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1. Black Rot, Botryosphaeria obtusa
I.
Introduction: Black rot is a fungal disease that can cause serious
losses in apple orchards, especially in warm, humid areas. Three forms
of the disease can occur: a fruit rot, a leaf spot known as frogeye leaf
spot, and a limb canker. Infected fruit become unmarketable, severe leaf
spotting can result in defoliation which weakens the tree, and limb
cankers can girdle and eventually kill entire branches.
II.
Symptoms: Leaf symptoms first occur early in the spring when the leaves
are unfolding. They appear as small, purple specks on the upper surface
of the leaves that enlarge into circular lesions 1/8 to 1/4 inch (3-6 mm)
in diameter. The margin of the lesions remains purple, while the center
turns tan to brown. In a few weeks, secondary enlargement of these leaf
spots occurs. At this time, the lesions assume a characteristic "frog-eye"
appearance (photo 2-30). As they age, a series of concentric rings
develops around the original infection point. Occasionally, small black
pycnidia (asexual fungus fruiting body) can be found in the center of
the lesion. Heavily infected leaves become chlorotic and defoliation
occurs. If defoliation occurs on a yearly basis, tree vigor is greatly
reduced and the tree becomes stressed. Fruit
infection, of which sepal infection is the most common form, can occur
early in the season. These infections result in blossom-end rot later in
the season. Early fruit infection usually appears at the calyx end of
the fruit. These lesions begin as reddish spots which later turn purple
and are bordered by a red ring. Infected areas on mature fruit become
black, are irregular in shape, and are occasionally surrounded by a red
halo. As the rotted area enlarges, a series of concentric bands of
uniform width form which alternate in color from black to brown. The
flesh of the rotted area remains firm and leathery. Black pycnidia are
often seen on the surface of the infected fruit. Eventually, a dry mummy
is produced that may remain attached to the tree.
Lesions resulting in canker formation usually are associated with a
wound in the bark. In the early stages, the bark is slightly sunken and
reddish-brown in color. Some cankers remain small and may die out by the
end of the year, while others enlarge from year to year. Some cankers
are observed to be merely a superficial roughening of the bark (photo
2-31). In other cases, the canker can kill the bark to the wood and the
area becomes cracked (photo 2-32). By the end of the second year,
fruiting bodies of the fungus can be observed in the cankered area.
Limbs can be completely girdled by this time. The black rot fungus often
can be found on wood previously killed by fire blight or damaged by cold
temperatures.
III.
Disease Cycle: The fungus overwinters in cankers, especially in those
initiated by fire blight, in dead bark, and in mummified fruit. These
overwintering structures provide an important source of disease inoculum.
The black rot fungus covers a wide geographical range and can infect
many hosts other than apple. The role these hosts play in the spread and
development of the disease is not known. In the spring, spores are
released during rainfall. The amount and duration of rainfall, as well
as temperature, are the main factors influencing spore release,
germination, and infection. Conidia are primarily waterborne and
continue to be produced during wet periods throughout the summer.
Ascospores are primarily airborne and are most common during the petal
fall period. Ascospores and conidia germinate after four hours of
wetting over a temperature range of 61 to 90 F (16-32 C). Below
temperatures of 61 F (16 C), longer wetting periods are needed for
infection to occur. The optimum temperature for leaf infection is 80 F
(27 C). At this temperature, four and a half hours are necessary for
infection. Leaf infection will not occur, however, at 46 F (80 C) even
when leaves have been wet for 48 hours. For fruit infection to occur,
temperatures between 68 and 75 F (20-24 C) with at least a nine hour
wetting period are required. During rain, conidia ooze out by the
thousands and are disseminated by splashing rain, wind, and insects.
Spores attach themselves to the plant, germinate in a film of moisture
within five to six hours and penetrate the leaf surface through stomata.
Early season infection of fruit also occurs through stomata. Later in
the season, infection of fruit occurs through cracks in the cuticle or
via wounds and possibly lenticels. Often, harvest injuries may become
infected and the fruit may decay during or after storage. Throughout the
growing season, infections occur through wounds in the bark or on killed
wood.
IV.
Monitoring: Monitor each tree for cankers (photos 2-31, 2-32, 2-35).
Cankers are a source of inoculum which can initiate leaf, fruit, and
wood infections. Old fire blight cankers, winter-injured wood, and dead
prunings left in the tree often serve as sources of inoculum. Remove
cankered wood from the orchard or mulch the brush so that it decays over
the period of a year. Inspect trees for apple mummies and remove them
from the orchard if possible, since mummies remaining in the trees from
the previous season can also serve as a source of inoculum.
Observe 25 fruit and leaves on each sample tree from mid-season through
the preharvest period. Be observant for the presence of frogeye leaf
spots (photo 2-30) caused by the black rot fungus. Leaf spots may
indicate an inoculum source for fruit rot infections. The most
susceptible varieties to leaf spots are 'Rome', 'Jonathan', 'Stayman',
and 'McIntosh'. Sports of 'Golden Delicious' and 'Delicious' are not as
susceptible. If leaf spots are observed it may be possible to locate
mummies, dead wood, or cankers (photos 2-31, 2-32, 2-35) higher in the
tree; these should be removed if possible. Fruit is most commonly
infected (photo 2-36) at an injury, but infection can occur without the
fruit being injured. Although there are some differences in fruit
susceptibility among the varieties, all are susceptible. Record the
location of trees with an abundance of dead wood and cankers (photos
2-31, 2-32, 2-35) so these can be scheduled for intensive pruning during
the dormant period.
