Chromosomal location of a gene for resistance to Corynebacterium michiganense ssp. nebraskense

Leaf Freckles and Wilt (LFW) (Goss's Wilt), caused by Corynebacterium michiganense ssp. nebraskense, is a foliar maize disease occurring throughout Nebraska and spreading in surrounding states. A set of 27 reciprocal translocation stocks in the M14 background (resistant to LFW) were crossed to susceptible inbred A632. The F1's were selfed, and plants grown from F2 seed were classified for pollen and selfed. F3 seed from the normal pollen plants were separated by the presence or absence of the translocation-linked waxy gene marker and were planted in paired rows. Some translocation stocks had insufficient segregation for waxy and normal F3 seed from different normal pollen F2 plants. For these stocks, a waxy-marked row was paired with two rows still segregating for the heterozygous translocation/homozygous normal conditions. Inoculation of C. m. ssp. nebraskense was performed at the five-leaf stage, and individual plants were scored from 0 to 5 for disease reaction (0 = no symptoms to 5 = dead) 7, 17, and 42 days later. Mean disease readings were compared between paired rows and t-tests were performed. The 42-day readings were determined unreliable due to secondary pathogens and environmental stress. The only significant differences detected between paired rows involved the T7-9a and T7-9b translocations. Each stock has a break on the different arms of chromosome 7 and a break on the short arm of chromosome 9. Stocks marking similar breakpoints in chromosome 9 but having translocations with chromosomes other than 7 did not show significant differences. This indicates that a gene or genes for resistance may be located on chromosome 7. The present data are preliminary and confirmation of these findings is needed, particularly for translocation T7-9b, which expressed significance for only one reading. No significant differences between paired rows for 25 of 27 translocation stocks indicates resistance may be quantitative, which is consistent with earlier research. This year planned experiments are designed to evaluate whether resistance genes can be detected that have effects only in certain environments or only at certain time intervals after inoculation.

Table.

Torbert R. Rocheford, Charles O. Gardner and Anne K. Vidaver


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