Intraracial variation inPhytophthora infestans and field resistance to potato blight

Summary

Newly isolated strains ofPhytophthora infestans belonging to the same physiological race differ in their rate of spread and depth of penetration on tubers and in their rate of growth on three synthetic media. They also differ in their speed of sporulation on leaves. Their growth on tubers is in general faster on the variety from which the strains were isolated in the field than on any other variety. Furthermore, the isolates which on average sporulate fastest are those obtained from varieties on which all isolates on average sporulate the fastest. These differences between isolates, however, are modified or lost following prolonged maintenance in the laboratory on a synthetic medium although at the same time their performance on this medium improves.

Nothing definite is known about the nature of the differences between the isolates or of the changes they undergo in the laboratory. The evidence available, however, favours an explanation based on the selection of nuclear differences rather than one based on cytoplasmic adaptation.

Commercial varieties of potato with the same immunity genes differ in the resistance they offer to the spread of the newly isolated strains of fungus on their tubers and to sporulation on their leaves. Both the differences between the fungal isolates and between the potato varieties have important consequences for the development of “field resistant” varieties of potato.