Pythium spp. and Rhizoctonia solani are extremely common in soils in both tropical and temperate regions. Seedling plants of almost all fruit, vegetable, field and ornamental crops are liable to attack, with soil and weather conditions playing a major role in development of the disease. Plants can be attacked following direct sowing in the field, in outside seedbeds and in glasshouse seedling-production areas.
Also known as 'wire stem' in seedlings with more rigid stems (e.g. cabbages).
Damping-off is the destruction of seedlings by pathogens. The pathogens most commonly responsible for damping-off are Pythium species and Rhizoctonia solani. Other pathogens that may be involved are Phytophthora, Fusarium and Aphanomyces. Many other fungi and some bacteria can cause damping-off symptoms when carried in or on the seed.
There are two main types of damping-off. Pre-emergence damping-off results in the rot of seed or seedling sprouts before emergence.
Post-emergence damping-off occurs after plants have emerged from the soil. A soft decay of the taproot or rootlets causes collapse of seedlings. Infection by R. solani results in a water-soaked, sunken lesion at ground level, causing the plant to fall over. Surviving plants are stunted, and affected areas often show uneven growth.
Seedlings with more rigid stems (e.g. cabbages) do not fall over, but the stem is thin, discoloured and may be bent or twisted without breaking, giving the disease the name 'wire stem'. Eventually, the stems are girdled and the plant dies.
|How does it spread?|
Pythium species are widely distributed in soil and water, living as saprophytes or as parasites attacking fibrous roots of plants. Different species are adapted to different temperature regimes. The pathogens thrive under wet conditions, and seed or seedlings growing in wet soil are most likely to be attacked, particularly when temperatures are unfavourable for the host.
Rhizoctonia solani is very common in soil, surviving as sclerotia or mycelium. The fungus survives between crops on plant trash. The plant residues provide a ready source of inoculum from which disease can develop in subsequent crops. Moderately wet rather than waterlogged soil favours this fungus. Infection is more likely in plants growing poorly, whereas rapidly growing, vigorous plants often escape infection.
Both Pythium and R. solani are spread by rain, irrigation, water splash, contaminated tools, soil or potting mix, and infected plants.
Seedling plants of almost all fruit, vegetable, field and ornamental crops.
Raise seedlings in fumigated soil or soil-less potting mix on elevated benches. Prevent contamination of treated soil by avoiding water splash, contaminated tools or storage in dirty areas.
Treat water supplies before use if dams are the source of water. Avoid excessive watering, and thin seedlings to allow good air circulation.
Treat seed with the recommended fungicides before planting and apply as necessary if outbreaks of damping-off occur.
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