Black rot and frogeye leaf spot are phases of a widespread and
damaging disease of apple and crabapple. The fruit rot phase is
called black rot and on the leaf it is called frogeye leaf spot. The
disease can result in losses from
- a rotting of fruit before harvest and in storage.
- a weakening of the tree from defoliation,
and
- a blighting and dieback of twigs and limbs caused by girdling
cankers. The premature dropping of infected leaves can result in
small, poor-quality fruit and reduces crop yield the following year.
All apple varieties appear to be equally susceptible to fruit rot.
Jonathan and Winesap appear to have the greatest susceptibility to
leaf infection.
Symptoms
Fruit
The disease usually starts at the calyx end of the
fruit. The fungus usually enters the fruit through wounds caused by
insects, hail, growth cracks, or an open calyx tube. At first, a
light brown spot forms on the fruit. Usually only one spot occurs per
fruit. With time, the spots enlarge and commonly develop a series of
brown and black concentric bands or rings. The rotted fruit finally
turns black. The decayed tissue remains firm to leathery, and holds
its original shape until the entire fruit is rotted. The completely
decayed fruit finally dries and shrivels into a wrinkled black
"mummy" which may remain on the tree a year or longer. Black,
pimple-like fruiting bodies (pycnidia) of the causal fungus appear on
the surface of rotted fruit. In cold storage, the flesh of black
rot-infected fruits remains firm, in contrast to several other apple
rots.
Leaves
Starting at petal-fall or somewhat later, small, purple specks
appear on infected leaves. These specks enlarge to form spots l/8 to
l/4 inch in diameter. The round to irregularly lobed spots develop a
light brown-to-gray center surrounded by one or more dark-brown
concentric rings and a purple margin giving it a "frogeye"
appearance. Black pycnidia, like those that appear on rotted fruit,
may develop on the upper surface in the centers of the older leaf
spots. These pycnidia help to distinguish frogeye leaf spots from
similar spots caused by spray injury.
Twigs, Limbs, and Trunks
Small, slightly sunken, reddish-brown areas
develop in the bark. These areas slowly enlarge and darken to form
cankers. Cankers may continue to expand a little each year, and may
extend down the limb for 3 feet or more. These areas remain somewhat
sunken, except for the slightly raised and lobed margin. Cankers may
appear as a superficial roughening of the bark; or the bark may be
killed and conspicuously cracked, especially at the margins. In
recently killed areas, the bark is firmly attached to the wood; but
after a year or so, it cracks and falls away and can be easily
removed from the wood. Black pimple-like pycnidia and another very
similar fungal fruiting structure (perithecium) are usually abundant
in older cankers.
Causal Organism and Disease Cycle
Black rot and frogeye leaf spot are caused by the fungus, Physalospora
obtusa. The fungus overwinters in cankers, mummified fruits, and the
bark of dead wood. In the spring, the black fungal fruiting bodies
(pycnidia and perithecia) release conidia and ascospores,
respectively. These two types of spores spread the disease to healthy
leaves, fruit, and wood. The heaviest discharge of spores occurs
around blossom time, but the production of conidia may continue
during wet periods throughout the summer. The conidia can remain
viable for at least one year. Leaf infection usually occurs during
the petal-fall period. Conidia become attached to the leaf and may
germinate in a film of moisture within 5 or 6 hours. After
germination, the fungus penetrates the leaf through natural openings
in the under surface or through insect, hail or other wounds. Spore
germination and infection are most rapid at 75 to 80 degrees F. Fruit
infection can occur as early as petal fall; however, symptoms are
usually not visible until mid to late-summer as the apple approaches
maturity.
Control
Control of black 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," and
backyard growers are referred to Bulletin 780 "Controlling Diseases
and Insects in Home Fruit Plantings." Homeowners are encouraged to
emphasize use of the previously described cultural practices in order
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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