Mapping components of partial resistance to Northern leaf blight of maize using reciprocal translocations

Maize F2 populations were constructed by crossing a series of northern leaf blight (NLB, caused by Exserohilum turcicum) susceptible reciprocal translocation stocks with the partially resistant inbred line Mo17, followed by one generation of selfing. Translocation breakpoints were marked with the waxy (wx) recessive gene and F2 seed were classified as waxy or nonwaxy before planting. F2 plants inoculated with E. turcicum were evaluated in the field and greenhouse for infection efficiency, incubation period, lesion length, lesion expansion rate, and sporulation. F2 plants in field experiments were also evaluated for disease severity on adult plants. Ratings of waxy and nonwaxy F2 plants were compared using Student's t tests to determine if one or both of the chromosome arms involved in the reciprocal translocation carried a gene or genes for NLB resistance. Partial resistance (as expressed by disease severity on adult plants in the field) in Mo17 was consistently associated with factors on chromosome 3, the short arm of chromosome 4, and the long arm of chromosome 6. Measures of components of partial resistance on greenhouse-grown seedlings were rarely significant and were not in agreement with ratings of disease severity on adult plants in the field. Based on measurements on older but still vegetative plants in the field, the gene or genes on the short arm of chromosome 4 and the long arm of chromosome 6 appeared to be related to a reduction in lesion numbers and an increase in incubation period, both of which were correlated with disease severity. Lesion length, lesion expansion rate, and sporulation intensity were not associated with partial resistance in Mo17. The lack of consistency in expression of components of partial resistance in seedling plants may be due to environmental and developmental regulation of genes for resistance.