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How to recognize gray rot?
If young seedlings die in large numbers, what is called damping-off of seedlings, if in wet weather a greyish felting is visible on leaves, stems, flowers, fruits and even seeds, this is a manifestation of gray mold. Similarly, the sign of a soft, damp and brownish rot on picked fruit ... The tissues affected by this mold wither and die quickly.
Gray mold and its consequences
Gray mold is a cryptogamic disease, caused by a filamentous fungus, of the genus botrytis. This fungus behaves like an opportunist, that is to say that it can attack a large number of different plants, annuals, perennials or shrubs. On the same subject, it can infest all types of organs, from seed to fruit. During the winter, it keeps in the soil, at a shallow depth, on the remains of dead plants, it is said to be saprophytic. It is a particularly severe disease during wet years. The mycelium (filaments of the fungus) develops on and in the plant, following injuries by hail and insects for example, or after an attack of powdery mildew. The fungus then releases substances that damage the tissues and cause the stems, leaves, flowers to wilt and rot the fruit. Botrytis multiplies from June to the end of October, thanks to the production of very numerous spores. This specific mushroom, Botrytis cinerea, however plays a beneficial role in a particular climatic context (relative heat and humidity), that is to say when it develops late on the epidermis of the grapes in vineyards, which promotes a concentration of sugars in the bay and leads to the birth of sweet wines. It is said in this case that it is "noble rot" ...
Prevention and control
Botrytis produces its harmful action especially on a plant weakened by damaging cultivation practices. We must therefore first analyze what seems objectionable in the way of farming. Ensure above all a good preparation of the soil, fertilize reasonably, weed if necessary, practice a mulching in connection with the climate, possibly keep insects away with a natural product like nettle manure ... This will help to obtain healthy subjects, therefore more resistant to disease. Greenhouse cultivation means paying particular attention to good ventilation around the plants and also, if necessary, pruning or leaf removal regularly during growth. If the fungus attacks in spite of everything, it is important to cut the sick parts early and gently, this to avoid the dispersion of too many spores, then it is necessary to burn this waste and disinfect the cutting material. Then treat with natural plant-based products such as a horsetail or garlic decoction, to which sulfur or copper (Bordeaux mixture) can be added, depending on the case.
The main plants affected by gray mold
More than two hundred plants are affected by this fungus. It is particularly feared in the vegetable patch, especially for strawberries, tomatoes, beans, melons. In orchards, it is feared especially for its attacks on apples, pears, peaches ... In viticulture, botrytis, in its "gray rot" form, can considerably decrease the quantity and quality of grapes. Many flowering plants such as chrysanthemums, roses, dahlias and many others are also susceptible to this disease. By C. Schutz Croué