Biological control of Rhizoctonia diseases of potato

Abstract

Potato (Solanum tuberosum L.) is one of the most important crops in New Zealand and internationally. Potato plants are susceptible to a range of pathogenic diseases, including the Rhizoctonia diseases, which are globally ubiquitous. Since synthetic fungicide control of the causal pathogen, Rhizoctonia solani Kühn, is not always efficacious, combined with a current drive to reduce synthetic agrichemical inputs in New Zealand agriculture, alternatives should be sought. The present study examined the potential for biological control (biocontrol) of Rhizoctonia diseases of potato from a New Zealand agricultural perspective. The study included investigations into inundative biocontrol (introduction of single/multiple strains of soil microorganisms) and conservation biocontrol, including organic matter amendments and crop rotation practice. To investigate inundative biocontrol, isolations were conducted from New Zealand potato cropping soils for members of the genera Trichoderma, Pseudomonas and Bacillus, identified as strong potential biocontrol candidates from previous published studies. Comparisons between the bacterial isolates in dual culture with two R. solani strains suggested that one soil (from the Pukekohe region) harboured Pseudomonads with greater suppressive potential than those from three other soils. In greenhouse assays, selected isolates (89) were applied to plant growth medium as fungal spore or bacterial cell suspensions, to test for their capacity to suppress Rhizoctonia diseases of potato. Initially, 22 isolates demonstrated positive impacts on either Rhizoctonia canker symptoms and/or on plant parameters (e.g. tuber number or weight). However, no isolates gave strong and consistant suppression of Rhizoctonia diseases in repeated assays. The results indicated that inundative biocontrol was insufficient to achieve strong, consistent suppression of Rhizoctonia diseases of potato, when applied in this commercially realistic manner. Investigations into conservation biocontrol included testing several organic matter (OM) amendments previously suggested as potential suppressors of Rhizoctonia diseases of potato. These OMs were based on pine bark compost, chitin or biochar. Very slight disease suppression was detected. Results from analysis of soil community substrate utilisation showed that increases in activity or diversity as a result of OM amendment, at commercially viable concentrations, were insufficient to suppress Rhizoctonia diseases. The soil population of R. solani AG 3 and AG 2-1, and fungal and bacterial soil community structure, as a result of four crop rotation treatments (including potato monoculture) from the Pukekohe region were assessed. No rotation treatment increased pathogen inoculum. Soil bacterial community was not affected by rotation treatment. Fungal community was affected by the most recent crop, but not crop sequence. The impact of the four rotation treatments on Rhizoctonia disease expression was also assessed. Soils from all four treatments gave similar disease expression. Culture dependent and independent techniques used to determine soil fungal and bacterial community richness, diversity or metabolic activity demonstrated that increases in these parameters did not correlate with decreases in Rhizoctonia disease measures. The study concluded that consistent biocontrol of Rhizoctonia diseases of potato probably requires the presence and stimulation of a specific, small group of suppressive organisms. For biocontrol of these diseases to be a reliable option for potato growers, biocontrol agents will probably require appropriate and economically feasible nutritional support when introduced into crop disease management strategies.