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Powdery mildew attacks leaves, blossoms, fruits, buds, shoots and twigs. Primary mildew occurs in spring on flower trusses and shoots, emerging from infected flowers or vegetative buds. These organs are completely covered with a floury layer of fungal mycelium and conidia. The phenological development of infected flower buds is retarded. They are significantly reduced in size, flower stalks are stunted, and petals are fleshy, distorted and likely to be greenish-white. Diseased blossoms fail to set fruit.
Shoots emerging from infected buds remain short. The leaves are narrower than normal and folded longitudinally in the form of a gutter. During the season the basal leaves wilt, becoming stiff and brittle. These primary infections on leaves and shoots often turn brown, and the leaves abscise by mid-summer. Only a brush of very small leaves may remain at the apex. Several shoots may emerge from one infected terminal bud, giving raise to a baleen structure.
Secondary infections appear on leaves as small grey to white felt-like patches of fungal mycelium and spores, frequently on the underside, most commonly near the midrib. This often results in leaf curling and crinkling. The lesions appear on the upper surface as chlorotic spots. Leaf symptoms either remain localized or may develop further. The leaves on the top of the shoots roll upwards and rise up. The mycelium grows along the midrib to the petiole and the shoot, and from there to the terminal bud. Infected terminals may be silvery-grey, misshapen and more susceptible to winter frost.
Infection of the fruit is most common on severely infected trees; affected fruits become russeted. On apples the surface is covered with a network pattern of fine lines. On pear, white mycelium remains visible on the fruits for a few weeks, then sloughs off giving rise to a russeted area, which expands as the fruit enlarges (Hickey and Yoder, 1997).
The management of powdery mildew on apple and pear is based on the importance of primary inoculum, cultivar susceptibility, weather conditions, other diseases and the anti-resistance strategy for the available fungicides. Mildew control is very important on young trees. In nurseries, it may provoke severe stunting of the vegetative terminal growth.
Although commercial apple varieties differ in the degree of susceptibility to powdery mildew, they will all become infected if large amounts of inoculum are present or if weather conditions are favourable for the spread of the disease (Aldwinckle, 1974). Highly susceptible varieties include: Jonathan, Idared, Baldwin, Cortland, Rome Beauty, Monroe, Gravenstein Holly, Stayman Winesap, Granny Smith, Paulared, Prime Gold, Britemac and Ginger Gold. Moderate susceptibility is observed on: Jonagold, Golden delicious, Cox's Orange Pippin, Gold Rush, McIntosh, Mutsu and Summerred. The newer varieties like Braeburn, Fuji and Gala are classified either as very susceptible (Grove, 1999) or tolerant (Hickey et al., 1997).
Only a few apple cultivars with Vf resistance to scab (Venturia inaequalis) also carry resistance towards powdery mildew (Goerre et al., 1999). The identification of the first molecular markers for mildew resistance (Pl1 from Malus robusta and Pl2 from Malus zumi) offered breeders the opportunity to select seedlings that carry more than one resistance gene. Ariwa is an example of a resistant apple cultivar that carries both Vf and Pl1 genes. However, physiological races have been isolated from scab and powdery mildew that are able to overcome these resistance genes. Other breeding strategies are, therefore, needed in order to develop more durable resistance for cultures with rotations of generally more than 15 years.
Pear cultivars are less susceptible to powdery mildew. The economic damage concerns skin russeting of the fruits (Spotts, 1984). It may be necessary to spray cultivars like Doyenne du Comice, d'Anjou and Louise Bonne. Pear orchards situated more than 200 m from an infested apple orchard must not be treated against powdery mildew because overwintering inoculum is not observed on pears.
Pruning infected shoots in winter and again in the following spring removes visible primary infections on emerging buds and improves the efficacy of chemical control of secondary infections later in the season. Although this practice is labour intensive and may interfere with crop production and training the tree structure, it is necessary as part of a durable control strategy.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Economic losses from powdery mildew vary with climatic conditions, cultivar susceptibility and cultural practices in orchards and nurseries. Leaf attack reduces the photosynthetic area and leads to smaller fruit and consequent loss in yield. Yoder and Hickey (1983) harvested 26 kg fruit/tree from untreated plots and 95 kg/tree from plots where mildew was controlled. Infected trees have an increased water requirement (Müller, 1957). Heavily mildewed trees are weakened and are more susceptible to other pests and winter injury (Ellis, 1994). The most critical loss in commercial value is through skin russeting of the fruits. Powdery mildew can be particularly severe in nurseries. Good control at this stage is needed to prevent control failures later in the commercial orchard.