Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2009
Publication Date: N/A
Citation: Interpretive Summary: Cowpea [Vigna unguiculata] is a valuable food crop that is cultivated in warm soils throughout the world. Cowpea, which includes southernpea, blackeye pea, crowder pea, and field pea types, is an important vegetable crop in the Southeastern U.S., California, and Texas. It is estimated that 80,000 acres are planted to cowpea annually in the U.S., where cowpeas are grown as a processing crop, as dry edible beans, and as a market garden crop. Cowpeas are tolerant to drought, high temperatures, and infertile soils. However, cowpea plantings in cool, moist spring soils are very susceptible to seedling damping-off and root rot caused by a soil-borne fungus, Rhizoctonia solani. This ubiquitous fungus is highly virulent to cowpea causing stand losses and subsequent yield losses. Although fungicides are currently used to control this disease, there are concerns about the environmental effects of pesticides moving into the soil and groundwater. The most economical and environmentally friendly method of controlling Rhizoctonia seedling disease in cowpea would be planting resistant varieties of cowpea. However, resistance has not been discovered in cowpea. Therefore, in the studies described here, we developed methods to evaluate cowpea for resistance to Rhizoctonia seedling disease and identified several wild cowpea lines that are moderately resistant. These resistant lines may be useful in developing resistant cowpea cultivars that have high and potentially durable resistance.
Technical Abstract: Rhizoctonia solani is an important pathogen of cowpea (Vigna unguiculata) seedlings causing substantial yield losses worldwide. Fungicide seed treatments are the current method of control for cowpea seedling diseases. The use of resistant cultivars would provide an improved control measure over current fungicide seed treatments. A method to evaluate cowpea germplasm for seedling resistance to R. solani was developed using inoculum cultured on dent corn kernels. The optimal inoculum rate for evaluating germplasm for seedling resistance was determined by comparing varying inoculum concentration x inoculum particle size treatments. All available accessions (684) from the core subset of the USDA Cowpea Germplasm Collection were evaluated. Seedlings were rated for disease on a 1 to 5 scale where 1 to 2.9 = resistant and 3 to 5 = susceptible. Disease indices (DI) ranged from 2.4 to 5.0 with six accessions exhibiting moderate resistance (DI equal to and less than 3.0). Cowpea accessions exhibiting moderate resistance in the unreplicated evaluation were re-evaluated in a replicated test. Moderately resistant accessions exhibited significantly lower (P less than 0.0001) DI (mean = 2.4) than susceptible accessions (mean = 4.2) in the replicated test. The optimized ground corn kernel method is an efficient and effective technique for evaluating cowpea germplasm for seedling resistance to R. solani.