Host Preference Correlated with Chlorate Resistance in Macrophomina phaseolina. C. A. S. Pearson, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506. J. F. Leslie, and F. W. Schwenk. Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan 66506. Plant Dis. 71:828-831. Accepted for publication 7 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0828.

Isolates of Macrophomina phaseolina from maize and soybean field soils and plant tissues were collected from 13 states in the central and eastern United States. The chlorate phenotype of each isolate was determined after subculture on a minimal medium containing 120 mM potassium chlorate. In general, maize isolates were chlorate-resistant, whereas soybean isolates were chlorate sensitive. Isolates from soil populations of the fungus could be partitioned into three phenotypic classes (dense, feathery, and restricted). Isolates with a dense phenotype (chlorate-resistant) preferentially colonized maize tissues, whereas isolates with feathery and restricted phenotypes (chlorate-sensitive) preferentially colonized soybean tissues. The effects of a 2-yr crop rotation on soil and tissue populations of M. phaseolina also were examined in a field in southeastern Kansas. Only isolates with the feathery and restricted phenotypes were detected in this field. The M. phaseolina soil population was significantly lower following a maize-after-maize cropping sequence (37.5 propagules per 5 g of soil) than it was following a soybean-after-soybean cropping sequence (68.3 propagules per 5 g of soil). Quantitatively, higher levels of fungus were obtained from soybean tissue (25.6 propagules per 100 mg) than from maize tissue (2.4 propagules per 100 mg). Host preference appears to be exhibited in both the M. phaseolina:maize and the M. phaseolina:soybean pathosystems.

Keyword(s): charcoal rot, crop rotation, Glycine max, Zea mays.