Resistance of potato cultivars to <i>Synchytrium endobioticum</i> in field and laboratory tests, risk of secondary infection, and implications for phytosanitary regulations

Abstract

Laboratory (Spieckermann) tests, pot tests and field tests provided concordant evidence for the partial nature of resistance of potatoes to pathotypes 1 (D1) and 6 (O1) of Synchytrium endobioticum . Susceptible potato cultivars produced large warts (&gt; 16 mm in diameter) in Spieckermann tests and had low field resistance levels (1–6). Field‐resistant cultivars (levels 7–9) produced small warts or no warts at all in Spieckermann and field tests. In pot tests, at low inoculum levels (1 sporangium per 25 g soil) susceptible cultivars still developed warts, whereas field‐resistant ones did not develop any warts below 25 sporangia per g soil. Above 35 sporangia per g soil, 100% disease incidence was observed in susceptible cultivars but only minimal wart development in field‐resistant ones. Tests with continuous cultivation of potato cultivars in infected soil during three consecutive years showed that field‐resistant cultivars will not support build‐up of inoculum in soil. It is concluded that field‐resistant cultivars do not create a risk of secondary infection, the criterion given for resistance in EU Directive 69/464/EC.