This fungus is classified into a quite separate group of fungi, the Plasmodiophorales.
Damage. It causes serious damage to most members of the Cruciferae family, which includes cabbage, cauliflowers, Brussels sprouts, stocks and Alyssum. Infected plants show signs of wilting and yellowing of older leaves, and often severe stunting. On examination, the roots appear stubby and swollen (see Figure 15.12), and may show a wet rot.
Life cycle and spread. The club root organism survives in the soil for more than five years as minute spores which germinate to infect the root hairs of susceptible plants. The fungus is unusual in forming a jelly-like mass (plasmodium), not hyphae, within the plant's root tissues. The plasmodium stimulates root cell division and causes cell enlargement, which produces swollen roots. The flow of food and nutrients in phloem and xylem is disturbed, with consequent poor growth of the plant. With plant maturity the spores produced by the plasmodium within the root are released as the root rots.
The disease is favoured by high soil moisture, high soil temperatures and acid soils. Although this fungus does not spread much in undisturbed soils it can be easily carried on infected plants, or on tools and wheels of machinery. In peat soil-growing areas, high winds may carry the disease a considerable distance.
Control. Several preventative control measures may be used by the amateur gardener and professional grower. Rotation greatly helps by keeping cruciferous crops away from high spore levels in the soil. Liming of soil greatly inhibits spore activity (see soil fertility). Recently released cultivars of late-summer cabbage are claimed to have strong resistance to club root. Autumn-sown plants establish in soil temperatures unfavourable to the disease and are normally less infected. Compost made from infected brassica plants should be avoided. The professional grower using transplants can prepare a seedbed previously sterilized with a granular product containing dazomet, which would ensure healthy transplants.
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