Assessing the infectivity of the winter sporangia of Synchytrium endobioticum (Schilb.) Percival the causal agent of potato wart

Potato wart is caused by the obligate biotrophic, soil-borne fungus Synchytrium endobioticum (Schilb.) Perc., which is regulated as quarantine pest in the European Union. Winter sporangia of S. endobioticum are extremely persistent and resistant to environmental conditions. Plots where the pathogen is detected must be demarcated (“scheduled”) by the national plant protection authority and farmer must comply with significant cultivation restrictions. A common procedure for descheduling is the direct microscopic examination of soil for the presence of viable resting spores of S. endobioticum. However, there is a controversy in the scientific community whether the viability/infectiousness of resting sporangia can be assessed visually. The objective of this study was to resolve this dispute. As autoclaving is a standard procedure to inactivate pathogenic microorganisms, soil contaminated with S. endobioticum was autoclaved before analyzing the infectivity and viability of resting sporangia by bioassay and microscopic examination in comparison with non-autoclaved soil. Although autoclaved and non-autoclaved substrates contained visually viable resting sporangia, symptoms of potato wart were observed only in the non-autoclaved soil. Hence, autoclaving renders winter sporangia harmless, and microscopic examination is not sufficient to assess the viability and infectivity of winter sporangia.