ALTERED ROOT EXUDATION AND SUPPRESSION OF INDUCED LIGNIFICATION AS MECHANISMS OF PREDISPOSITION BY GLYPHOSATE OF BEAN ROOTS (PHASEOLUS-VULGARIS L.) TO COLONIZATION BY PYTHIUM SPP

Several possible mechanisms for the glyphosate-induced predisposition of bean roots (Phaseolus vulgaris L.) to colonization by Pythium spp. were investigated. Glyphosate at 0.1 and 1.0 mu g ml(-1) from the surf actant-containing formulation Roundup(R) and the non surfactant-contai ning formulation Accord(R) did not affect mycelial growth of Pythium u ltimum and Pythium splvaticum on water agar and cornmeal agar. One mic rogram per millilitre of glyphosate from both formulations significant ly stimulated germination of sporangia of P. ultimum. Germination and growth of germ tubes of P. ultimum were significantly greater in root exudates from bean plants whose primary leaves had been treated with g lyphosate than in exudates from non-treated plants. The lignin content of roots was increased significantly when P. ultimum or P. sylvaticum was added to the hydroponic system in which the roots were growing. W hen glyphosate was applied 2 days prior to Pythium, deposition of lign in in response to Pythium in the bean roots was significantly reduced. These results suggest that predisposition by glyphosate of bean roots to colonization by Pythium spp. may involve changes in root exudates that enhance germination and growth of pathogen propagules, and suppre ssion of a pathogen-induced lignification response by plant roots. (C) 1997 Academic Press Limited.