GLYPHOSATE TREATMENT OF BEAN SEEDLINGS CAUSES SHORT-TERM INCREASES INPYTHIUM POPULATIONS AND DAMPING-OFF POTENTIAL IN SOILS

The effect of bean root residues on populations of known isolates and unidentified Pythium species in soils was assessed, with special regar d to herbicide treatment of bean seedlings. The general Pythium popula tion in a muck soil was significantly increased 6 days after foliar tr eatment of bean seedlings with glyphosate, and by soil amendment with heat-killed bean roots, but not by amendment with roots of healthy bea n seedlings. The enhanced populations returned to near initial levels 4 days Later. Isolated populations of Pythium ultimum (a glyphosate-sy nergistic isolate) and Pythium coloratum (a non-synergistic isolate) i n a sandy loam soil were enhanced approximately 10-fold over control b y roots of healthy bean seedlings and by roots of seedlings killed wit h either glyphosate or paraquat. Distinct peaks in the population resp onses of P. ultimum and P. coloratum occurred at 9 and 18 days after t reatment, respectively. Strong positive correlations were observed bet ween the population estimates obtained by dilution plating and damping off of sunflower for both P. ultimum and P. coloratum. These results suggests that herbicide treatment of plants can cause temporary increa ses in both Pythium, populations and damping off potential of soils. ( C) 1998 Elsevier Science B.V. All rights reserved.