Determination of Resistance to Phytophthora infestans on Potato Plants in Field, Laboratory and Greenhouse Conditions

An experiment was conducted to determine the host resistance of potato against Phytophthora infestans for twenty-five potato genotypes in 2010 and 2011 at Khumaltar, Lalitpur, Nepal using four assays- three for foliage resistance (field, whole-plant and detached leaf) and one for tuber resistance (tuber slice). An isolate of P. infestans collected from Lalitpur (LPR-1) was used for inoculation at a concentration of 3 x 103 sporangia ml-1 in all assays. Infected foliage area in the field and whole-plant assays, lesion size on detached leaves, and colony growth on tuber slice were all individually converted to 0-9 interval scale for susceptibility. Field assessment, considering the most robust measure of resistance, was used as benchmark for comparing the other assays. Sixteen of the genotypes had very little disease in the field (scale value <1) indicating they were probably expressing race-specific resistance, which has historically been short lived. Susceptibility levels measured in the whole-pant assay were highly correlated (r = 0.90) with converted field scale values, although the correlation was lower for the detached leaf assay (r = 0.63) and least for tuber-slice assay (r = 0.46). Low correlation in the detached leaf assay was assumed to represent lower resolution of the single-cycle assay. Low correlation in the tuber assay may have also reflected genetic differences as foliage and tuber blight resistance are not always correlated. Genotypes with extreme resistance in the field were frequently identified as having partial resistance in the other assays, which could mistakenly be interpreted as more durable field resistance. The consequences for selecting durable resistance are discussed. Â