VIEW ARTICLE

Resistance

The Effect of Host Resistance on Relative Parasitic Fitness of Helminthosporium maydis Race T. M. H. Royer, Graduate assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; R. R. Nelson, Evan Pugh Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 71:351-354. Accepted for publication 11 August 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-351.

Eight isolates of Helminthosporium maydis race T were passed through one conidial generation on the susceptible Texas male sterile corn hybrid PA887 × B14 T-cms, followed by six serial conidial generations on the resistant, normal (N) cytoplasm hybrid Pioneer Brand 3306 N. A part of each generation was preserved in dry leaf material. Relative disease efficiency (number of lesions per unit of inoculum), lesion size, and sporulation of the first (T₁) and seventh (N₆) generations of each isolate were compared on the T-cms hybrid. Significant decreases occurred in at least one of the fitness attributes for each isolate. The effects of two generations (T₂) on the T-cms hybrid after two (N₂) or 10 (N₁₀) generations on the N hybrid were tested for five isolates. Some isolates exhibited a significant decrease in relative disease efficiency after only two passages on the resistant host. Relative disease efficiency lost during 10 generations on the N hybrid was partially restored in some isolates after two passages on the T-cms hybrid (N₁₀–T₂).