Genetics of reaction to maize dwarf mosaic virus strain A in several maize inbred lines

Nine maize inbred lines resistant to maize dwarf mosaic virus strain A (MDMV-A) were studied in hybrid combinations for inheritance of reaction to MDMV-A in F1 and F2. Plants growing in a field free of natural inoculum were mechanically inoculated at the two- to four-leaf stage. Three to four weeks later they were scored as healthy or symptomatic in response to virus. This was done to detect early symptoms not always apparent after anthesis. After anthesis plants were scored again but on a scale of 1-7; 1 = no infection, and 7 = nearly completely mottled. Plants that had symptoms before but not after anthesis were scored 2. Resistant inbred line B68, Oh1EP, Oh7B, and Pa405 (the B68 group) behaved homogeneously for virus reactions as lines and the F1 of crosses with susceptible line Va50 produced only type 1 plants. In the F2, resistance was monogenic and completely dominant. Since no infected plants were observed in F2 of crosses of lines within the B68 group, resistance occurred at one locus. The symbol Rmd1 is suggested for this locus. The resistant inbred lines A239, Va53, Va85, and VaOM73 (the Va53 group) produced only type 1 plants, but in the F1 of crosses with Va50, produced some susceptible plants; however, in F2 all segregated monogenically. In the F2 of crosses within the Va53 group and between the Va53 and B68 groups, susceptible segregates appeared. However, since the Va53 group behaved variably in F1 combination with VAa50 it could not be ascertained that genes of the Va53 group were allelic with or different from the resistance gene of the B68 group. Va35 is unstable as a line and produced inconclusive results; in the F2 of Va50 X Va35, a 1:1 ratio was obtained instead of the expected 3:1. Inconclusive results were also obtained in the crosses Va35 X Pa405 and Va35 X A239.