There are three races of this pathogen race O, race T, and race C. Race T no longer considered a threat since the transition to normal cytoplasm corn 2. Lesion size can range from a barely visible speck up to 1. The elongated lesions along the veins are tan in color 2 and can have purplish to brown borders 1. The lesions reduce the photosynthesis leaf surface area compromising the plant normal physiology 3. Ecology and Spread Spores and mycelium of the pathogen survive the winter on plant debris.
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Accepted for publication 16 May DOI: The recessive gene rhm conditions resistance in corn to Illinois isolates of Helminthosporium maydis races O and T. Its expression, in terms of lesion type, lesion size, and extent of sporulation, depended upon the level of resistance contributed by other genetic systems, plant cytoplasm, and race of H.
Monogenic-resistant plants with normal or cms-T cytoplasm, infected with race O, formed small chlorotic lesions with reduced sporulation, in contrast to large necrotic lesions with abundant sporulation on susceptible plants. The increase in resistance to race O due to rhm was less in corn plants having other genetic systems for resistance, than in more susceptible plants. These effects of rhm against race O were consistent in both greenhouse-and field-grown plants. The expression of rhm against race T depended upon plant cytoplasm.
Plants with normal cytoplasm exhibited chlorotic lesions. Fewer conidia of race T formed in necrotic lesions of normal cytoplasm plants than of race O. Seedling plants with cms-T cytoplasm homozygous for rhm, and infected with race T, expressed chlorotic-shot-hole-type lesions in the greenhouse. Gene rhm did not appear to affect sporulation of race T in lesions on plants having cms-T cytoplasm. Additional keywords: Zea mays, southern leaf blight of corn, cms-T, gene rhm, epidemiology.
Class Ascomycota, subclass Dothideomycetidae, order Pleosporales, family Pleomassariaceae, genus Helminthosporium. Biological group. It is a facultative parasite. Morphology and biology. The disease affects leaves, leaf sheaths, ear, and maize grains. The lesions on leaves and ears are the main symptoms of the disease. On leaves of adult plants the grayish-red or stramineous long lesions with dark brown center appear along leaf veins, being spindle-shaped or elliptical.