This fungus belongs to the Zygomycota group of fungi.
Damage. This soil-inhabiting fungus is most commonly a problem in nursery stock production nurseries. It causes the foliage of plants to turn grey-green, then brown and eventually to die off completely (see Figure 15.14). Sliced roots show a chestnut brown rot, with a clear line between infected and non-infected tissues. Two hundred plant species, including Chaemaecyparis, Erica and Rhododendron species may be badly attacked.
Life cycle and spread. The disease is commonly introduced on infected stock plants or contaminated footwear. It multiplies most rapidly under wet conditions, within a temperature range of 20°C and 30°C, infecting the root tissues and producing numerous asexual spores, which may be spread by water currents to adjacent plants. Sexual oospores produced further inside the root are released on decay and allow the fungus to survive in the soil for several months without a host.
Control. Preventative control (see hygienic growing, p269) is important. Reliable stock plants should be used. Water supply should be checked to avoid contamination. The stock plant area should be elevated slightly higher than the production area to prevent infection by drainage water. Rooting trays, compost and equipment, e.g. knives and spades, should be sterilized (e.g. with formalin) before use. Placing container plants on gravel reduces infection through the base of the pot. The chemical, etridiazole, incorporated in compost protects the roots, but does not kill the fungus. Some species, such as Juniperus horizontalis, have some tolerance to this disease.
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