Biological and chemical control and their combined use to control different stages of the Rhizoctonia disease complex on potato through the growing season

Abstract

Abstract Rhizoctonia solani causes stem canker and black scurf diseases on potato and negatively affects the yield in all potato‐growing areas. While seed‐borne infection can be efficiently controlled by dressing with fungicides, few means of effective control are available against soil‐borne infection. In this study, commercially available antagonistic fungi and bacteria, and the combination of antagonistic Trichoderma harzianum and seed dressing with flutolanil, were tested for their efficacy in the control of soil‐borne infection of R. solani in the field. Combined use of flutolanil and T. harzianum was found feasible because even the highest tested concentration of flutolanil [13.0 μg active ingredient (a.i.) mL−1] had little effect on the growth of T. harzianuminvitro, whereas over 100‐fold lower concentrations (0.1 μg a.i. mL−1) were sufficient to strongly inhibit the growth of R. solani (EC50 0.045 ± 0.0068 μg a.i. mL−1). The variables under focus in plants inoculated with R. solani were the relative stem lesion index; sprout/stem number; stolon number, weight and incidence of symptoms on stolons; total yield and the yield of marketable sized tubers, and incidence of black scurf on the marketable‐sized tubers. Flutolanil and its combined application with T. harzianum reduced the damage to sprouts and severity of stem canker at the early stages of growth (up to 30 days postplanting). Towards the end of the growing season, T. harzianum was required to reduce disease severity. When applied in‐furrow alone or in combination with flutolanil‐dressed seed potatoes, T. harzianum increased the proportion of marketable‐sized tubers in yield from 35% to 60% and decreased the incidence of black scurf on progeny tubers from 31% to 11%, which was not achieved using flutolanil alone. The number and weight of stolons and the yield of plants remained lower in the inoculated plants than un‐inoculated control plants regardless of the method of control used. The other two antagonists tested, Streptomyces griseoviridis and Gliocladium catenulatum, showed no consistent control of R. solani. Taken together, the results suggest that combining the application of the antagonist T. harzianum with seed dressing with flutolanil may provide the best protection of the potato crop against damage caused by R. solani throughout the growing season.