Masters Theses

Date of Award

8-1973

Degree Type

Thesis

Degree Name

Master of Science

Major

Agricultural Biology

Major Professor

Jim Hilty

Committee Members

L.M. Josephson, Stelmon E. Bennett

Abstract

Following the 1970 epiphytotic of southern corn leaf blight a new race of the causal pathogen, Helminthosporium maydis Race T, was designated. This new race is host specific for corn lines produced in the Texas (T) male-sterile cytoplasm. The studies presented in this paper were undertaken to assess the variability that exists in the H. maydis population. The objectives of the study were twofold. The first objective was to measure the cultural variability among several single spore isolates of H. maydis as to sporulation and linear growth using four media. The second objective as to estimate the variation in pathogenicity to corn lines with six male-sterile and normal cytoplasms among the same isolates, a) as to crude and concentrated pathotoxin production by seedling bioassay, and b) seedling inoculation of the lines in the greenhouse.

Significant variability in linear growth and sporulation existed for the H. maydis isolates tested. There was a significant, negative correlation between linear growth and sporulation. Lactose casein hydrolysate medium was further substantiated as a good medium for inducing sporulation in Helminthosporium maydis.

The variation among the isolates in optimum harvest date for pathotoxin production reiterates the need for this type of determination previous to any studies dealing with the pathotoxin produced by H. maydis. There were significant differences in cytoplasm reaction to both the crude and concentrated pathotoxin; however, the reactions were too variable for meaningful patterns. These results support two conclusions. First, at the present time, seedling inoculation and field reaction is the best method for determining the susceptibility or resistance of male-sterile cytoplasms to H. maydis. Second, pathotoxin studies should be done with highly purified pathotoxin preparations and should be based on several replications using large quantities of seed. Also, the reaction of the cytoplasms to the pathotoxin should be studied using the same cytoplasms in several genotypes.

The isolates used in the seedling inoculation studies were all H. maydis Race T. Based on seedling reaction under the conditions in this study, male-sterile cytoplasms C and W were as susceptible as T cytoplasm to the isolates tested. Cytoplasms G, K, and M were as resistant as N cytoplasm to the same isolates. These results indicate that the H. maydis Race T population contains biotypes that are more virulent to particular male-sterile cytoplasms than to others.

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