WHAT IS DAMPING OFF DISEASE?
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Damping off is one of the most common and troublesome types of horticultural diseases. It can affect all types of seedlings, but is most problematic on fast growing ornamental seedlings such as antirrhinums, lobelias, nemesias, petunias, salvias and stocks, or vegetable seedlings like cabbages, cress, lettuces, tomatoes, peas and beans.
Symptoms can be varied but without treatment all result in plant death. Often, young seedlings are seen rapidly collapsing in small - roughly circular - patches, or the seedlings may just become progressively weaker with shrivelled stems. Sometimes, the root system simply rots away. In larger seedlings, and even young plants, you may also witness leaf spotting or other discoloration, and sometimes grey mould displayed on the stems or leaves.
There are a number of organisms that cause damping off, which is why the symptoms are varied. The most common ones that cause dying out in patches are the fungi Pythium and Rhizoctonia solani, surviving as spores in the soil. Stem lesions are often caused by soil-borne species of Alternaria and leaf spots are generally associated with soil-borne Phyllosticta and Pseudomonas fungi. The grey mould that is often seen accompanying damping off is caused by Botrytis cinerea.
Treatment is difficult in the garden environment. Though some chemical controls have been employed in commercial practices, they are not yet available for use on a small scale. For the amateur or small-scale grower hygiene at all stages of propagation is essential. If your seedlings are prone to damping off then only use cleaned and disinfected pots and seed trays, and make sure that greenhouse benches are sterile. Mains water and a proprietary sterilised seed compost, which is moist but not over wet, should also be used. Don't assume all bought compost is sterile, as most is not.
Small quantities of compost for seed sowing can be sterilised by 'cooking' in an oven at 150C for an hour or so. Be aware that care should be taken when using water other than tap water as this may be another source of fungal infection. All storage tanks should be regularly cleaned and disinfected regularly, and preference should always be given to tap water on susceptible plants.
Avoid stressing plants and seedlings by preventing waterlogging and high humidity as this will make them more vulnerable and prone to attack. Sow seed thinly and prick out as soon as possible. Also, handle the seedlings by their leaves, and not the stems. Do not re-use compost that has been affected by damping off disease and if only part of the seed tray has shown symptoms remove all the affected seedlings - including a barrier of a few extra seedlings - and the affected compost. Water with Cheshunt Compound or similar fungicide to help prevent any further spread. Cheshunt Compound can also be used as a soil drench prior to seed sowing as a preventative measure, but this will not completely eradicate all problems. It can only be used as a preventative aid.