Resistance to Potato Late Blight (Phytophthora infestans (Mont.) De Bary) in Wild Solanum Species

Resistance to the potato late blight fungus in cultivated varieties appears to be either nondurable monogenic resistance or durable field resistance. The 11 genes known so far for non-durable race-specific resistance, have been derived from the Mexican wild species Solanum demissum and S. stoloniferum. They were overcome by compatible races of the pathogen soon after their introduction in the first half of the twentieth century. Complex races with as many as seven virulence genes are now common in potato growing areas. Potato cultivars differ in their reaction to compatible races; some cultivars were found to succumb in a very short time, whereas other cultivars are more resistant and disease progress on these was found to be much slower. This type of resistance is known as field resistance and is considered to be non-race-specific. For some older cultivars this resistance does not seem to have diminished during several decades of cultivation. Wild Solanum species are a valuable source of durable resistance to late blight. Many species are described as highly resistant in gene bank inventories. It is likely that many of these will contain resistance genes that will turn out to be race-specific once they have been introduced into the cultivated potato. The major task is not to find resistant species, but to identify those that carry durable resistance.