Interactions of fungicide and insecticide combinations against Rhizoctonia damping‐off and root rot in canola*

Abstract

SummaryIn growth chamber pot experiments with soilless mix infested with an isolate of Rhizoctonia solani AG2‐1, two seed‐dressing insecticides (cloethocarb and gamma‐HCH) and two granular insecticides (carbofuran and terbufos) were tested alone and in combination with five seed‐dressing fungicides (carboxin, cyproconazole, furmecyclox, iprodione and tolclofos‐methyl) against pre‐emerg‐ence damping‐off and post‐emergence seedling root rot in canola cv. Westar (Brassica napus). Each fungicide provided almost complete control of damping‐off, but, against post‐emergence root rot, furmecyclox, iprodione and tolclofos‐methyl gave far better control than carboxin and cyproconazole. Cloethocarb and carbofuran increased the incidence of damping‐off and, when used in combination with fungicides, lowered the disease control against damping‐off and root rot. Gamma‐HCH and terbufos decreased the incidence of pre‐emergence damping‐off, and terbufos also reduced post‐emergence root rot. Combined treatments with gamma‐HCH and terbufos gave better control against damping‐off than either gamma‐HCH or terbufos alone. Disease control by fungicides against both damping‐off and root rot was enhanced by the use of either gamma‐HCH or terbufos together with fungicide treatments. Fungicide combinations with gamma‐HCH and terbufos were at best additive in their activity for overall disease control. Maximum disease control with insecticide‐fungicide combinations was achieved when terbufos granules were applied at the time the pots were sown with fungicide + gamma‐HCH‐treated seed. Adding terbufos granules to the treated seed 1–4 wk before seeding, resulted in a considerable loss of efficacy of each of the five fungicide + gamma‐HCH seed treatments.