Brassicas Resistant to Clubroot

Disease Basics

• The fungal pathogen Plasmodiophora brassicae thrives in moist soil and affects root crops more than those growing above ground. The soil-borne pathogen causes roots to swell into galls, abnormal swellings that interfere with the roots' ability to absorb nutrients and water. The pathogen typically forms galls in six to eight weeks from the time of infection. Immature galls are firm and white, turning soft and grayish-brown as they mature and decay. Infections cause yellowing leaves, wilting and clubs, root galls that swell up to 20 times their normal size. Severe infection can kill the plant. Clubroot spores can survive in the soil for up to 20 years. However if they have nothing to feed on, they live only about four years.

Disease-Resistant Cultivars

• The clubroot resistant Brassica cultivars developed so far are for Brassicas that grow above ground and include green cabbage (Brassica oleracea var. alba), Napa cabbage (Brassica rapa subsp. pekinensis) and cauliflower (Brassica oleracea Botrytis). The green cabbages are “Kilaton,” “Kilaxy,” “Kilaherb,” "Kilazol" and “Tekila.” The Napa cabbages are “Bilko,” “China Gold,” “Deneko,” “Emiko” and “Yuki.” The cauliflower cultivars are “Clarify” and “Klapton.”

Cultural Practices

• Do not plant brassicas in infested soil. Since the pathogen can be transferred from infected soil on shovels, spades and other equipment, disinfect any tools you have used in another area. The pathogen thrives in soil depressions where water collects, in soil with high clay content and in soil that does not drain well. To increase soil drainage, grow your brassica in raised beds or plow your field deeply. Since clubroot fungi develop best in cool, wet soil, spread clear plastic sheets over infected soil surrounding your plants during the warmer months. Leave it there for four to six weeks. Thin sheets absorb more heat than thick sheets.

Managing Soil Acidity

• Clubroot fungi especially like acidic soil with a pH less than 6.5. Soil pH measures the acidity or alkalinity of soil. Acidic or “sour” soil, typically found in moist climates, has a pH below 7; alkaline or “sweet” soil, typically found in arid climates, has a soil pH above 7. Since clubroot likes low soil pH, add calcitic lime to raise the soil pH to above pH 7.2 Use more expensive dolomitic lime if your soil is low in magnesium. Calcitic lime is pure calcium carbonate. Dolomitic lime contains equal parts of calcium carbonate and magnesium carbonate. Most garden supply centers sell kits that will measure soil pH.

Chemical Controls

• Rototilling the fungicide PCNB (pentachloronitrobenzene) into the soil before you plant brassica does not control clubroot completely, but does reduce the number of root clubs. PCNB in an emulsion form will burn the roots. You can also apply PCNB 4 inches deep in bands on either side of your plants, but the benefit of the chemical is lost in the second year by this method. Calcium cyanamide fertilizer is showing promise as a way of controlling clubroot. When you mix it into into soil, calcium cyanamide metabolizes into lime (calcium dihydroxide) and hydrogen cyanamide. Hydrogen cyanamide has both herbicidal and fungicidal and herbicidal properties, killing weeds as it reduces fungal spores. It also degrades into urea, releasing nitrogen into the soil over a period of six to eight weeks.