Rhizoctonia solani AG-3PT causes stem canker and black scurf disease of potato which results in economic yield loss of up to 35-50% around the world. Until now, progress in the management of these diseases has been slow. This study looked at the effects of biological control strategies for stem canker and black scurf of potato caused by R. solani AG-3PT using soil organisms alone or in combination with resistant potato varieties or fertilizers or intercropping systems with Brassica plants.

Trichoderma species were studied because they have a known role in minimising the impacts of pathogens and improving plant growth. Eight isolates of Trichoderma obtained from healthy tubers and soil reduced the severity of disease and promoted growth of potato plants in laboratory and glasshouse experiments. T. harzianum strain T5 and T. hamatum strain T8 had the best performance in prevention of disease and enhancing the plant growth and potato yield.

The information on comparative sensitivity of varieties of potato will assist farmers to make informed decisions in terms of black scurf disease management. Six potato varieties utilized in this study showed a range of sensitivity reactions to R. solani AG-3PT, but none of these varieties were completely resistant to the disease. The Sapphire and Royal Blue varieties showed the lowest level of infection of tubers by sclerotia of R. solani AG-3PT at harvest, whereas Sebago and Desiree varieties showed the highest level of sclerotia on tubers.

Laboratory experiments showed that Sebago sprouts contained materials that reduced the pathogen growth and prevented the growth of biocontrol agents compared with other potato varieties. The interaction between two Trichoderma isolates (T5 and T8) and the pathogen in glasshouse experiments with three potato varieties (Sebago, Desiree and Sapphire) showed the same disease reduction and plant growth promotion for all varieties. There was therefore no interaction between plant resistance and biocontrol.

In culture experiments, there were small but significant effects of seven nutrients (N, P, K, Ca, Mg, Fe and Mn) on the interactions between Trichoderma species and the pathogen. One series of pot trials on the effects of N, K and Mn on biocontrol did not detect an interaction between the nutrients and disease control or growth promotion. However, a field trial did show that the effect of T. hamatum T8 on growth promotion of potato in the presence of the pathogen was greater at low levels of NPK fertilizer. As a result, farmers may reduce fertilizer level when the biocontrol agent is applied.

Biofumigation has been used as an alternative method for controlling soilborne plant pathogens. Biocontrol agents T5 and T8 were not pathogenic to broccoli or cabbage and promoted their growth. R. solani was sensitive to compounds from the root tissues of broccoli and cabbage, whereas Trichoderma isolates, especially T8, were tolerant to compounds from root tissues. In a glasshouse experiment either intercropping with cabbage or broccoli, or inoculation with T8, prevented disease symptoms on potatoes. In a field trial, intercropping with cabbage or treatment with T8 controlled the stem canker and black scurf disease caused by R. solani AG-3PT. Isolate T8 also greatly increased the growth rate of the cabbages. The growth of potato plants was reduced by competition with the cabbage, indicating the need for further work on refining the intercropping system.

Overall this study provides insight on the efficacy of Trichoderma isolates for biological control, and the effect of Trichoderma isolates on potato varieties resistance, fertilizers and cropping system to control stem canker and black scurf of potato caused by R. solani AG-3PT.