White rot of apple fruits is also referred to as "Bot Rot" or
Botryosphaeria rot. The fungus that causes fruit rot can also cause a
canker on limbs and other above-ground woody portions of the tree.
The canker phase of the disease is most severe in trees weakened by
drought, winter injury, sunscald, poor pruning, low or unbalanced
nutrition, and other plant diseases. The fruit rot phase can be
sporadic in appearance, being serious one season and difficult to
find in the following season. The Botryosphaeria fungus attacks a
wide range of woody plants that are common in Ohio. Dutchess, Golden
Delicious, Grimes Golden, Gallia Beauty, Rome, and Yellow Transparent
apple varieties are all very susceptible to fruit rot. Jonathan and
Red Delicious are less likely to be affected than other varieties.
Symptoms
Fruit
At first, small, reddish-brown spots appear around the
lenticels. The spots enlarge and become slightly depressed. On
yellow-skinned varieties, these spots may be bordered by one or more
red "halo" rings. Spots on red-skinned apples often become bleached.
The tissue under the spots is soft and egg shaped, with the long axis
parallel to that of the core. Several spots may 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.
Beads of exudate appear on the surface of fruits completely rotted by
white rot. Small, black fruiting bodies (pycnidia) that are filled
with spores (conidia) develop on the surface of rotted fruits during
warm, moist conditions. Mature fruits are most susceptible to the
disease. Apple fruits often become infected from mid-summer on
without showing external symptoms. Thus, fruit may be infected
without symptoms appearing in the orchard. Disease development is
checked when these apples are placed in cold storage. However, when
they are removed from cold storage an internal rot develops and the
fruit may deteriorate very rapidly at room temperature.
Twigs and Limbs
Small, circular spots or "blisters" appear on the
twigs in June and July. The spots enlarge, become somewhat sunken,
and fill with a watery fluid. The fungus may grow rapidly through the
tissues to form slightly sunken, dark-colored cankers that may 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 outer bark becomes tan to orange and papery,
and the margins of the canker crack and fissure. The outer bark
sloughs off and the underlying tissue appears slimy. In the fall,
twig and limb cankers stop growing and may split along the edges.
Rings of small, black, spore-producing bodies (pycnidia and
perithecia) are formed on the surface of the cankers or under the
papery outer bark. The following spring, a canker may resume growth
or be corked off and become inactive.
Casual Organism and Disease Cycle
Botryosphaeria canker and fruit rot
(white rot) is caused by the fungus Botryosphaeria dothidea. The
fungus overwinters as black pycnidia and perithecia in a wart-like
stroma on living and dead cankered limbs and in rotted fruits. The
fungus is also commonly found on fire-blighted twigs or cankers.
Wounds or breaks in the epidermis are necessary for the fungus to
penetrate. Spores (ascospores) are forcibly discharged from
perithecia during spring rains. Another type of spore (conidia) is
produced within pycnidia and ooze out in tremendous numbers. They are
then washed and rain-splashed to other parts of the tree throughout
the summer. Apple fruits may become infected fairly early in the
season, but rotting does not develop much until the fruit is almost
mature. At temperatures above 75 degrees F (24 degrees C), mature
fruit may rot completely within a few days after infection. The
development of Botryosphaeria canker and fruit rot is favored by any
condition that reduces tree vigor.
Control
Control of white rot is best achieved through an integrated
program of cultural practices and chemical control measures.
- Sanitation is critical for effective control. Piles of prunings are
an important source of inoculum and should be removed from the
perimeter of the orchard or burned. Prunings can be left on the
orchard floor if they are chopped with a flail mower, which removes
much of the bark and allows them to decompose faster. Removal of
mummified apples and pruning out dead wood in the tree are important
for reducing the inoculum within the tree. Pruning out current-season
shoots infected with fire blight is also important, because they can
be colonized and serve as an inoculum source during the same growing
season.
- Any practice that helps to maintain trees in a healthy,
vigorous condition is critical for controlling the canker phase of
the disease. Cankers generally develop only on stressed or weakened
trees. Prune trees annually and maintain a balanced fertility program
based on soil and foliar nutrient analysis. Cankers generally develop
rapidly on winter-injured trees.
- The use of fungicides combined
with good sanitation is beneficial for controlling the fruit rot
phase of the disease. Fungicides are not effective for controlling
the canker phase of the disease on weakened trees. For the most
current fungicide recommendations, commercial growers are referred to
Bulletin 506-A2, "Ohio Commercial Tree Fruit Spray Guide;" backyard
growers are referred to Bulletin 780, "Controlling Diseases and
Insects in Home Fruit Plantings." Homeowners are encouraged to
practice the previously described cultural recommendations to reduce
or eliminate the need for fungicide use. These bulletins can be
obtained from your county Extension agent or The Extension
Publications Office; The Ohio State University; 385 Kottman Hall;
2021 Coffey Road; Columbus, OH 43210-1044.
-----------------
All educational programs conducted by Ohio State University Extension
are available to clientele on a nondiscriminatory basis without regard
to race, color, creed, religion, sexual orientation, national origin,
gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and
Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868
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