Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Cherry Leaf Spot
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Cherry
leaf spot is one of the most serious diseases of both sweet and sour cherries
in the Midwest. The disease mainly affects the leaves, but lesions may also
appear on fruit, petioles, and fruit stems (pedicels). Diseased leaves drop
prematurely, and severely affected trees may be defoliated by mid-summer.
Early and repeated defoliation can result in: 1) dwarfed and unevenly ripened
fruit with poor taste; 2) devitalized trees that are more susceptible to
winter injury; 3) death of fruit spurs; 4) a reduction of fruit set and size;
5) small and weak fruit buds; 6) reduced fruit growth; and 7}eventual death
of the tree. Symptoms During
the latter part of May and the first half of June, small circular purple
spots appear on the upper surface of the leaf Spots gradually enlarge to about
¼ inch in diameter and turn reddish-brown. Lesions may emerge to produce
large, irregular spots. Whitish-pink masses of sticky spores (conidia) form
within the spots on the undersides of infected leaves during periods of damp
weather. After six to eight weeks, the centers of the spots may dry up and
drop out, giving a "shot-hole" appearance. The
"shot-hole" effect is more common on sour than on sweet cherries. The
most conspicuous symptom, especially on sour cherries, is the golden
yellowing of older infected leaves before they drop off. Although this
symptom does not occur every season, the spotting of infected leaves is
always visible. Spots
similar to those on the leaves may also form on leaf petioles and fruit pedicels,
causing fruit to ripen unevenly. Spots usually do not form on fruit. |
Cherry leaf spot symptoms on upper surface of cherry
leaf. |
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Causal Organism Cherry leaf spot is caused by the fungus, Blumeriella jaapii (previously
called Coccomyces hiemalis).
The fungus over winters in dead leaves on the ground. In early
spring (about petal fall), fungal fruiting bodies called apothecia
develop in these leaves. Spores (ascospores) are produced in the
apothecia and are forcibly discharged during rainy periods for about six
to eight weeks, starting at petal fall. The optimal temperatures for
ascosporic discharge are 61 degrees F (16 degrees C) and higher.
Very few ascospores are discharged at temperatures below 46 degrees
F (8 degrees C). These ascospores are spread by wind or splashing rain
drops to the green, healthy leaves and serve as primary inoculum for
disease. The ascospores stick to the leaf surface, germinate in a film
of water, and within several hours at the proper temperature penetrate
the leaf through stomata (natural openings) on the underside of the leaf. The
small purple spots soon appear on the upper surface. Incubation time, from
fungus penetration to the appearance of the spots varies with temperature.
Under damp conditions and with temperatures between 60 and 68 degrees F
(15-20 degrees C), the period may be as short as five days. When rain and
dews are absent and at lower temperatures, as long as 15 days may be required
before symptoms appear. |
Infected leaves usually turn
yellow or gold before they drop prematurely. |
Once
lesions have developed, masses of secondary or summer spores (conidia) are
formed from the slightly concave eruptions (acervuli) on the underside of the
leaf This mass of conidia provides the white appearance to the under leaf
lesions. Conidia are spread to other leaves by splashing raindrops and are
capable of causing new infections (each producing thousands of additional
conidia) under the temperature and wetness conditions listed in Table 1.
Serious leaf spot damage usually occurs in years with numerous rainy periods
throughout late spring and summer, when repeated secondary infection cycles
allow the disease to snowball into an epidemic.
Control
- Collect and destroy the
fallen cherry leaves in late autumn. The fungus over winters in these
leaves. This, practice should be quite beneficial for backyard growers
with one or a few trees, but is generally not considered practical for
large commercial plantings.
- Select a planting sight that
is always exposed to direct sunlight and has good air circulation and soil
drainage. Proper pruning to open the canopy will increase sunlight
penetration and air circulation. Any practice that promotes faster drying
of leaves will reduce -the risk of infection.
- Especially in commercial
plantings, the disease is controlled primarily with fungicide sprays.
Information obtained through the Ohio State Extension Factsheet HYG 3021-95
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES