Reaction of Red Clover to Aphanomyces euteiches. J. E. Tofte, Department of Agronomy, University of Wisconsin-Madison and Agricultural Research Service, U.S. Department of Agriculture, Madison 53706. R. R. Smith, and C. R. Grau. Department of Agronomy, University of Wisconsin-Madison and Agricultural Research Service, U.S. Department of Agriculture, and Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Plant Dis. 76:39-42. Accepted for publication 23 July 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0039.

Aphanomyces euteiches is an economically important root rot pathogen of pea (Pisum sativum). Red clover (Trifolium pratense) is known to be a host but expresses low susceptiblity to A. euteiches. Recently, an isolate (Ae-572) of A. euteiches was observed to be highly virulent on red clover. A seedling assay was used to characterize the reaction of red clover to A. euteiches (isolate Ae-572). One-week-old red clover is most susceptible to A. euteiches. Two- and 3-wk-old seedlings were significantly less susceptible. A reduction in forage dry matter and root weight of inoculated plants was observed at 5 and 8 wk of age. An inoculum concentration of 333 zoospores per milliliter at 24 C was optimum to determine the reaction of red clover to A. euteiches. Tetraploid red clover developed by asexual or sexual methods was not significantly different from diploid red clover in its reaction to A. euteiches. No common cultivars of red clover adapted for the northern Midwest have a high level of resistance to the isolate Ae-572 of A. euteiches.