This fungus is classified in the Deuteromycota group of fungi.
Damage. This disease is most commonly recognized by the fluffy, light-grey fungal mass which follows its infection. In lettuce, the whole plant rots off at the base. The plant turns yellow and dies. In tomatoes, infection in damaged side shoots, and yellow spots (ghost spots) on the unripe and ripe fruit are found. In many flower crops, e.g. chrysanthemums, infected petals show purple spots which, in very damp conditions, lead to mummified flower heads. This disease may affect many crops.
Life cycle and spread. Grey mould normally requires wounded tissue for infection, which explains its importance in crops which are de-leafed, e.g. tomatoes, or disbudded, e.g. chrysanthemums. Damp conditions are essential for infection and spore production. The millions of spores are carried by wind to the next wounded surface. Black sclerotia, about 2 mm across, produced in badly infected plants, often act as the overwintering stage of the disease after falling to the ground, and are particularly infective in unsterilized soils on young seedlings and delicate plants, e.g. lettuce.
Control. Preventative control may involve soil sterilization. Strict attention to greenhouse humidity control (particularly overnight) reduces the dew formation which is so important in the organism's infection. Cutting out of infected tissue is possible in sturdy stems, e.g. in tomatoes. Amateur gardeners at present have no effective chemical control against this disease. The professional grower has two protectant fungicide ingredients, iprodione and chlorothalinal. As well as being sprayed, iprodione can also be applied as a paste to cut plant surfaces.
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