This fungus belongs to the Ascomycota group of fungi.
Damage. Powdery mildews should not be confused with downy mildews. Powdery mildew is distinguished by its dry powdery appearance, most commonly found on the upper surface of the leaf, and by its preference for hot, dry weather conditions (see examples of powdery mildew in Figure 15.7).
Life cycle and spread. The disease survives the winter as mycelium within the buds which often appear small and shrivelled on twigs which often have a dried, silvery appearance. The emergence of the mycelium with the germinating buds in spring results in a white bloom over the young leaves (primary mildew). As the spring progresses, asexual spores produced in chains on the upper surface of the leaf are spread as individual spores by wind and cause the destructive secondary mildew. This stage of the fungal life cycle involves an external mycelium covering the leaf surface, but not entering into the internal leaf tissues other than to sink small peg-like structures (which suck out the leaf's moisture and may cause premature leaf drop). Flowering normally occurs before the secondary infection stage, but infection in young apple fruit may produce a rough skin (russeting). This organism may affect other species of fruit such as pears, quinces, medlars and ornamental Malus.
Powdery mildews in autumn may produce sexual, dark-coloured spore cases (cleistothecia) on the leaf, about 1 mm in size. (Although not important in most horticultural crops as an overwintering stage, they may assume a vital role in powdery mildew of cereals, the sexual spores having the potential to form new strains resistant to fungicides, and new strains capable of overcoming the plant's genetic resistance.)
Control is achieved by two main 'methods'. Pruning of silvered shoots can eliminate a good proportion of the primary mildew. The amateur and professional horticulturist can use a systemic fungicide ingredient such as myclobutanil for spring and summer sprays, whilst the professional also has a protective fungicide option such as dinocap.
Other species of powdery mildew commonly occur in horticulture, e.g. Sphaerotheca pannosa on rose (Figure 15.7) and S. fuliginea on cucumber. Cross-infection between these crops does not occur.
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