Linkage Relationships of a Gene in Corn Determining Susceptibility to a Helminthosporium Leaf Spot 1

Appropriate crosses were made between a series of translocation stocks in which sugary endosperm was used as a marker for the chromosomal interchange, and two inbred lines of yellow dent corn, Pr and K:6r, homozygous for susceptibility ̄ to the disease4. The F1 plants of these several crosses were backcrossed with pollen from double recessives (sugary, susceptible). The kernels of backcross progenies were divided into sugary and starchy groups and planted in the greenhouse. When the seedlings attained the 3 to 4 leaf stage they were inoculated by atomizing a spore suspension of H. carbonum, race r, over the leaves. One week later seedlings grown from starchy and sugary kernels were classified for disease reaction. Symptoms were clear cut, showing either distinct well-developed lesions in case of susceptible segregates, or small yellowish flecks characteristic of the resistant plants. Since preliminary results had indicated that the gene, hm, controlling susceptibility probably was located in the first chromosome, crosses also were made between the homozygous susceptible, red cob, inbred line Pr, and a linkage tester stock that was white cob, resistant, brachytic, and fine stripe-I. Multiple ̄ recessives--white cob, susceptible, brachytic, and fine stripe--in the F2 generation were selfed and crossed subsequently with red cob, resistant, normal stocks. The ]71 plants of the latter cross were backcrossed with multiple recessives and the resulting progenies were planted in the field and inoculated with a spore suspension of the pathogen. The disease’reaction, plant ct{aracter, and cob color were recorded for each plant at appropriate intervals after inoculation.