Major Diseases of Apple in Georgia
Black Rot (fungus Botryosphaeria obtusa)
Primary infection occurs before or during bloom, but rot symptoms do not become noticeable on the fruit until late summer. The rot first appears on the calyx end of the fruit as a firm, black, metallic-like spot with concentric rings. When dead wood is present in the tree, secondary black rot infections can occur as small black spots late in the summer. These spots have a reddish border and appear anywhere on the apple surface. The leaf spot phase of this disease is generally associated with dead wood which has been infested by the fungus. Frog-eye leaf spot can be confused with foliar chemical injury and isolation is necessary for a positive identification.
Fire Blight (bacterium Erwinia amylovora)
The bacterium responsible for fire blight usually attacks blooms first. Blossoms wilt and turn brown. Bees which are essential for pollination can spread the bacterium from bloom to bloom. As the disease progresses down into the woody tissue, water-soaked bark is noticeable. Infected branches die back and leaves turn brown. A bacterial ooze frequently occurs on infected branches and occasionally the ooze can be seen on immature fruit. A heavily infected tree looks as if it has been damaged by fire. Dead wood produced by fire blight provides areas for secondary fungus infection by other fruit rotting organisms.
Scab (fungus Venturia inaequalis)
On the leaf, scab causes an olive green spot somewhat darker than the leaf tissue. Spots later become dark, thicken and cause leaves to curl. On the fruit, grayish, scabby-like spots develop causing fruit to become disfigured and crack. Spots are always hard and never turn into a soft rot. Primary leaf and fruit infection occurs very early in the season as soon as the first green tissue is present.
Bitter Rot (fungus Glomerella cingulata)
The fungus responsible for bitter rot can infect the fruit anytime after petal fall. The disease is severest as fruit reaches maturity. Small, brown, circular areas develop anywhere on the fruit surface. Spots later become sunken, forming a saucer-shaped depression. Under wet weather conditions, pink fruiting bodies of the fungus develop in the center of the rotten area.
White Rot (fungus Botryosphaeria dothidea)
Two types of rot occur on the fruit. One type appears to rot the fruit from the inside outward. The entire apple becomes soft but retains its shape and takes on a light brown color. When dropped, the rotten fruit falls completely apart. The second type symptom of white rot is small, brown, circular spots on the fruit, similar to bitter rot. The spots are softer than black rot and more cupped-shaped than bitter rot. Infection occurs about six weeks before harvest or about mid-July.
Sooty Blotch (fungus Gloeodes pomigena)
Sooty blotch is a superficial fungal growth on the apple surface. Spots are a dull black with an indefinite outline resulting in a sooty appearance. The entire surface of the fruit may be covered by the fungus. Sooty blotch infection can occur anytime during fruit development but it is most noticeable during late season.
Fly Speck (fungus Leptothyrium pomi)
Fly speck consists of very small, black, superficial circular spots on the fruit surface. This disease downgrades the fruit because of appearance but does not otherwise affect fruit quality. This disease usually occurs near harvest when cover sprays are discontinued too soon
Cedar Apple Rust (fungus Gymnosporangium juniperi-virginianae)
Rust occurs on leaves, twigs and fruit. On the leaves, pale yellow spots develop on the upper leaf surface. The spots later enlarge and turn an orange color. Lesions on the fruit usually occur at the calyx end. The spots first appear to be light green but later turn a dark green to orange. Fruit becomes rough and misshapened on the calyx end. Rust infection occurs early in the season as soon as new growth starts in the spring.
