IDENTIFICATION AND ROLE OF PYTHIUM SPECIES AS GLYPHOSATE SYNERGISTS ON BEAN (PHASEOLUS-VULGARIS) GROWN IN DIFFERENT SOILS

Five Pythium species, P. ultimum, P. sylvaticum, P. irregulare, P. col oratum, and Pythium 'HS' group, were identified using morphological ch aracteristics out of 65 isolates obtained from roots of glyphosate-tre ated bean seedlings grown in five different soils. Various genotypes w ithin the Pythium species were determined from RFLP patterns of total DNA. There were six RFLP types represented in P. sylvaticum, three in P. ultimum, and two each for P. irregulare, P. coloratum and P. 'HS' g roup. The potential of a representative isolate from each RFLP group t o enhance the herbicidal action of glyphosate was quantified by estima ting glyphosate LD,, values on bean seedlings growing in sterilized so ils amended with each isolate separately. The LD,, values were compute d by logistic regression using plant mortality data gathered 4 wk afte r treatment of 2-wk-old seedlings with different doses of glyphosate. Twelve of the 15 isolates of Pythium tested were glyphosate synergists on beans. The efficacy of the different isolates as glyphosate synerg ists varied both between species and among different RFLP types within the same species. The pathogenicity of the representative isolates to beans without glyphosate treatment was also determined. All Pythium s pecies tested were pathogenic to varying degrees on germinating bean s eeds and on 2-wk-old bean seedlings. The results indicate that several Pythium species can function as glyphosate synergists and that five d ifferent soils all yielded glyphosate synergistic Pythium isolates.