Tillering and resistance to MDMV

--C. W. Roane and S. A. Tolin

In our work on genetics of breeding for resistance to maize dwarf mosaic virus (MDMV), we observed that there is a strong tendency toward tillering (suckering) in some segregating populations. Usually tillering occurs in relatively thin stands when there has been excellent distribution of moisture prior to anthesis. The year 1985 was such a year and it was noted that the reaction of a tiller to MDMV was sometimes different from that of the main stalk. In F1 of crosses of MDMV-resistant x susceptible lines where resistance was dominant, 51 tillered plants were observed; main stalks and tillers were healthy (= reaction type 1) in 46 plants. The other five plants, a through e, were scored main stalk and tiller, respectively, a-1,4; b-1,6; c-1,7; d-2,1; e-6,1 (1 = healthy, 2 = narrow stripes of mosaic tissue below ear, 3 = narrow stripes above ear, 4 = narrow stripes most leaves, 5 = broad stripes or mosaic below the ear, 6 = broad stripes or mosaic above the ear, 7 = broad stripes or mosaic above and below the ear; only types 6 and 7 are considered to be susceptible). Plants a-d were considered resistant by virtue of their main stalk behavior, but the tillers of plants b and c created an element of doubt. Plant e was in a row behaving atypically for its cross, was presumed to be a rogue because resistance of the resistant parent, Oh7B, is completely dominant and F1 plants remain healthy (= type 1). However, it illustrates what may happen. In F1 of crosses of resistant x resistant lines, 106 tillered plants were observed. All main stalks and tillers were healthy. Thus, only plants heterozygous for resistance behaved inconsistently.

The genetics of MDMV reaction has proven to be a difficult and controversial problem. When one is trying to determine the number of genes contributing to resistance, inconsistency of main stalk-tiller behavior, if of high frequency, can create yet another problem for the researcher. Fortunately, in our case the frequency of inconsistency was very low. In maize breeding, tillered plants are usually discarded; in genetics they must be regarded. Our observations raise the interesting question that if resistance of a plant is not expressed in its tillers, would it be expressed in plants produced by clonal propagation?

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