The Evaluation of Relative Parasitic Fitness of Isolates ofHelminthosporium maydisRace T

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Abstract
Results of previous research demonstrated that the relative parasitic fitness of isolates of H. maydis race T could be reduced by serial passage on a resistant host. Three of these isolates were used to evaluate the consequences of reduced parasitic fitness on disease spread, disease severity and relative parasitic survival under field conditions. Three original field isolates and the same isolates with reduced parasitic fitness after 10 serial passages on normal cytoplasm corn [Zea mays] were used to initiate epidemics resulting from a point source of inoculum in field plots of corn cv. PA887P .times. B14 T-cms. Late in the epidemics, whole-plot disease severity was significantly lower in plots inoculated with isolates having reduced parasitic fitness than in plots inoculated with original field isolates. Disease severities at point sources and at distances from a point source reflected similar trends. Use of orthogonal comparisons to test combinations of isolates revealed significant differences among field isolates and isolates conditioned on normal cytoplasm. Reductions in disease severity among comparisons of conditioned isolates most likely reflects the effect of a reduced sporulation capacity. This intraracial source of pathogen variability should be considered in disease management strategies involving host resistance and yield loss assessment.