VIEW ARTICLE

Ecology and Epidemiology

Interactions Between and Among Grain Sorghum, Sorghum Downy Mildew, and the Seed Herbicide Antidotes Concep II, Concep, and Screen. J. B. Szerszen, Research associate, Department of Plant Pathology and Microbiology, Texas Agricultural Experiment Station, Texas A & M University, College Station 77843; R. A. Frederiksen(2), J. Craig(3), and G. N. Odvody(4). (2)Professor, Department of Plant Pathology and Microbiology, Texas Agricultural Experiment Station, Texas A & M University, College Station 77843; (3)Plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, College Sation 77843; (4)Assistant professor, Texas A & M Research and Extension Center, Corpus Christi 78410. Phytopathology 78:1648-1655. Accepted for publication 1 July 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1648.

The herbicide antidote Concep II significantly increased the incidence of sorghum downy mildew in susceptible grain sorghum hybrids inoculated with oospores of Peronosclerospora sorghi, pathotypes 1 and 3. Other antidotes, Concep and Screen, had no effect, or Screen significantly decreased disease incidence. Concep and Concep II retarded growth of young sorghum seedlings when used at >0.5 g a.i./kg seed. Higher rates further inhibited plant growth and increased disease incidence. The length of time that Concep II inhibited seedling growth increased with dosage. Scanning electron microscopy revealed that Concep II affected sorghum root growth and development. A major morphological effect of Concep II appears to be that it delays natural maturation of root tissue, which may prolong occurrence of pathogen-susceptible juvenile tissue. Concep II had no bactericidal effect in vitro but had a fungicidal effect on microorganisms naturally associated with oospores of P. sorghi. There was no evidence that Concep II enhanced oospore germination. Seed treatment with metalaxyl at 0.5 g a.i./kg seed completely controlled the iatrogenic effect of Concep II. The fungicidal effectiveness of metalaxyl in controlling sorghum downy mildew was reduced when rates of Concep II higher than 1.25 g a.i./ kg seed were used.