Aggressiveness of Phytophthora infestans on detached potato leaflets in four Nordic countries

Potato fields in Denmark, Finland, Norway and Sweden were sampled for single-lesion isolates of Phytophthora infestans. The aggressiveness of the isolates was determined on detached leaflets of potato cvs Bintje (susceptible) and Matilda (moderately resistant). The aggressiveness tests were carried out in the respective home countries of the isolates, with the exception of the Danish isolates. Fifteen Danish isolates were studied in each of the other three countries, including five isolates tested in all three laboratories. Results obtained from the Danish isolates revealed substantial differences between the test laboratories for infection efficiency, lesion growth rate and sporulation capacity on detached leaflets. When the laboratory effect was taken into account, the differences in aggressiveness between the countries were generally small or inconsistent between the test cultivars and epidemiologically insignificant. By contrast, variation among isolates within countries was substantial. The magnitude of the variation depended on country and cultivar. Maximal variation for the means of the isolates was between 89 and 185 h for latent period, between 100 and 1297 sporangia mm(-2) for sporulation capacity and between nearly zero and 6 mm day(-1) for lesion growth rate. Typically less than 1% of sporangia were able to cause infections, except in Norway. These extraordinarily low values may be an artefact of the testing method. High variation in results between the test laboratories emphasizes the need for caution when comparing results obtained by different research groups.