Caring for your fruit trees from common diseases in Spring

Spring is just around the corner, and your fruit trees are waking up from their winter’s rest. Help them get off to a good start to the year by monitoring for these common springtime diseases.

Scab

Apple Scab and Pear Scab are found in most regions, although it is more common where heavy or frequent spring rains occur. The two types of scab are not related, and cannot cross-infect tree species, however the symptoms and causes are similar. Scab is caused by fungus that lives in dead leaves and releases spores during spring rains. It can continue to infect trees during summer rains, but later infections are less common or severe. Initial symptoms are dark velvety spots on fruits and leaves. As the season progresses, the spots become scab-like and the leaves and fruits may become deformed. Prevent scab from spreading by removing fallen leaves and disposing of them (not by composting).

Outbreak forecasts may be available from your local agricultural extension office, which are calculated based on temperature and moisture conditions. If no forecast is available but rust is a problem in your area, the University of California Extension recommends you spray your trees preventatively with lime sulfur, micronized sulfur, or fixed copper beginning at the green tip stage. These fungicides may also be used to treat infected trees. Follow the frequency and application rates on the package label, and talk to your certifier if you are an organic grower.

Bacterial Blossom Blast

Bacterial Blossom Blast is caused by the bacterium Pseudomonas syringae. It can affect fruit trees and other garden plants, and is common in all regions. Infection only occurs with freezing temperatures during bloom. Infected fruit buds turn brown, stop growing, and may fall off. The fruiting wood may also die, however the infection rarely moves further. Dead fruit buds may resemble fire blight in apples and pears, however blast does not present other fire blight symptoms such as cankers, leaf damage, and shepherd hook branches. To learn more about fire blight, read the article Controlling Fire Blight Organically and watch our video. The best way to deal with this disease is with prevention. If the temperature is expected to drop below 32°F, cover young trees with frost blanket bags; bigger trees can be protected from freezing temperatures with Frost Shield. The University of California Extension recommends that infected trees be treated with streptomycin, however this product should be applied with care to prevent resistance; fixed copper can also be used. If you are an organic grower, check with your certifier before spraying. Treatments are made preventatively from green tip through bloom, any time a freeze is likely, where blast has been a problem in the past. Treatment after symptoms occur is ineffective.

Powdery Mildew

Powdery Mildew is a common problem in trees and many other types of plants. The disease is caused by a fungus which causes powdery growth on leaves and shoots, stunted growth of shoots and fruit, russetted fruit, and reduced harvest. This fungus over winters in the tree’s buds; infected buds are white and may be misshapen.

The exact species of fungus varies by the variety of tree infected; for correct identification, check out the University of California Extension website. The best course of treatment is to prune off infected buds during dormancy. Pruning for thinning is also helpful to reduce the infection by allowing more fungus-killing sunlight and air circulation to the branches and fruits.

There are many fungicidal sprays that can be used to treat powdery mildew as well, including lime sulfur, micronized sulfur, and horticultural oil. Typically these are applied at pink bud stage. If this does not result in sufficient control, additional treatments may be made until terminal shoot growth ends for the season. Treatments once terminal growth ceases are ineffective at controlling this fungus. For more details on this and other fruit tree pests and diseases, check out the University of California Integrated Pest Management Program website.

For more information on sun damage for plants, see our companion guide in the Resource Center.