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. coloratum, and Pythium 'HS' group, were identified using morphological characteristics out of 65 isolates obtained from roots of glyphosate-treated bean seedlings grown in five different soils. Various genotypes within 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' group. The potential of a representative isolate from each RFLP group to enhance the herbicidal action of glyphosate was quantified by estimating glyphosate LD,, values on bean seedlings growing in sterilized soils amended with each isolate separately. The LD,, values were computed by logistic regression using plant mortality data gathered 4 wk after 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 synergists 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 species tested were pathogenic to varying degrees on germinating bean seeds and on 2-wk-old bean seedlings. The results indicate that several Pythium species can function as glyphosate synergists and that five different soils all yielded glyphosate synergistic Pythium isolates.