TOLERANCE TO BLACKEYE COWPEA MOSAIC POTYVIRUS NOT CORRELATED WITH DECREASED VIRUS ACCUMULATION OR PROTECTION FROM COWPEA STUNT DISEASE

Progeny from a near-isogenic cowpea line responded heterogeneously whe n infected with a cowpea stunt-derived isolate of blackeye cowpea mosa ic potyvirus (BlCMV). One group of plants developed a delayed, mild re action to BlCMV while sister plants rapidly exhibited strong systemic mosaic symptoms. Conversely, enzyme-linked immunosorbent assay (ELISA) results indicated that BlCMV generally accumulated to the same levels and at the same rates in these two plant groups. Similar results were obtained for two commercial varieties that expressed different BlCMV symptoms. Symptom analyses and ELISA were used to demonstrate that one of these commercial varieties was highly resistant to this virus isol ate. All genotypes responded with similar, mild reactions when inocula ted with cowpea stunt-derived cucumber mosaic cucumovirus (CMV). Both symptoms and ELISA-detectable levels of CMV decreased as plants aged. Mixed infections with BlCMV and CMV resulted in severe cowpea stunt di sease symptoms and high concentrations of CMV coat protein 20 days aft er inoculation in all plants that did not express extreme resistance t o BlCMV. Interestingly, at early time points after inoculation, differ ences in symptom severity between singly and dually infected plants we re not consistently correlated with significant differences in relativ e CMV concentrations. The results indicate that (i) resistance to BlCM V, as determined through visual observation, is not adequate when eval uating germ plasm for cowpea stunt disease resistance, and (ii) rapid development of severe symptoms on dually infected plants may not be du e solely to increased CMV concentrations.