Reports on Plant Diseases

Botryosphaeria canker and fruit rot, commonly called white rot or "bot rot", is caused by the fungus Botryosphaeria dothidea, formerly known as B. ribis (imperfect stage, Dothiorella and Macrophoma). This fungus causes one of the most widespread and important summer diseases of apples in the Midwest. The damage from dead limbs and twigs can be so extensive that some orchards have had to be removed. The disease is most severe in trees weakened by drought, winter injury, sunscald, poor pruning, low or unbalanced nutrition, fire blight, and black rot. Bot rot can be sporadic in appearance, being serious one season and difficult to find in succeeding years.

The Botryosphaeria fungus attacks a wide range of woody plants, including blackberry, blueberry, currant, gooseberry, grape, pear, quince, all stone fruit trees, catalpa, dogwood, elm, flowering currant, flowering quince, forsythia, hickory, horsechestnut, linden, London plane tree, maple, mountain ash, pecan, persimmon, pyracantha, redbud, rhododendron, rose, sumac, sweetgum, tree of heaven, tulip tree, tupelo, willow, and yellowwood.

Duchess, Golden Delicious, Grimes Golden, Gallia Beauty, Rome, and Yellow Transparent apple trees are all very susceptible to bot rot. Jonathan and Red Delicious are less affected by this disease than are other varieties.

Figure 1. Young botryosphaeria cankers on an apple twig. Note the stain on the bark where a watery blister has ruptured.

Figure 2. Botryosphaeria canker on a limb. Note the rings of black, spore-producing bodies (pycnidia and perithecia).

Figure 3. Young botryosphaeria rot infections around the lenticels.

Symptoms

Twigs and Limbs. Small circular spots or "blisters" usually appear on the twigs in late June or July (Figure 1). The spots enlarge, become somewhat sunken, and are filled with a watery fluid. The watery blisters rupture, allowing the sticky liquid to ooze over the bark. The fungus usually grows rapidly through the wet tissues to form somewhat sunken, girdling, dark-colored lesions–ones that extend to the cambium on very susceptible apple varieties. Under favorable conditions, several cankers may fuse to girdle and kill large limbs. On older cankers, the papery outer bark sloughs off with the underlying tissue, often appearing slimy.

The twig and limb cankers stop growing in the fall and may split along the edges. Rings of minute, black, spore-producing bodies (pycnidia and perithecia) are formed on the surface of the lesions or under the papery outer bark (Figure 2). The following spring, the canker may be corked off and become inactive, or it may resume growth and produce spores (conidia) periodically throughout the summer. The conidia serve as the principal source of infection for twigs, limbs, and fruit.

Fruit. Infections start as small, reddish brown spots around the lenticels (Figure 3). The enlarging, circular spots become slightly depressed; on yellow-skinned varieties, these spots may be bordered by one or more red "halo" rings (Figure 4). Spots on red-skinned apples often become bleached, giving rise to the common name of "white rot." Infected flesh under the spots is soft and egg-shaped, with the long axis parallel to that of the apple core. Several spots commonly merge to involve all or much of the fruit. As the rot progresses, the skin color becomes dark brown and superficially resembles black rot –except that with black rot the decayed tissue is firm, instead of soft and mushy.

Syrupy beads of exudate appear on the surface of fruits completely rotted by the Botryosphaeria fungus. Small, black fruiting bodies (pycnidia) that are filled with conidia may develop on the surface of rotted fruits during warm (above 75 F or 24 C), moist conditions. Mature fruits are the most susceptible.

Fruit rot may not appear in the orchard. Apples often become infected from mid-summer on without showing external symptoms. The internal rot starts to develop again after the fruit is removed from cold storage (about 33 F or 1 C). Such apples deteriorate rapidly when left at room temperature for several days.

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Disease Cycle

The Botryosphaeria fungus survives the winter as black pycnidia and perithecia in wart-like stromata on living and dead cankered limbs and in rotted, or "mummified" apple fruit. The fungus is commonly found on fire blighted twigs or cankers. Wounds or breaks in the cuticle are necessary for infection. The perithecia forcibly discharge ascospores during spring rains, while the conidia produced within the pycnidia ooze out in tremendous numbers and are washed and rain-splashed to other parts of the tree. The spread of the conidia may continue throughout the summer. Apple fruit may become infected fairly early in the season, but the rotting does not develop much until the fruit is almost mature. At temperatures above 75 F or 24 C, mature fruit may rot completely within a few days after infection.

Botryosphaeria canker and fruit rot infection is favored by any condition that reduces tree vigor–drought, high summer temperatures, winter injury, sunscald, poor pruning practices, low or unbalanced nutrition–and by such diseases as fire blight and black rot. When the moisture in the soil is adequate and the trees are vigorous, the cankers on twigs and limbs may be sloughed off, exposing healthy tissue underneath.