Resistance Responses in Cowpea to Southern Bean Mosaic Virus Based on Virus Accumulation and Symptomatology

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Abstract
Southern bean mosaic virus accumulation and various other virus-host interactions were evaluated in cowpea. All cowpea lines became infected, but four major categories of disease reaction were observed: local chlorosis followed by systemic severe mosaic and stunting, local chlorosis followed by systemic mottle, no symptoms, and necrotization. Infrequently, one normally symptomless cultivar, Iron, exhibited local chlorosis. Virus accumulation was high in inoculated leaves of genotypes (e.g., California Blackeye) with severe symptoms (susceptible), and it was restricted about 10- to 25-fold in Early Pinkeye, which reacted with a mottle and little or no stunting (moderately resistant). Symptomless lines were divided into three subcategories; virus accumulation was restricted 40- to 50-fold in Iron (resistant), about 500-fold in Worthmore (highly resistant), and 10,000- to 1,000,000-fold in two plant introductions (147562 and 186465) (extremely resistant). Local necrotization occurred in two lines; virus accumulation was restricted over 300-fold in Clay (small local lesions) and was relatively unrestricted in PI 399419 (large local lesions). Systemic necrosis occurred frequently in PI 399419. Virus accumulation was similar in both inoculated and uninoculated tissue of three susceptible control cultivars; however, in all lines with resistance, virus production was less in uninoculated tissue than in inoculated. Also, spread of virus within inoculated leaves was more restricted in resistant lines than in susceptible ones. Plant growth and symptomatology were related to virus production. Both biological and physicochemical properties of virus from cowpea lines with different virus-host interactions were similar, if not indistinguishable.