Response of potato differentials to natural population of Phytophthora infestans under Shimla (India) conditions

The resistance of individual differential of potato late blight to Phytophthora infestans was not overcome with equal frequency. The level of resistance depends on inoculum load and presence of R gene. The resistance of differentials R8, R9 and R1.4 was overcome least frequently. Inoculum load less than 20,000 sporangia/ml could not defeat resistance in R8, R9 and R1.4 differentials under natural conditions despite of presence of all 11 virulence factors in Phytophthora infestans population. The values for AUDPC ranged from 0.00 to 3.80. When the same set of differential was challenge inoculated at different inoculum concentration under in vitro conditions, the values for lesion size varied significantly among the differentials. In general, progression of lesion size decreased with decrease in inoculum concentration. Lowest lesion size was recorded in differential R8 (0.58 cm²) while highest was recorded in R11 (4.91 cm²). No sporulation was recorded on differential R8 and R9. No significant differences were observed in the amount of spore production and inoculum concentration.