Evaluation location: California, United States
Seeds were first germinated at 10 degrees C in the dark for 48 hours in a growth chamber, and grown at 20 degrees C with a 16-hour photoperiod. Seedlings were inoculated at 2, 3 and 4 weeks after sowing by saturating the soil in each plug tray well with a 3-ml suspension containing 2 x 106 conidia/ml in sterile, distilled water. Seedlings were incubated for another 1 to 2 weeks after the third inoculation and transplanted into 0.5 L foam-insulated cups filled with a pasteurized sand:potting soil mixture (3:1 v/v). When all plants within a PI had reached flowering, plants were uprooted, roots were cleaned of soil and cut longitudinally to evaluate disease severity (DS) by rating root discoloration on a 0 - 5 scale where 0 = no vascular discoloration, 1 = 1 to 25% of the vascular tissue exhibiting discoloration; 2 = 26% to 50%; 3 = 51% to 75%; and 4 = 76% to 100% discoloration in the absence of foliar symptoms; and 5 = 100% discoloration and the presence of foliar symptoms typical of Verticillium wilt. Plants with less than 100% root discoloration and foliar symptoms were not observed since the disease progress acropetally. Plants with DS greater than 1 were considered symptomatic, and the disease incidence (proportion symptomatic plants) was calculated as the sum of plants with DS > 1 divided by the total plants evaluated. Root and crown tissues of inoculated and non-inoculated plants were periodically sampled and placed on a potato dextrose agar medium to confirm the presence or absence of the pathogen.