Selection for Phytophthora field resistance in the F2 generation of organic outdoor tomatoes

Tomatoes are the most important vegetable, globally as well as in Germany. Outdoor tomato production is seriously impaired due to increasing infections with evolving late blight (Phytophthora infestans) populations. Within organic agriculture, research is being conducted to develop regionally adapted and open pollinated cultivars of outdoor tomatoes with late blight field resistance. In the present experiment, three crosses, including wild, cocktail, and beefsteak tomatoes, were selected for field resistance against late blight in F₂ at one location per cross. The comparison of positive and negative selection in F₃ revealed the selection of single F₂ plants to be efficient in all three crosses. F₂ selection has proved to be a robust and efficient tool for breeding programs. The correlated response to selection in other traits, including yield, fruit weight, days to maturity, harvest period, and plant height, depended on the cross. It was evident that selection for desired traits combined with field resistance against late blight is promising, even in wide crosses. The most undesired attribute of wild tomatoes is the formation of shoots on leaves and in inflorescences. No correlation was observed between field resistance and shoot formation, allowing the selection of genotypes with improved field resistance and yield, but without morphological disadvantages.