soybean in the United States. The response by agencies within the United States Department of Agriculture, industry, soybean check-off boards, and universities was immediate and complex. The impacts of some of these activities are detailed in this review. The net result has been that the once dreaded disease, which caused substantial losses in other parts of the world, is now better understood and effectively managed in the United States. The disease continues to be monitored yearly for changes in spatial and temporal distribution so that soybean growers can continue to benefit by knowing where soybean rust is occurring during the growing season.

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.

Response of Soybean Cultivars to Bean pod mottle virus Infection

Bean pod mottle virus (BPMV) has become increasingly common in soybean throughout the north-central region of the United States. Yield loss assessments on southern soybean germplasm have reported reductions ranging from 3 to 52%. Currently, no soybean cultivars have been identified with resistance to BPMV. The objective of this study was to determine the impact of BPMV infection on soybean cultivars representing a broad range of northern soybean germ-plasm by comparing inoculated and noninoculated soybean plants in paired row studies. In all, 30 and 24 cultivars were evaluated in Nebraska (NE) in which soybean plants were inoculated at the V3 to V4 growth stage. Eleven cultivars from public and breeding lines were inoculated at the VC and R5 to R6 growth stages in Ohio (OH). Disease severity, yield, and percent seed coat mottling were assessed at both locations, whereas protein and oil content also were assessed at NE. Yield and percent seed coat mottling was significantly reduced following inoculation at the VC (OH) and V3 to V4 (NE) growth stages. In addition, seed oil and protein composition were impacted in 1 of the 2 years of the study. This study demonstrates that substantial yield losses can occur in soybean due to BPMV infection. In addition, protein and oil may be affected depending on the environment during the production season.