Potato blackleg is a seedborne disease that can cause significant economic losses for growers. Disease development depends mainly on two drivers, namely seed inoculum and local climatic conditions. To better establish the relationship between these two drivers, blackleg development was monitored in Swiss field trials at multiple locations from 2010 to 2013 involving three sets of naturally infected seed lots planted in each of three locations. The seed lot itself was thereby the most important factor explaining differences in disease development, rather than environmental factors. In a further on‐farm project conducted at various locations in Switzerland and southern Germany from 2013 to 2015, the implementation of a seed‐testing procedure was investigated. A total of 177 seed lots were tested for natural latent infection with soft rot Pectobacteriaceae and the corresponding blackleg incidence was tracked in 242 fields. The reliability of the relationship between latent infection and field incidence was found to be strongly linked to the bacterial species. Dickeya spp. field infection could be predicted with an acceptable reliability, whereas Pectobacterium carotovorum subsp. brasiliense, even when detected as latent tuber infection, was not consistently expressed as visual blackleg. Moreover, commonly found mixed latent infections with several bacterial species made it even harder to predict which bacteria would cause blackleg symptoms. Finally, variability in the reliability of seed testing may also be explained by differences in local farming practices. These trials over several years with naturally infected potato seed highlight the usefulness and limits of seed testing to manage blackleg.
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 Kühn (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.
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