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 when infected with a cowpea stunt-derived isolate of blackeye cowpea mosaic potyvirus (BlCMV). One group of plants developed a delayed, mild reaction 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 isolate. All genotypes responded with similar, mild reactions when inoculated 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 disease symptoms and high concentrations of CMV coat protein 20 days after inoculation in all plants that did not express extreme resistance to BlCMV. Interestingly, at early time points after inoculation, differences in symptom severity between singly and dually infected plants were not consistently correlated with significant differences in relative CMV concentrations. The results indicate that (i) resistance to BlCMV, as determined through visual observation, is not adequate when evaluating germ plasm for cowpea stunt disease resistance, and (ii) rapid development of severe symptoms on dually infected plants may not be due solely to increased CMV concentrations.