There are three fungal leaf spots
of strawberry, Leaf Spot, Leaf Blight, and Leaf Scorch. All three
fungal leaf spots are similar in life cycle. They do not generally
cause severe damage, but may weaken plants enough to cause overwintering
problems. Yield is not usually affected. These diseases can affect
plants from the time the first leaves are unfolding until dormancy.
All are favored by warm, moist weather.
Leaf Spot is caused
by the fungus Mycosphaerella fragariae, which infects only
strawberry. Middle aged leaves are most susceptible to this disease.
It is favored by temperatures of 68o to 77o
F. Leaf spots on caps can make them unattractive and unmarketable.
Symptoms. Round, deep purple spots that enlarge until the centers are grayish to white on older leaves or light brown on younger leaves. A distinct reddish-purple to rusty brown border surrounds the spot. Spots can also develop on fruit (causing 'black seed disease'), stems, petioles, caps, and runners in favorable years.
Angular
Leaf Spot is a bacterial
disease caused by Xanthomonas fragariae. This bacterium
infects only strawberries. It is sporadic in New England, but
it can be important when it strikes. This disease causes leaf,
petiole and calyx spots in New England, but has been reported
occasionally to kill plants in California. It is favored by wet,
cool (65o
F in day, 35o F at night) weather.
Symptoms. Tiny water soaked spots appear on the lower
surface of the leaves, which are angular in shape because they
are bordered by veins. When the leaves are held up to the light,
the spots are translucent.
When viewed normally, they are dark green. Later, the spots will
grow together to form larger, reddish-brown irregularly shaped
spots, which may become surrounded by a yellow ring. These larger
spots often follow veins. The leaf will have a ragged appearance.
Spots can also appear on the petioles and on the calyx of the
fruit, darkening them and making the fruit less attractive. In
wet weather, a thick fluid can appear on the undersides of the
infected leaves, which will dry to a shiny brown varnish-like
film. This fluid and film is diagnostic for this disease.
Prevention.
If this disease has been a
problem, rotate away from strawberries for at least one year.
Remove as much leaf debris from fields as possible at the time
of renovation. Space plants widely in the row and plant narrow
rows. Avoid overhead irrigation. Avoid working in the fields when
the plants are wet. Scouting should begin in fields that have
a history of the disease as soon as buds extend from the crown,
and should continue until bloom. If symptoms are seen, discontinue
irrigation unless needed for frost protection or if weather is
very dry. See current recommendations for chemical control measures.
Leaf Blight is caused by Phomopsis obscurans. It, too, affects only strawberry. It is found worldwide, and occurs sporadically in New England.
Symptoms. One
to six round reddish-purple spots appear on the leaflet, usually
on older leaflets in the later part of the season. The spots follow
major veins as they grow, to become V-shaped, light brown inside
with dark brown edges. The spots may also have a purple, red,
or yellow border. Tiny black dots may be seen in the center of
the spots. The whole leaflet may turn brown. Stolons, petioles,
and fruit stems may be infected, and, if a spot grows completely
around them, they may be killed. Fruit caps and fruit may also
be infected.
Leaf Scorch, the
third of the fungal leaf spots, is caused by Diplocarpon earliana.
Middle-aged and older leaves are most susceptible. It is favored
by warm (68 o
to 86o F), wet weather.
Most of its spores are formed in mid-summer, as the leaves age
and the weather is favorable. This disease can affect how successfully
plants will overwinter.
Symptoms: Numerous small, irregular purplish spots
on leaves, that can grow together. The centers become brownish.
When the entire leaflet is covered, it appears purplish to reddish
to brown. Tiny black dots appear in the spots. The leaves will
eventually dry up, and their edges curl up, making the leaf look
scorched.
Prevention of All Fungal Leaf Spots.
Plant disease-free plants where there is good air circulation
and in well-drained soil. Plant narrow rows with well-spaced plants
to keep the canopy dry. Irrigate in the morning so leaves can
dry quickly. Remove old tissue at the end of the season. If the
fields are mown at the end of the season, the mowings must be
either removed from the field or plowed under.
Resistant varieties are available for Leaf Spot and Leaf Scorch;
see current recommendations for a list of resistant varieties.
No resistant varieties are available for Leaf Blight; see current
recommendations for chemical control measures.
Powdery Mildew is another fungal disease of strawberry, caused by Sphaerotheca macularis. It occurs worldwide, wherever strawberries are grown. It can affect leaves, flowers, and fruit. This disease is favored by dry weather, 58o to 68o F. Severe foliar infection happens late in the season, and usually does not result in lower yields. Flower and fruit infection can cause losses. It is significant on only a few highly susceptible varieties.
Symptoms. Gray powdery growth on bottom of leaves, causes leaves to curl upward. Purplish or reddish blotches are sometimes seen on the undersides of leaves. Flowers and ripe fruit may be covered with the powdery growth as well. If green fruit is infected, it may fail to ripen, and will remain hard.
Prevention. Control of this disease is only to prevent high levels of disease and fruit infection the following season. Resistant varieties are available for this disease. See current recommendations for chemical control measures.
By Pamela
S. Mercure, IPM Program Assistant, University of Connecticut
References.
Cooley, D., D. Handley,
S. Schloemann, and W. Wilcox. 1998. Disease Management and Physiological
Disorders. Chapter 9 in Strawberry Production Guide for the
Northeast, Midwest, and Eastern Canada. Cooperative Extension,
Ithaca, NY.
Maas, J.L., ed. Compendium of Strawberry Diseases. 1984. APS Press,
St. Paul, MN
New England Small Fruit Pest Management Guide, 1996-1997.
S. Schloemann, ed. University of Massachusetts Cooperative Extension
System.
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