Location: Foreign Disease-weed Science ResearchTitle: Development of an inoculation technique and the evaluation of soybean genotypes for resistance to Coniothyrium glycines Author
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/14/2017
Publication Date: 5/25/2017
Citation: Tooley, P.W. 2017. Development of an inoculation technique and the evaluation of soybean genotypes for resistance to Coniothyrium glycines. Plant Disease. 101:1411-1416. Interpretive Summary: Red leaf blotch of soybeans (caused by the fungus Coniothyrium glycines) is a plant disease not yet present in the U.S., but which could be introduced via import of soybeans or other means of international trade or human activity. A method of testing soybeans for resistance to red leaf blotch is needed so that resistance can be identified in worldwide soybean accessions and bred into U.S. cultivars. At USDA-ARS, Ft. Detrick, MD we developed an assay that can be used with soybean seedlings to identify resistance. We used a known susceptible soybean variety to study the conditions that affect the assay, such as length of time at high relative humidity, amount of fungus used to inoculate plants, and temperature, to learn how to obtain optimal disease levels. The assay can now be used to screen larger numbers of soybeans from worldwide sources to look for effective resistance to red leaf blotch.
Technical Abstract: A screening assay was developed for the pathogen Coniothyrium glycines which causes red leaf blotch of soybeans. We investigated the effects of inoculum density, temperature (20 and 25°C), and dew chamber incubation period (2-5 days) on disease expression in Williams 82 soybeans inoculated with C. glycines. Inoculum density and incubation period had significant effects on disease severity, but no significant difference was observed between inoculations performed at 20°C or 25°C. The highest levels of disease were obtained at an inoculum density of 6.45 cm2 inoculum/10 ml water and 5 days at 100% relative humidity produced the highest level of disease. Similar results were obtained for studies performed in dew chambers and in a humidity tent used in the greenhouse. Twenty-three soybean genotypes that represent nearly 90% of the genes present in US soybeans were evaluated and found to be susceptible. The assay will prove useful in determining the reactions of additional soybean germplasm to red leaf blotch.