Question

Describe an experiment, either at the molecular/cellular level or at the physiological level to gain knowledge on the impact the biotic pathogen Phytaphthora infestans has on potato plants. Please explain methodology and what can be learned from the experiment.

Answer 1

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.