Fungal Leaf Spots

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 68° to 77° 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.

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° to 86° 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. If overhead irrigation is used, irrigate in the morning so leaves can dry quickly. Drip irrigation is preferred. 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.

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 (65° F in day, 35° 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 overhead irrigation unless needed for frost protection or if weather is very dry. See current recommendations for chemical control measures.

Powdery Mildew

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, 58° to 68° 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.

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.

By: Pamela S. Mercure, IPM Program Assistant, University of Connecticut

Updated by: Mary Concklin, UConn IPM. 2012

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