Apple scab (Venturia inaequalis) is a problem wherever apples are grown; it's more serious, however, in temperate areas that experience cool, wet weather in spring and early summer. The fungus affects cultivated apples and ornamental crab apples (Malus spp.), as well as hawthorns (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp), and loquat (Eriobotrya spp.). The principle source of infection is the fruiting bodies that overwinter on fallen leaves and fruit. When the litter is moistened by rain or heavy dew in late winter, the fungus begins to grow and eventually discharges windborne spores, which land on emerging leaves and flowers. The spores are capable of traveling 300 to 600 feet from their source.
Depending on temperature and the duration of leaf wetness, it may take 9 to 17 days before the first apple-scab lesions appear on the leaves and fruit. Spores are discharged over a five- to nine-week period, with the peak infection period occurring between the pink to petal-fall stages of apple blossoming. Infected leaves and fruits become sources for secondary infections that can occur throughout the summer and early fall. Mature leaves, however, are less susceptible to infection than young ones.
SYMPTOMS: Velvety brown to olive spots develop on the leaves as they emerge in the spring, and turn greenish black to black. Severely infected leaves are shed, sometimes leaving the plant defoliated. Infected fruits have lesions that eventually turn brown and corky. Often the lesions occur at the blossom end of the fruit; later in the season, they occur anywhere on the fruit surface, resulting in deformed, cracked fruit that stores poorly. Scab will not kill apple trees, but defoliation, especially in midsummer, can affect next year's flowering because of the lack of flower-bud formation on trees.
CONTROL: Reduce future infections by collecting and disposing of fallen leaves. Pruning to improve air movement and sunlight penetration will help to dry off the leaves quickly. Chemical fungicides for apple scab include chlorothalonil (3336 WP), Captan, Ferbam, myclobutanil (Spectracide Immunox), lime-sulfur (calcium polysulfide), and sulfur. The most important time for application is from tight cluster to three weeks after petal fall. General-purpose orchard spray mixtures contain these fungicides; however, better protection can be achieved when you apply these fungicides separately.
There are more than 50 apple cultivars that are genetically resistant to apple scab. These include ‘Enterprise‘. ‘Goldrush’, ‘Liberty‘, ‘Jonafree’, ‘Prima‘, ‘Priscilla’, ‘Sir Prize‘, and ‘William's Pride’. The newly released Rezista series of apples also offer broad disease resistance. They include Rezista Early Jonathan (‘Resi‘), Rezista Gold Granny (‘Goldstar’), Rezista Gala (‘Releika’), and Rezista Rome (‘Rajka’) from Vanguard Nursery (800–386–5600; www.forfruittrees.com). In addition, there are many scab-resistant ornamental crab apples, from‘Donald Wyman’ to ‘Sugar Tyme’. For more information about scab-resistant crab apples, visit the International Ornamental Crabapple Society Web site (www.malus.net).