Identifying common deseases in your orchard

There are over two-dozen diseases that are associated with specific types of apple trees.
Listed below are the more common diseases you may find on your tree.

Apple Scab
Apple scab is a fungal disease that infects apple trees causing lesions on the leaves and fruit. It begins in early spring and often seen on the underside of the leaves as light brown areas. The brown areas turn dark brown and black as the cells die. It spreads to other parts of the tree as the sores can be found on leaves, blossoms, sepals, petioles, pedicels, shoots and bud scales. Young leaves begin to curl, twist, dwarf and become deformed.

The fungus that causes apple scab (V. inaequalis) over winters in the infected trees. Most apple cultivars are susceptible to scab and will require fungicidal sprays to prevent the disease.

Fire Blight
Fire blight is a disease caused by the (bacterium Erwinia amylovora) and is very difficult to control. The disease develops rapidly in early spring during rainy weather when temperatures are above 60 °F and the tree is in bloom. Blossoms and young leafy twigs show the first symptoms, appearing wilted or shriveled and turning brown to black. The tips of infected young twigs wilt and die, forming a shepherd's crook as the disease moves down the branch. Dead leaves often remain attached to the branch. During wet weather, a milky-like, sticky liquid containing bacteria can be seen on the stems and branches.

Prevention & Treatment: Remove all infection sources, such as blighted twigs and cankers, before growth starts in the spring. Pruning cuts should be made 12 to 18 inches below any sign of infected tissue. Disinfect all pruning tools between each cut, using a solution of 1 part household bleach to 9 parts water.

Powdery Mildew
Powdery mildew (Podosphaera leucotricha) is a disease that affects many apple trees in mild climates. It doesn't need wet or rainy conditions to proliferate. The fungus over winters in dormant blossoms. Longitudinally folded leaves identify this disease in the spring, as well as a gray-white powder coating on the twigs, resulting in stunted twig growth. If left untreated, it will result in blossoms dropping prematurely from the trees and overall stunted growth of the tree.

The disease can be treated by implementing a mildewcide program and by pruning away whitened terminal shoots on the trees.

Flyspeck & Sooty Blotch
These two diseases are caused by the fungi Schizothyrium pomi and Gloeodes pomigena, respectively, infect the surface of the fruit and are mainly cosmetic problems. They often occur together, even though they are each a distinctive disease. Although unsightly, the fruit is still edible. The sooty blotch will wipe off of the fruit and fly speck will not.

Flyspeck's name describes it well, since this disease looks like groups of very small superficial black dots on the surface of the fruit. The dots are slightly elevated and occur in groups of six to 50. Sooty blotch looks like a brown or black blotch (¼-inch in diameter) on the fruit. Spots may coalesce to cover the entire fruit. During the summer these diseases develop during cool rainy weather, particularly in dense, unpruned trees with poor air circulation.

Prevention & Treatment: Maintain good air circulation by pruning, to keep trees from becoming too dense. Thinning of fruit is also important. If disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apples: thiophanate-methyl, basic copper sulfate captan (must wait 14 days to harvest), ferbam, sulfur or lime sulfur. Apply all chemicals according to directions on the label.

 

Apple tree diseases can be avoided in many cases by selecting and planting healthy, disease free rootstock. Carefully monitoring trees in the spring can help gardeners identify many of the diseases so treatment can begin early. Finally, sanitation is of great importance in any orchard. Prompt removal of dead leaves and rotting fruit as well as a regular pruning program can help stave off many infectious diseases.