These two fungi belong to the Zygomycota.
Damage. These two similar genera of fungi cause considerable losses to the delicate seedling stage. The infection may occur below the soil surface, but most commonly the emerging seedling plumule is infected at the soil surface, causing it to topple (see Figure 15.13). Occasionally the roots of mature plants, e.g. cucumbers, are infected, turn brown and soggy, and the plants die. Rose plants often have high levels of Pythium around their roots as they age. Although the mature plant is not seriously affected, it is a common experience that on removal of the plant and replacement with a young rose plant, there is a quite rapid decline in its vigour, called rose-sickness.
Life cycle and spread. Both Pythium and Phytophthora occur naturally in soils as saprophytes, but under damp conditions they produce the asexual spores that cause infection. These spores are spread by water. Sexual spores (oospores) are produced in infected roots (mostly in autumn) and may survive several months of dry or cold soil conditions.
Control. Prevention control is best achieved (both for the amateur gardener and the professional grower) against
these diseases by providing a disease-free growing medium. This may be produced by using fresh compost, by partial sterilization of soil with heat, or (for the professional grower) by a sterilant such as dazomet. Seed producers often coat crop seed with a protective seed dressing (see also p284) such as thiram to prevent early infection.
Water tanks with open tops, harbouring rotting leaves, are a common source of infected water and should be cleaned out regularly. Sand and capillary matting on benches in greenhouses should be regularly washed with hot water. The use of door mats soaked in a sterilant such as dilute formalin may prevent foot spread of the organisms from one greenhouse to another. Waterlogged soils should be avoided, as these fungi increase most rapidly under these conditions. The amateur gardener may use a copper formulation (known as Cheshunt mixture) as a drench to slow down the increase of damping off. Professional growers use a product containing etridiazole, which may be mixed in with composts, or drenched on to seed trays, pots or border soil growing young plants.
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