Effect of spore density, cultivar resistance and Phytophthora infestans isolate on tuber blight under field conditions

Survival of P. infestans in soil was limited to 5 weeks at high inoculum density. With a ten fold dilution of the infection pressure survival of P. infestans in soil was limited to two weeks. Differences between isolates concerning survival in the soil were small. From these experiments we conclude that survival of P. infestans in soil depended on spore density rather than P. infestans isolate used. Tuber blight infection rate depends on density of the sporangia in the soil, tuber blight resistance of the cultivar and to a lesser extent the P. infestans isolate used. Isolate of type EU 13 A2 was not more aggressive to tubers than IPO 428-2, regardless of the resistance level of the cultivar used. A mixture of isolates was found to be more aggressive than two single isolates tested on cultivar Bintje only. Thus measures to avoid infection of the soil with P. infestans sporangia lower the tuber blight infection risk, especially on the susceptible cultivar Bintje