VIEW ARTICLE

Ecology and Epidemiology

Oospores of Sclerospora sorghi in Soils of South Texas and Their Relationships to the Incidence of Downy Mildew in Grain Sorghum. R. G. Pratt, Assistant Professor, Texas Agricultural Experiment Station, Texas A&M University Agricultural Research and Extension Center, P.O. Box 10607, Corpus Christi, TX 78410, Present address of senior author: Science and Education Administration, U.S. Department of Agriculture, Mississippi State, MS 39762; and Department of Plant Pathology and Weed Science, Mississippi State University, P.O. Drawer PG, Mississippi State, MS 39762; G. D. Janke, Research Associate, Texas Agricultural Experiment Station, Texas A&M University Agricultural Research and Extension Center, P.O. Box 10607, Corpus Christi, TX 78410. Phytopathology 68:1600-1605. Accepted for publication 8 June 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1600.

Oospores of Sclerospora sorghi from soil were concentrated among silt particles following removal of most sand and clay particles from aqueous suspensions of soil by washing and sedimentation. Oospores were counted in aliquots of silt suspensions at ×25-50 magnification. An average of 97% of oospores added to soils at three densities were recovered. In three naturally-infested soils, oospores also were counted in pieces of organic debris collected on sieves and cleared in potassium hydroxide. At least 97% of oospores were free in soil 5 mo after harvest. Soil samples from 18 sorghum fields were assayed to determine the relation between densities of oospores of S. sorghi and incidence of downy mildew. Population densities of oospores ranged from 8-95/g soil and incidence of downy mildew was 1-53% in the field and 0-47% in sorghum seedlings grown in the soils in pots. Oospore densities were not significantly correlated with incidence of downy mildew in plants in the field (r = 0.33, 16 df) or in pots (r = 0.45, 16 df) because little or no disease developed in some soils with high oospore densities. However, ratios of infected plants in the field to oospore densities were significantly correlated (r = 0.72, 16 df) with clay contents of soils; the highest ratios occurred in soils with the least clay. These results indicate that incidence of downy mildew in sorghum seedlings in the field is related to oospore densities and soil texture.