Notes on the inheritance of field resistance of the foliage of Solanum tuberosum to Phytophthora infestans

Abstract

During the very wet summer of 1956 observations were made on the damage done to the foliage by Phytophthora infestans in about 2,000 small seedling-clones of 35 cross-combinations. In part of these crosses the genes R ₁ and R ₃ for hypersensitivity were involved.

The degree of field resistance of the material devoid of R-genes seems to be governed by a series of minor genes and it is difficult to draw a clear-cut conclusion as to dominance relations.

The R-material did not show any lesion during the initial stage of development of the epidemic. However, with the rapid growth of the population of the parasite physiologic races came to the fore and ultimately also the R-material was more or less heavily attacked.

The data indicate that the degree of field resistance of the R-clones to the new biotype is also governed by a set of minor genes and probably by the same set as present in the material devoid of R-genes. As with the common varieties, in the R-material there is also a close relation between earliness and a low degree of field resistance. The value of the complex of genes for field resistance that may be present in the wild Mexican species and notably in S. demissum is discussed. Finally the influence of the plasticity of the Phytophthora population is considered.