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Summer Fruit Rots
White Rot (left) and Bitter Rot (right)
Several different rots can occur on apple fruits, especially as they approach maturity. The following are the most common and important "summer rots" which occur in Kentucky.
SYMPTOMS: Infections are usually not apparent until fruit begin to ripen. The disease may first be noticed as a brown to black spot at the blossom end of the fruit, or around a worm hole or some other wound. Usually, only ONE spot appears per fruit. This is a characteristic which helps distinguish black rot from some other fruit rots.
As the infected area enlarges, a series of brown or black concentric rings sometimes develops, producing an alternating "bulls-eye" or target-shaped pattern. Small black dots will frequently be seen within older rotted areas. The infected tissue remains firm as the rotted area expands throughout the entire fruit. At this point the whole fruit typically turns black. Such fruit will eventually shrivel and harden into mummies which remain attached to the tree and become a source of spores for future infection.
DISEASE CYCLE: Same as "Frogeye Leaf Spot", discussed previously.
SCOUTING: For each tree observed record the number of fruits with black rot per 100 examined (20 fruit per 5 limbs). Note the presence of mummified fruit in the tree in the comments section of the form. This disease is more likely to occur in old and in poorly pruned trees that have poor air and light penetration than in young trees.
SYMPTOMS: One to several small brown circular spots may first appear anytime after fruit are half grown. These spots expand rapidly in warm weather, becoming dark brown or black and somewhat sunken in the center (saucer-shaped). After the spot has enlarged to about 3/4" in diameter, a number of slightly raised dark "cushions" appear near the centers. As the spots continue to enlarge, these cushions frequently radiate outward in rings to give target-shaped appearance. In warm, humid weather, masses of slimy salmon-pink spores may be seen oozing from the cushions. If warm, moist weather prevails, several spots may expand and fuse together to rot the entire fruit.
DISEASE CYCLE: The bitter rot fungus persists between crops in partially mummified fruit on or beneath the tree, or in dead wood. Initial infection can occur anytime that warm (about 700 F or higher), rainy weather occurs. The salmon-pink spores are then produced as the fruits approach maturity and the infected regions expand. These spores are subsequently splashed onto healthy fruits by raindrops, and can cause a rapid secondary spread of disease under warm, humid conditions. Pruning, tree shape and weed control will effect the degree of severity.
SCOUTING: Record the number of fruits with bitter rot per 100 examined from each tree. This disease can be distinguished from black rot by the slimy salmon-pink spores oozing from cushions (use a hand lens to see), as well as the appearance of saucer-shaped depression towards the center of each spot. The presence of more than one spot per fruit, and their lack of consistent association with the blossom end or wounds are other characteristics which may distinguish bitter rot lesions from those of black rot. In addition, bitter rotted tissue can be cleanly separated from healthy tissue using a knife to give the appearance of an inward cone. This is not possible in fruit with black rot.
White Rot (Bot Rot)
White rot has the potential to cause serious losses especially during hot summers. Like most of the fruit rot diseases, the fungus is capable of causing limb or branch cankers, thus providing a ready source of spores to initiate fruit infections.
SYMPTOMS: White rot lesions begin on fruit as small, circular tan spots that are sometimes surrounded by a red halo. As the rot expands, it extends in a cylindrical shape toward and surrounding the core. Eventually, the entire fruit becomes soft, watery, and light tan, or darker under cool weather conditions. Branch infections start out as reddish brown bark lesions that expand and sometimes exude fluid.
DISEASE CYCLE: Like other fruit rots, the fungus overwinters in infected branches and limbs. The fungus produces new spores which are disseminated and capable of causing new infections. Twigs and branches recently killed by fire blight or having injured bark are especially susceptible to infection, and they, in turn become sources for new infections. Fruit infections occur under warm, humid weather conditions at any time, but decay does not begin until fruits are nearly ripe. Trees growing under stressful conditions such as drought are more likely to develop cankers.
SCOUTING: Record the number of fruits with white rot per 100 fruit examined from each sample tree.
White or "Bot" Rot
Apple IPM web site created by Kerry Kirk - 2001 - Maintained by Pat Dillon