Diagnosis of Synchytrium endobioticum: the importance of sample pretreatment to determine the infectivity of resting spores in bioassays

Abstract

Potato wart disease, caused by the soil-borne, obligate biotrophic fungus Synchytrium endobioticum, is one of the most important diseases of cultivated potatoes. The fungus does not produce hyphae, but produces very thick-walled sporangia (resting spores), so an infection with S. endobioticum results in a long-term contamination of fields. Chemical control of the disease is not possible; consequently, potato production on infested fields is prohibited for many years. Reliable diagnosis of S. endobioticum is essential to preserve cultivation areas. A critical step is to determine the infectivity of resting spores. The new implementing regulation (EU) 2022/1195 recommends a bioassay based on soil samples in addition to microscopic analysis. The aim of this study was to evaluate soil-based bioassays for their ability to determine the infectivity of resting spores in infested soils. It was shown that the resting spores could be in a dormant state, in which they do not infect potatoes. In some cases, this dormancy could be broken by a certain sequence of moistening and aerating the soil. Reactivation of soil is therefore recommended prior to soil-based bioassays. Microscopic analysis could not distinguish between dormant and infectious resting spores.