CHARACTERIZATION AND PATHOGENICITY ON SEEDLINGS OF PYTHIUM SPECIES ISOLATED FROM SOYBEAN ROOTS IN THE TOULOUSE AREA

Isolation frequencies and in vitro pathogenicity are given for several species of Pythium isolated from roots of soybean plants growing near Toulouse (SW France). Approximately half of all isolates were pathogenic on soybean seedlings (cv Kingsoy) at 20-degrees-C. Some differences among isolates could be detected, but the variability among species was greater in the isolates studied. The most pathogenic species was P ultimum which reduced root elongation to approximately 6% of the control but made up only 10% of the total isolates in 2 years of sampling, slightly varying from one year to the next. P sylvaticum and P irregulare were isolated more frequently (26 and 13%, respectively) and were pathogenic on soybean, reducing root elongation by 30 to 50% of the control in 2 tests. The most commonly isolated group (38%) did not form oospores and was classified as HS (hyphal swelling). This group appeared to have little effect on soybean root growth and caused no visible symptoms. P ultimum was only isolated from young plants; P irregulare was isolated primarily during the first two months of growth; and P sylvaticum and the HS group were isolated in a fairly uniform manner throughout the first 3 months of the season.