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Effect of Wirestem Severity on Survival and Head Production of Transplanted Broccoli and Cabbage

Field experiments were conducted with transplants of Brassica oleracea with known severity levels of wirestem caused by Rhizoctonia solani anastomosis group 4. Seedlings of broccoli and cabbage were grown in steamed soil infested with R. solani at 5 to 25 sclerotia/kg. Two weeks after inoculation, plants were separated into five severity classes based on wirestem symptoms, then transplanted into fumigated field plots in the spring and fall of 1995. The percentage of plants with and without aboveground symptoms was assessed at 14 and 42 days after transplanting. Marketable-sized heads were harvested eight times. In both seasons, percentages of symptomless plants, surviving (symptomless plus symptomatic) plants, and plants producing a marketable-sized head decreased as wirestem severity increased. Only 33 and 29% of cabbage transplants with >75% of the stem circumference girdled survived and produced a marketable head, respectively, compared with 95 and 83% of healthy transplants, respectively. For broccoli, only 15% of transplants with girdled stems survived and produced heads in spring but, in the fall, 74 and 72% of transplants with girdled stems survived and produced heads, respectively. Percentage of plants producing a marketable-sized head was highly correlated (P = 0.0001) with percentage of symptomless plants at 14 days after transplanting and percentage of surviving plants at 42 days after transplanting.