Apple Scab

Superficial, velvety dark-olive-to-black spots on fruits and leaves are the initial symptoms of apple scab. These fruit spots become scablike with age and tissues may become misshapen.

The fungus survives in dead leaves on the ground. Primary spores are discharged during spring rains and infect young leaves and fruits during prolonged moisture. Secondary spores may spread disease from established infections on trees. A lack of spring rains in most years reduces its importance in California in the southern Central Valley, but as a rule, this disease requires yearly treatments in coastal and foothill districts. Ornamental crab apples are also hosts. As plant parts mature and the weather gets warmer, susceptibility to this disease decreases, but pinpoint scab can occur during extended periods of moisture during summer.

The main objective in scab management is the reduction or prevention of primary infections in spring. Extensive primary infections result in poor fruit set and make scab control during the season more difficult. If primary infections are successfully controlled, secondary infections will not be serious. The key to success in scab control is exact timing and full coverage. Wet periods, temperature, and relative humidity are important factors. Because scab control often is part of a combination treatment aimed at other diseases and insect control, choice of materials and timing are also extremely important.

A fall foliar fertilizer application of zinc sulfate and urea hastens leaf fall and speeds decomposition of fallen leaves. This in turn reduces the level of overwintering apple scab inoculum.

Treatments with lime and sulfur, sulfur alone, or with approved fixed copper products are organically acceptable.

Apple scab outbreaks can be predicted based on temperature and moisture conditions. The table below, derived from research by Mills and La Plante, gives hours needed at various temperatures under constantly wet conditions for primary spores (ascospores) to cause infection in spring. This system for forecasting scab and timing sprays has been validated for apple-growing regions in the northern areas of California, but not for the central and southern San Joaquin Valley where temperatures tend to be higher in spring than those in the table.

How to use the table: Figure the average temperature for the rain period by adding the maximum and the minimum temperatures and dividing by 2. If wet periods are intermittent, total their durations until there is a period of at least 6 hours of continuous dryness. You will need a wetness recorder to do this efficiently. If the dry period is sunny, and drying is quick and thorough, it is assumed that 6 hours after the trees have dried, the danger is passed. If drying is slow, and humidity remains high, then the 6-hour dry period is extended by a safety margin of 3 to 4 hours.

Treatment Decisions

Unless wetness periods are being monitored as outlined in the section above, apply protective fungicides at regular intervals beginning with green tip. Be sure to monitor wetness periods throughout the spring to ensure that trees are adequately protected.