Management:
Removing dead wood, mummies, and cankers from the trees are important
cultural practices that may help reduce the incidence and severity of
the disease. Current-season prunings should be either removed from the
orchard and burned or raked and then chopped with a flail or rotary
mower. Piles of prunings on the orchard perimeter can serve as sources
of fungal spores. The main method of control is application of
fungicides from silver tip through harvest. Apple cultivars do not vary
greatly in their susceptibility to B. obtusa; however, Empire and
Cortland may be slightly more susceptible than others.
Text prepared by J.W. Travis, J.L. Rytter, and A.R. Biggs www.caf.wvu.edu
I.
Introduction: Black rot is a fungal disease that can cause serious
losses in apple orchards, especially in warm, humid areas. Three forms
of the disease can occur: a fruit rot, a leaf spot known as frogeye leaf
spot, and a limb canker. Infected fruit become unmarketable, severe leaf
spotting can result in defoliation which weakens the tree, and limb
cankers can girdle and eventually kill entire branches. 
II.
Symptoms: Leaf symptoms first occur early in the spring when the leaves
are unfolding. They appear as small, purple specks on the upper surface
of the leaves that enlarge into circular lesions 1/8 to 1/4 inch (3-6 mm)
in diameter. The margin of the lesions remains purple, while the center
turns tan to brown. In a few weeks, secondary enlargement of these leaf
spots occurs. At this time, the lesions assume a characteristic "frog-eye"
appearance (photo 2-30). As they age, a series of concentric rings
develops around the original infection point. Occasionally, small black
pycnidia (asexual fungus fruiting body) can be found in the center of
the lesion. Heavily infected leaves become chlorotic and defoliation
occurs. If defoliation occurs on a yearly basis, tree vigor is greatly
reduced and the tree becomes stressed.
Fruit
infection, of which sepal infection is the most common form, can occur
early in the season. These infections result in blossom-end rot later in
the season. Early fruit infection usually appears at the calyx end of
the fruit. These lesions begin as reddish spots which later turn purple
and are bordered by a red ring. Infected areas on mature fruit become
black, are irregular in shape, and are occasionally surrounded by a red
halo. As the rotted area enlarges, a series of concentric bands of
uniform width form which alternate in color from black to brown. The
flesh of the rotted area remains firm and leathery. Black pycnidia are
often seen on the surface of the infected fruit. Eventually, a dry mummy
is produced that may remain attached to the tree.
III.
Disease Cycle: The fungus overwinters in cankers, especially in those
initiated by fire blight, in dead bark, and in mummified fruit. These
overwintering structures provide an important source of disease inoculum.
The black rot fungus covers a wide geographical range and can infect
many hosts other than apple. The role these hosts play in the spread and
development of the disease is not known. In the spring, spores are
released during rainfall. The amount and duration of rainfall, as well
as temperature, are the main factors influencing spore release,
germination, and infection. Conidia are primarily waterborne and
continue to be produced during wet periods throughout the summer.
Ascospores are primarily airborne and are most common during the petal
fall period. Ascospores and conidia germinate after four hours of
wetting over a temperature range of 61 to 90 F (16-32 C). Below
temperatures of 61 F (16 C), longer wetting periods are needed for
infection to occur. The optimum temperature for leaf infection is 80 F
(27 C). At this temperature, four and a half hours are necessary for
infection. Leaf infection will not occur, however, at 46 F (80 C) even
when leaves have been wet for 48 hours. For fruit infection to occur,
temperatures between 68 and 75 F (20-24 C) with at least a nine hour
wetting period are required. During rain, conidia ooze out by the
thousands and are disseminated by splashing rain, wind, and insects.
Spores attach themselves to the plant, germinate in a film of moisture
within five to six hours and penetrate the leaf surface through stomata.
Early season infection of fruit also occurs through stomata. Later in
the season, infection of fruit occurs through cracks in the cuticle or
via wounds and possibly lenticels. Often, harvest injuries may become
infected and the fruit may decay during or after storage. Throughout the
growing season, infections occur through wounds in the bark or on killed
wood.
IV.
Monitoring: Monitor each tree for cankers (photos 2-31, 2-32, 2-35).
Cankers are a source of inoculum which can initiate leaf, fruit, and
wood infections. Old fire blight cankers, winter-injured wood, and dead
prunings left in the tree often serve as sources of inoculum. Remove
cankered wood from the orchard or mulch the brush so that it decays over
the period of a year. Inspect trees for apple mummies and remove them
from the orchard if possible, since mummies remaining in the trees from
the previous season can also serve as a source of inoculum.
Management:
Removing dead wood, mummies, and cankers from the trees are important
cultural practices that may help reduce the incidence and severity of
the disease. Current-season prunings should be either removed from the
orchard and burned or raked and then chopped with a flail or rotary
mower. Piles of prunings on the orchard perimeter can serve as sources
of fungal spores. The main method of control is application of
fungicides from silver tip through harvest. Apple cultivars do not vary
greatly in their susceptibility to B. obtusa; however, Empire and
Cortland may be slightly more susceptible than others.
