Evaluation location: Wisconsin, United States
Twelve plants of each population were planted in the fall of 2008 for winter greenhouse tuber generation. One tuber from each of 12 individuals was randomly selected. They were rinsed with distilled water and dried overnight before inoculation. Pectobacterium carotovorum isolate WPP14 (provided by Dr. Amy Charkowski, UW-Madison) was cultured on agar plates and 10 ?l of prepared bacterial suspension (1.0 ? 108 CFU/ml, OD value at 0.20, which was measured with the wavelength at 600 nm) was used as the inoculum source (Yap et al., 2004). A sterilized pipette tip was used to make a 7 mm deep hole in the middle of each tuber, avoiding lenticels. Then, the inoculum was injected into the hole using a new pipette tip. A new tip was used for each inoculation. After a 72-h incubation period in the dark at > 80% relative humidity and room temperature (23?C), each inoculated tuber was cut in half along the inoculated hole. Lesion diameter on the cut tuber was measured.