The Effect of Host Resistance on Relative Parasitic Fitness ofHelminthosporium maydisRace T

Abstract

Eight isolates of H. maydis race T were passed through 1 conidial generation on the susceptible Texas male sterile corn [Zea mays] hybrid PA887 .times. B14 T-cms, followed by 6 serial conidial generations on the resistant, normal (N) cytoplasm hybrid Pioneer Brand 3006 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 1st (T1) and 7th (N6) generations of each isolate were compared on the T-cms hybrid. Significant decreases occurred in at least 1 of the fitness attributes for each isolate. The effects of 2 generations (T2) on the T-cms hybrid after 2 (N2) or 10 (N10) generations on the N hybrid were tested for 5 isolates. Some isolates exhibited a significant decrease in relative disease efficiency after only 2 passages on the resistant host. Relative disease efficiency lost during 10 generations on the N hybrid was partially restored in some isolates after 2 passages on the T-cms hybrid (N10-T2).