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807/232 Rhizoctonia Diagnostics and Epidemiology

Publication Date: 
16 August 2011
Author/Contact :
Author/Contact: 
Stuart Wale

Contractor :
Contractor: 
SAC

Full Research Project Title: Prediction and control of stem canker and black scurf of potato caused by soil-borne inoculum of Rhizoctonia solani
Duration: July 2002 - April 2004

Aim: To provide growers with a risk assessment tool for disease development caused by soil-borne R. solani AG3 from which control strategies can be decided.

Black scurf is a blemish disease of potato caused by the fungus Rhizoctonia solani and is characterised by black sclerotia on the tuber surface. With increasing emphasis on skin quality in the pre-pack and seed potato markets, this disease has increased in significance and is considered one of the three major tuber disease problems of the potato industry. In addition to producing a blemish on the tuber, the fungus can infect developing sprouts and stolons from the seed tuber prior to emergence resulting in the formation of cankers. This infection can result in delayed emergence, an uneven and reduced tuber set and an undesirable tuber size distribution.

The source of the fungal inoculum can be either the seed tuber or soil. The presence of black scurf or mycelial threads of the fungus on seed tubers can be established visually and microscopically and this permits a rational decision on the use of a Rhizoctonia-specific fungicide treatment. Where used, fungicide seed treatments have proved very effective at controlling seed-borne inoculum. However, soil-borne inoculum is not controlled by a seed tuber fungicide treatment.

Published studies on soil-borne inoculum have indicated that the fungus is not long-lived in soils and that during a long rotation the level of soil infestation declines markedly. Other studies have suggested that the impact of the disease is greatest in lighter soils. However, observations over the last decade suggest that these findings no longer hold true. Whilst light soils probably do represent the greatest risk, quality problems from black scurf development on tubers are common on a wide range of the best land for potatoes and an increase in prevalence of potato groundkeepers, which sustain a high level of soil-borne inoculum.

At the time this project was carried out, growers had no way to predict which fields harbour the greatest amount of the fungus. Three different methods were tested for their ability to detect the fungus in soil. Further work on DNA-based methods for detecting Rhizoctonia were carried out on project R253.

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