Common Leaf Spot of Strawberry

Common Leaf Spot of Strawberry

There are many leaf spot diseases that occur on strawberry. Common leaf spot, sometimes referred to as plain old "leaf spot", has worldwide distribution. Incidence of the disease is often high and it may be severe enough to cause economic injury on susceptible cultivars in wet years. Common leaf spot has been recognized as a problem on strawberry for a long time and has acquired a number of pseudonyms including rust, bird's-eye spot, gray spotness, white spot, leaf blight (now used for another disease), and Mycosphaerella or Ramularia leaf spot. The last two names come from the causal organism, Mycosphaerella fragariae (Tul.) Lindau (anamorph: Ramularia brunnea Peck). The fungus exists in several races. Cultivated and wild strawberry are the only known hosts of the pathogen. The perennial nature of strawberry provides excellent overwintering sites for the pathogen. In mild climates, including the southern U.S. Mycosphaerella fragariae survives on older infected leaves. In colder climates it survives to a lesser extent on green leaves and stolons (runners) covered with mulch. Perithecia may also be present on these structures in colder climates or the fungus may overwinter as sclerotia on leaf debris. Conidia on leaves or formed from germinating sclerotia are the main source of primary inoculum. Ascospores may also incite primary infection. Conidia are rain splashed and ascospores are wind blown to susceptible tissue. Infection occurs through the stomata on upper and lower leaf surfaces. Once the fungus is established, more conidia are produced and serve as inoculum f or the secondary spread of the disease. Leaf spot may also be introduced in to new plantings on infected runner plants used for establishment. Disease development occurs during periods of wet weather with an optimum temperature range of 65-75^oF.

Symptoms

Symptom development is most common on leaves but may occur on most above ground plant parts. Lesions are initially visible on the upper leaf surface as small purple spots. The spots enlarge slightly with age up to Ľ inch and may coalesce. The tissue in the center of the spot changes from purple to tan or gray and eventually become white. The margin of the spot remains purple or brown giving the lesion a "bird's-eye" appearance. Infection sites appear as indistinct tan or bluish areas on the lower leaf surface. Symptom expression is variable depending on the age of the leaf, susceptibility of the cultivar or species, fungal strain and environmental conditions. Lesion size, number and coloration are all affected by these factors. Middle-aged leaves are most susceptible and often develop numerous lesions and appear scorched. Petioles, stolons, fruit stems and calyxes may also become infected. Lesions resemble those produced on the upper leaf surface. Mycosphaerella fragariae also infects the fruit causing a condition known as black seed disease. A few achenes (seeds) on the outer surface of the fruit are infected and become discolored. Surrounding tissue and pulp also becomes brownish-black and is leathery. Affected fruit does not rot but receives a lower grade.

Plant Health Management

Resistant Cultivars.

Resistance among strawberry cultivars and Fragaria species was recognized almost as soon as common leaf spot was detected. Leaf spot resistance remains a component of strawberry breeding so many cultivars with this characteristic are available and should be used.

Fungicide Application.

While resistance is available, some susceptible cultivars of strawberry are still widely used due to their popularity with growers and consumers. The application of fungicides is necessary to keep leaf spot in check on these plants. Fungicide application may also be necessary when new stains of the pathogen become prevalent. Labeled compounds include: benomyl (Benlate), captan and liquid copper (including Bordeaux) 1997.

Use Pathogen-Free Nursery Stock.

References

Agricultural Research Service, U. S. Dept. of Agriculture. 1978. Strawberry diseases. Farmer's Bull. 2140. U. S Government Printing Office, Washington, D.C. 27 pp.

Anderson, H. W. 1956. Diseases of fruit crops. Mc-Graw Hill Book Company, Inc. New York, NY. 501 pp.

Maas, J. L., ed. 1984. Compendium of strawberry diseases. American Phytopathological Society, St. Paul, MN. 138 pp.

Useful Links

Texas A&M University

University of California - Davis

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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