QUANTITATIVE RESISTANCE TO LATE BLIGHT OF POTATO IN WILD SOLANUM CARIPENSE
International Potato Center (CIP), PO Box 1558, Lima, Peru
Background and objectives
S. caripense is a non-tuber bearing Solanum occurring throughout central South America. In its natural habitat it is affected by late blight of potato caused by Phytophthora infestans. I studied host-pathogen interactions using segregating host progenies and several unrelated pathogen isolates, to test the occurrence of race-specific vs. non-specific resistance.
Materials and methods
Resistance to late blight was tested on a series of six interrelated cross progenies and their parents, representing a total of 211 individuals of varying degrees of resistance. A detached-leaflet assay was applied, using four lateral leaflets of the top fully expanded leaves from greenhouse-grown plants in bud or flower. Seven potato isolates of P. infestans representing different levels of virulence complexity (from 0 to complex, for the 11 potato R genes) were used for spray inoculations of sporangia suspended in distilled water at a concentration of 20,000 sporangia/ml. The samples were incubated at 18°C and a 16 h daylength regime. The traits infection efficiency (number of leaflets with visible disease symptoms; IEF), percent area of diseased leaflet surface (A), degree of sporulation (S), ocurrence of necrotic spots (N), and tissue yellowing as a response to infection (Y) were recorded 96, 144, and 192 h after inoculation. Tests were repeated 2-4 times over 1-2 years.
Results and conclusions
Traits IEF, A, and S were significantly correlated (IEF-A, r=0.77; A-S, r=0.67). Resistant individuals of all progenies expressed only very few visible disease symptoms (0-20% A). Susceptible individuals always expressed lesions of varying size, covering 40-100% of the total area. Analysis of segregation of the trait A (expression 144 h after inoculation was chosen for the analysis) suggested that resistance resulted from complementary interaction of two independent, dominant genes; Lex1 and Lex2. Either one or both dominant alleles produced a resistant phenotype with all isolates used. Depending on the isolate, double-recessive genotypes (lex1 lex2) were not always fully susceptible, but expressed low to intermediate levels of quantitative resistance. The pattern of segregation observed suggested maternal cytoplasmic control of expression of allele Lex2. Resistance was quantitative because leaflets that appeared to be uninfected sometimes started to produce visible disease symptoms two weeks after inoculation. Resistant genotypes had a reduced lesion expansion rate (LER), when compared to susceptible genotypes. Sporulation was reduced as a pleiotropic effect of limited fungal growth. Traits N and Y were not consistently correlated with particular Lex-genotypes. The pair of complementary genes Lex1/lex1 and Lex2/Lex2 are different from the potato R genes  because their expression is quantitative and reduces LER.
It is concluded that S. caripense possesses high levels of race-specific, quantitative resistance to potato late blight. Implications and the usefulness of these resistance genes for breeding potato for late blight resistance are discussed.
1. Black W, Mastenbroek C, Mills WR, Peterson LC, 1953. Euphytica 2, 173-178.