Members of Academic Board, Plant Pest and Disease Research Institute, Tehran 19395-1454
Damping off caused by Rhizoctonia solani (isolate AG4) constrains growing soybean in the north Iranian province of Golestan. The pathogen was grown on malt agar containing 0, 50, 100 or 200 ppm trifluralin then its vegetative growth and virulence were evaluated. The response of soybean cv. Williams seedlings grown in soil containing 0, 0.5, 1 and 2 ?g ai g-1 trifluralin was investigated. On malt agar containing trifluralin, the radial growth of the fungal colonies was slower than that on untreated medium. There was no significant difference between these two types of colony in their abaility to cause disease symptoms on soybean seedlings. The incidence of soybean damping off in autoclaved soil treated with trifluralin was significantly higher than that in untreated soil. The results indicate that trifluralin enhanced the susceptibility of soybean radicles to the pathogen.