Simultaneous screening for resistance to blackeye cowpea mosaic virus and root-knot nematode in southernpea

Four southernpea (cowpea) (Vigna unguiculata (L.) Walp.) cultivars represent- ing various combinations of resistance and susceptibility to blackeye cowpea mosaic virus (BlCMV) and southern root-knot nematode (Meloidogyne incognita (Kofoid and White) Chitwood) were used to determine effectiveness of simultaneous screening of plants for resistance to both pathogens. Plants were inoculated with both pathogens simultaneously, each pathogen separately, or left uninoculated as controls. The resistance classification of the cultivars based on treatments with only one pathogen was not different from that based on the treatment with both pathogens. Virus × nematode interaction was not a significant source of variation in BlCMV symptoms and root-knot nematode galls. Simultaneous screening for both pathogens in southernpeas appears to be a feasible option. Nematode-virus interactions may be di- rect, such as transmission of viruses by nema- todes, or, indirect due to changes caused by a pathogen in a plant's defense mechanism as when the presence of a virus alters nematode development and/or multiplication (Taylor, 1990; Weischer, 1993). Inoculation of cow- peas with CMV and M. incognita simulta- neously or with CMV 10 days before nema- tode inoculation reduced root galling by 39% and 24%, respectively, compared to cowpeas inoculated with M. incognita only. When virus inoculum was applied after nematode inocu- lum, virus concentration was 23% lower than in plants inoculated with virus only (Goswami et al., 1974). Weischer (1975) detected a re- duction in nematode counts in tobacco (Nic- otiana tabacum L.) plants inoculated with seven viruses and two nematode species, de- pending on specific virus/nematode combina- tions. Watermelon mosaic virus did retard the establishment of M. javanica (Treub) Chitwood in Cucurbita pepo L. (Huang and Chu, 1984). Severity of infection with nematode species in an interspecific hybrid sugarcane ( Saccharum ×officinarum L.) was related to concentrations of free amino acid levels caused by host plant stress as a result of infection with sugarcane mosaic virus (Showler et al., 1990). The objective of this experiment was to evaluate the effectiveness of screening for resistance to BlCMV and root-knot nematode simultaneously as compared to screening for each pathogen separately.