Effect of aggressiveness level of Phytophthora infestans on over-winter survival of tubers

The aim of this study was to understand selection pressure on Phytophthora infestans populations, during winter, the inter-epidemic phase, and to address the question of a tradeoff between aggressiveness during the epidemic phase and survival capacity during the winter season in P. infestans. In France, P. infestans, the causal agent of potato late blight, behaves as an obligate biotroph. French populations are essentially thought to survive as mycelium in tubers remaining either in the soil or in refuse piles near potato fields. Its inability to survive in the absence of living plant tissue raises the question of possible negative selection against the most aggressive isolates during winter. Indeed, the most aggressive isolates are not necessarily the fittest isolates over the whole season or over several seasons: while such isolates were favored during the epidemic season, they might exhaust their nutrient base (infected tubers) too fast to successfully bridge seasons. Intermediate or weakly aggressive isolates would then tend to best survive, in contrast to highly aggressive isolates, which would probably kill tubers before they sprout. To investigate this hypothesis, we inoculated tubers with isolates of different aggressiveness levels (low, medium, high), leaving them in piles (outdoors) at three different sites during the winter, and scored the proportion of living tubers in the following spring. Our data indicated that over-winter survival of potato tubers was not related to the aggressiveness level of the isolates and thus did not lead to negative selection of the most aggressive isolates. A strong site effect was observed, but the absence of interaction between piles (i.e. aggressiveness levels) and sites indicated that the lack of trade-off was true for all three winter French conditions. These results suggest that the most aggressive isolates are not eliminated faster than less aggressive ones during over-winter survival in tubers. In absence of a trade-off between aggressiveness and over-winter survival, the selection process may occur at another stage of the life cycle.