Drycore Appears to Result from an Interaction between Rhizoctonia solani and Wireworm (Agriotes ssp.)-Evidence from a 3-Year Field Survey

Drycore is an important quality deficiency in Europe especially in organic potato production and after grass clover leys. The drycore symptom is attributed to Rhizoctonia solani Kuhn (teleomorph: Thanatephorus cucumeris (Frank) Donk). In the framework of a 3-year survey (2001-2003), data concerning potato quality, crop rotation, management, and site parameters were analysed from 185 potato fields. The hypothesis was tested that injuries on potatoes caused by wireworms facilitate the penetration of R. solani and favour the formation of drycore. Analysis of variance showed a significant influence of wireworm damage, seed quality, and grass clover leys in the crop on the level of drycore damage. On fields which had both a low occurrence of black scurf on the seed tubers and a low occurrence of wireworm damage at harvest, significant drycore damage was never observed. The relative risk for drycore damage on tubers was significantly higher if black scurf or wireworm damage was on the same tuber. In contrast, no higher risk for drycore was observed on tubers with slug damage. Abiotic factors like farm manure application, organic matter content, texture, and pH of the soil also had no significant influence on the level of drycore. Thus, the wounding of potatoes by wireworm could be confirmed as the major variable for drycore. The mode of action has to be clarified under controlled conditions.