2.2.73
VARIATION IN AGGRESSIVENESS TO POTATO TUBERS IN LOCAL POPULATIONS OF PHYTOPHTHORA INFESTANS IN THE NETHERLANDS

WG FLIERl, LJ TURKENSTEENl, A MULDER2 and GBM VAN DEN BOSCHl

lDLO-Research Institute for Plant Protection (IPO-DLO), PO Box 9060, 6700 GW Wageningen, The Netherlands; 2HL Hilbrands Laboratory for Soilborne Pests and Diseases (HLB), PO Box 323, 9400 AH Assen, The Netherlands

Background and objectives
A comparative study was performed on the variation in aggressiveness to tubers among 36 isolates of Phytophthora infestans, recently collected from three potato-growing regions in The Netherlands. In addition, the origin of interaction between potato cultivars and P. infestans isolates was investigated by means of whole tuber inoculation experiments [1] in two successive years. In 1996, four new population isolates and an old population reference isolate were tested for their ability to infect and invade tubers of the cultivars Astarte, Bintje, Eikana, Elles and Producent.

Results and conclusions
Considerable variation in tuber infection and successive invasion of infected tubers of variety Bintje was found between isolates of all three local populations of P. infestans. Tuber infection varied between 10.6 and 77.4% when 39 isolates were compared after inoculation of freshly harvested tubers of variety Bintje. Between populations, no significant differences for infection efficiency or disease severity were detected. The most aggressive isolate of the old population matched the average level of the recently introduced population with respect to its ability to infect tubers. The amount of tuber infection and the successive invasion rate was not correlated, which is in agreement with other reports [2,3]. Mating type was not correlated with either tuber aggressiveness or average number of virulence factors.

Interaction effects between isolates and varieties were considerable and highly significant (P<:0.001) for both tuber infection and the successive invasion of tuber tissues. The interference of hypersensitivity genes, the so-called R-genes, could be excluded through the use of compatible isolates. Hence, genes other than such R-genes are to be held responsible. In 1997, nine potato cultivars were inoculated with four isolates. Again the interaction terms for tuber infection and invasion were found highly significant (P<0.005 and P<0.017 respectively). It is concluded that aggressiveness to potato tubers is highly variable in the present population of P. infestans in The Netherlands. The resistance of potato tubers is controlled by both additive and interactive factors, the latter being controlled by factors that differ from the above-mentioned R-genes. This cultivar-isolate specificity may interfere with proper tuber resistance testing of potato cultivars and therefore reduce the practical value of resistance levels given in lists of recommended varieties.

References
1. Mooi JC, 1964. Proceedings of the Second Triennial Conference of the European Association for Potato Research, p. 195.
2. Pietkiewicz JB, Jellis GJ, 1976. Biui. 1. Ziemniak, pp. 129-134.
3. Wastie RL, 1991. In: Ingram DS, Williams PH (eds.) Advances in Plant Pathology, Vol. 7. London: Academic Press, pp. 193-224.