Fungal diseases of fruit trees often show the same symptoms as bacterial disease and the effects of stress caused by cold and drought. There are measures you can take to limit the possibility of infection. Some fungal diseases can be treated by fungicides; others cannot. What fungal diseases infect fruit trees depend on the particular fruit, the growing zone and the weather.
Causes and Types
Fungal infections of fruit trees are most often caused by high humidity, rain, cool weather or moist, soggy soil. Water-borne fungi can attack the roots and where the trunk meets the soil. Airborne fungi can enter a tree through cuts and other damage. Fungi can infect roots, bark, twigs, blossoms or cause scabs, spots and rots on fruit.
Fungal infections can cause cankers to grow on the bark of the trunk or limbs. They can cause powdery growth and spots on leaves, causing them to turn yellow, curl and die. They can cause scabs or rotten spots on fruit.
Brown rot, caused by the fungus Monilinia fructicola, affects all stone fruits. The rot appears as light brown spots on green fruits. In wet weather, brown, felt-like masses of fungus spores can cover ripe fruit.
Black knot, Dibotyron morbosum, attacks plum and sours cherry trees causing black swellings, or knots, up to a foot long to develop on smaller branches. These knots can kill branches.
Cherry leaf spot, Coccomyces hiemalis, causes leaves to turn yellow and fall prematurely. Cherry leaf spot is carried from year to year by fallen leaves.
Verticillum wilt, Verticillum dahlia, invades the root systems of all stone fruits. It causes leaves to wilt and drop and discolors the wood under the bark.
Powdery mildew, the airborne fungus Podosphaera oxycanthe, causes a powdery white growth on leaves of trees, distorting them and causing them to curl upward. Black spots develop on the leaves, and the fungus distorts and stunts shoots.
Scab, caused by the fungus Cladosporium carpophilum, is sometimes called black spot or freckles. It causes scabs on apricots, cherries, peaches and plums.
Leaf curl, caused by the fungus Taphrina deformans, causes leaves to puff and fold, turning inward. They turn yellowish-red to brown, eventually withering and falling during hot, dry weather.
Perennial canker fungi Valsa cincta and V. leucostoma can infect apricots, cherries and plumbs, forming oval cankers that circle and kill branches and trunks.
Many fungi spores, especially those that infect branches and blossoms, are spread by wind and rain splash in the spring. Fungi that infect roots and work their way up the trunk of the tree are carried by water in the soil.
Find out which fungal diseases are most likely to infect your particular fruit and learn their symptoms and preventative measures. Use botanical names where possible; popular names are often confusing and one name is often used to describe different fungi.
The best thing to do to prevent the growth of soil-borne fungi is to make sure the soil is kept well-drained.
Do not plant trees next to trees with cankers or older orchards.
Pruning can help prevent airborne fungi. Prune infected branches and burn them. Thin crowded tree interiors. The sun and circulating air both help dry trees and their fruit. Delay pruning until trees begin growing in the spring. Do not leave long pruning stubs. Clean pruning shears and saws with alcohol.
Not all fungi can be treated by fungicides. The Bordeaux mixture, a blend of copper sulfate and lime, is effective on many. Benomyl is recommended for brown rot; it will also kill powdery mildew and numerous other fungi. Captan is a useful fungicide, but it is a carcinogen and should be used with care.