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Genetics of Cowpea Reactions to Two Strains of Cowpea Mosaic Virus from Tanzania. P. N. Patel, Legume breeder/pathologist, IITA/ USAID/ Tanzania Grain Legume Research Project, Crop Development Division, Ministry of Agriculture, P. O. Box 9071, Dar es Salaam, Tanzania, Present address: Division of Mycology and Plant Pathology, Indian Agricultural Research Institute, New Delhi-110012, India; Phytopathology 72:460-466. Accepted for publication 8 May 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-460.

Inheritance and genetic relationships among the four host reactions, resistant or immune (R), susceptible (S), necrotic local lesion resistant (NLLR), and lethal susceptible (LS), produced by two strains of the cowpea mosaic virus, CPMV-LL and CPMV-TN, were studied in F1, F2, F3, and testcross populations from 82 crosses of cowpea (Vigna unguiculata). The crosses involved 15 R, seven S, seven NLLR, and four LS reacting cowpea lines. CPMV-LL induced R, NLLR, and S reactions. CPMV-TN induced R, LS, and S reactions. Plants in the segregating populations or of the parents classified as R or S, reacted similarly to both the strains, whereas those classed as NLLR to CPMV-LL were either S or LS in reaction to CPMV-TN. Reaction of F1s and segregation patterns in the hybrid populations indicated that interaction of two dominant genes, designated MVN and MVS, governed the four reactions to the two strains. MVN controlled the necrotic (hypersensitive) reactions, NLLR and LS. MVS governed the S reaction and its recessive allele, mvs, controlled the R reaction. MVN was epistatic to MVS, but hypostatic to mvs. Three different necrosis genes (MVN-1, MVN-2, and MVN-3) were identified on the basis of appearance of the lesions and reactions to CPMV-TN. They involved the same locus. MVN-1 was dominant over MVN-2 and MVN-3, and MVN-3 was suspected of being dominant over MVN-2. All 15 R parents had the same recessive gene controlling the R reaction. The 29 parents crossed represented seven genotypes (one allele of each gene mentioned): mvs mvn (R), mvs MVN-1 (R), mvs MVN-3 (R), MVN-1 MVS (NLLR/LS), MVN-2 MVS (NLLR/S), MVN-3 MVS (NLLR/S), mvn MVS (S). From the similarity in the properties of the two strains reported earlier and the results of the present genetic studies, it is possible that CPMV-TN evolved in nature by a mutation from the CPMV-LL strain to overcome necrotic local lesion type of resistance controlled by the MVN gene in cowpeas.