Author: Frank Louws, Jean Harrison, and Garrett Ridge, North Carolina State University
Scientific Name Synonym
Ramularia brunnea Peck [Anamorph]
Mycosphaerella fragariae is an ascomycetous plant pathogenic fungus that causes one of the most common diseases of strawberry. M. fragariae reproduces by forming perithecia that are black, partially embedded, globose (100 to 150 µm) and minutely ostiolate. Within each perithecium, asci (30 to 40 x 10 to 15 µm) that are cylindrical to clavate, have short stalks, and contain 8 spores, are formed in small clusters. Ascospores (12 to 14 x 3 to 4 µm) are hyaline and two-celled with a median septum with each cell generally containing two oil drops.
The anamorph of M. fragariae, Ramularia brunnea is actually more commonly found on strawberries during the growing season, especially in southern latitudes. This anamorphic state produces conidia that are elliptical to cylindrical (20 to 40 x 3 to 5 µm) hyaline, and zero- to four-septate (Figures 1-2). The conidia are often formed in short chains and are borne on short, hyaline, unbranched, and frequently curved conidiophores.
Hyphae of M. fragariae may aggregate and form sclerotia which resemble perithecia in size and shape but lack a cavity.
Hosts, Signs, and Symptoms
The host range of Mycosphaerella fragariae is limited to cultivated and wild Fragaria species .
Signs of infection include the development of small black sclerotia within mature lesions towards the end of the growing season. Perithecia of M. fragariae may also form within lesions on plants grown in northern latitudes. In southern latitudes tufts of conidia may be present within leaf lesions (Figure 3).
Symptoms begin as small round purple spots 3 to 6 mm in diameter on upper leaf surfaces (Figure -). Lesions may also develop on fruit (black seed), caps, fruit trusses, petioles, and runners. On older leaves the center of the lesion changes from tan or gray to white, with reddish purple to rusty brown margins (Figures 4-5), while on younger leaves the lesions stay light brown. Lesions may form on the undersurface of leaves, but the color is not as intense. Leaves may die if numerous lesions coalesce. Usually the lower leaves die out. Symptoms may vary with strawberry cultivar and strain of pathogen. Temperature may also affect the appearance of leaf spot. Atypical lesions, uniformly brown without darker borders or lighter centers, may form in warm humid weather on young leaves.
Common leaf spot on strawberry is a polycyclic disease, and under favorable conditions it will continue to reinfect the host and surrounding plants. M. fragariae may overwinter as sclerotia in the soil, lesions on living leaves, or on wild strawberry populations. However, common leaf spot problems have most typically been associated with transplant source. M. fragariae and its anamorph state, Ramularia brunnea, can generate two types of spores that infect newly-emerging leaves in the spring: ascospores and conidia, respectively. Conidia are the primary inoculum in southern latitudes and are produced in spring and early summer on upper and lower leaf surfaces in matted row systems. In annual systems, disease build-up has occurred in the late fall and early spring. Conidia are disseminated primarily by splashing rain water. High rainfall and warm temperatures (20 to 25°C) are most favorable for this pathogen and lead to rapid disease development.
Common leaf spot occurs worldwide on cultivated and wild strawberries. In the southern hemisphere, M. fragariae is uncommon in strawberry fields and the anamorph form R. brunnea is the predominant agent of common leaf spot.
It is important to select light, well drained soil with good air circulation and exposure. Many strawberry cultivars resistant to common leaf spot (e.g. Atlas, Cavendish, Earliglow, Jewel, Lateglow, Tennessee Beauty) are available for planting. Plant only disease free plants purchased from reliable nurseries. Carefully space runner plants in matted-row culture and control weeds in all plantings to improve air circulation and reduce drying time for leaves.
Growers generally do not treat for M. fragariae specifically, but broad spectrum fungicides used to control other diseases also control common leaf spot. Use of protective fungicides is only necessary on highly susceptible varieties and if common leaf spot symptoms are prevalent in the transplants. In most cases, symptoms in the fall have no impact on yield. If the winter is warm and wet, new growth may become infected with many lesions and a fungicide application is warranted. Some resistance to fungicides has been observed but not documented in the southern latitudes. Soil fumigation may successfully reduce inoculum levels in the field for matted row production systems but carry over from one year to the next has not been observed as a problem.
Lesions produced by M. fragariae differ depending on host cultivars and environmental conditions during infection. Symptoms of common leaf spot may be confused with those of Diplocarpon earliana, which produces purple blotches on the leaf surface. If signs of the pathogen are not evident on a symptomatic plant, individual leaves or leaflets may be incubated in a moist chamber for 24 to 48 hours to induce sporulation.
Mycosphaerella louisianae Plakidas is another Mycosphaerella species that causes leaf spot on strawberry. This pathogen has smaller perithecia, asci, and ascospores than M. fragariae and also occurs in the southeast.
Resources and References
- ↑ Maas, J. L. 1998. Compendium of Strawberry Diseases, 2nd edition. APS Press. St. Paul, MN.1.01.11.2
- ↑ Carisse, O., Bourgeois, G., and Duthie, J. A. 2000. Influence of temperature and leaf wetness duration on infection of strawberry leaves by Mycosphaerella fragariae. Phytopathology 90:1120-1125.2.02.1
- ↑Swift, C. E. Strawberry diseases. Fact Sheet No. 2.931. Colorado State University Extension.
Common Names and Diseases
strawberry leaf spot
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