Apple scab is a malignant tree disease Malus , like an apple tree, caused by the ascomycete fungus Venturia inaequalis . This disease manifests as a dull black or gray brown lesion on the surface of leaves, buds or fruit. Lesions may also appear less frequently in wooden tissue. The fruit and the lower part of the leaves are very susceptible. This disease rarely kills its host, but it can significantly reduce fruit yield and fruit quality. The affected fruit is less valuable because of the presence of black fungal lesions.
Video Apple scab
Life cycle
The infection cycle begins in spring, when appropriate temperature and humidity encourage the release of V. inaequalis ascospores from leaf litter around the base of the previously infected tree. These spores rise into the air and land on the vulnerable tree surface, where they sprout and form a germ tube that can directly penetrate the wax cuticle of the plant. A mushroom mycelium is formed between the cuticle and the underlying epidermal tissue, beginning as a yellowing point that grows and ruptures to reveal asexually formed black lesions as the conidia are released and germinate in the fresh area of ​​the host tree, which in turn produces another generation of conidia spores. This secondary infection cycle continues throughout the summer, until the leaves and fruits fall from the trees at the beginning of winter.
During the winter, V. inaequalis undergoes sexual reproduction in the leaf litter around the base of the tree, producing a new generation of ascospores released the following spring. Scab lesions located in woody tissues can also withstand winter in place, but will not undergo a sexual reproduction cycle; this lesion can still produce infective conidial spores in the spring.
Maps Apple scab
Control
In affected gardens, new infections can be reduced by removing leaf litter and ornaments containing infected tissue from the garden and burning them. This will reduce the number of new ascospores released in the spring. In addition, scab lesions on wood tissue can be cut from the tree if possible and also destroyed.
Chemical controls may include various compounds. Benzimidazole fungicides, for example, Benlate (now banned in many countries because they contain harmful chemical benzene) work well but resistance can emerge quickly. A number of other chemical classes include sterol inhibitors such as Nova 40, and Sovran-like strobilurins are used extensively; However, some of these are gradually removed due to resistance problems.
Contact fungicides are not susceptible to resistance, such as Captan, is a viable option. Potassium bicarbonate is an effective fungicide against apple scab, as well as powdery mildew, and is allowed for use in organic farming. Copper and Bordeaux blends are traditional controls but less effective than chemical fungicides, and can cause russeting of fruits. Wet sulfur also provides some control. Application times and concentrations vary between compounds.
The apple prognostic model called RIMpro is developed by Marc Trapman, who numerically assesses the risk of infection and can serve as a warning system. This allows better targeted spraying. Parameters for calculation are wet leaf, amount of rain fall and temperature.
Fifteen genes have been found in apple cultivars that give resistance to apple scabies. Researchers hope to use cisgenic techniques to introduce these genes into commercial cultivars and therefore create new resistant cultivars. This can be done through conventional breeding but it will take more than 50 years to achieve it.
See also
- Sooty and sooty spots
References
Source of the article : Wikipedia
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