Tuber late blight development caused by the different P. infestans genotypes on the tuber cultivars was evaluated at 10°C storage temperature using whole-tuber subperidermal inoculation. The washed, SB203580 cell line surface-disinfested tubers were inoculated by removing a 5-mm-diameter potato plug using a sterile cork borer and placing 2 × 10−5 ml of sporangia suspension (delivering zoospores released from approximately 20 sporangia per inoculation) with

a hypodermic syringe and needle at the apical end of the tuber approximately 1 cm from the dominant sprout to a maximum depth of 1 cm. The potato plug was returned to close the wound, and it was sealed with petroleum jelly. A complete randomized block design with three experimental repeats consisted of 10

tubers per cultivar, six different cultivars and 12 different isolates representing four genotypes. A total of three replicates were inoculated for each of the cultivar–isolate combination. Ten control tubers per cultivar were inoculated with cold (4°C) sterile distilled H2O. After inoculation, tubers were placed in the dark in sterilized, covered plastic crates and returned to controlled environment chambers [Percival Incubator (Model I-36LLVL; Geneva Scientific, LLC, Fontana, WI, USA)]. The chambers were set at 10°C BI2536 and 95% humidity, and the sample tubers were incubated for 30 days until evaluation. Tubers of three different cultivars with different responses to P. infestans were obtained from MSU potato breeding programme to evaluate periderm susceptibility. The three cultivars with different tuber blight ratings were Atlantic (S), Jacqueline Lee (MR) and Stirling (R) (Kirk 上海皓元医药股份有限公司 et al. 2009). Tubers were prepared as described above. Tubers were submerged in a sporangial suspension containing approximately 1 × 104 total sporangia/ml for 48 h at 18°C. After inoculation, tubers were placed in the dark in sterilized, covered plastic crates with damp towels to maintain high humidity and then placed in controlled environment chambers

at 10°C. The experimental design encompassed two different P. infestans genotypes (US-8F and Pi10-012), three cultivars and 12 tubers per cultivar. Arbitrary samples of three tubers per genotype–cultivar combination were sampled at 3, 6, 10 and 15 days postinoculation for evaluation. The experiment was conducted four times, and the replicates were analysed together, blocking by repeat. At each time, the number of eyes and the number of lenticels infected were assessed under the dissecting microscope at 20 ×  magnification (Olympus SZX10; Olympus America Inc., Lake Success, NY, USA) and light microscope at 200 ×  magnification (Olympus CX22; Olympus America Inc.). A digital image analysis technique (Niemira et al. 1999; Kirk et al. 2001b) was used to assess tuber tissue infection. The image files were analysed using SigmaScan V3.0 (Jandel Scientific, San Rafael, CA, USA).