Damaged strawberry leaves were common in some coastal strawberry plantings early this year. We have confirmed that most of these symptoms are caused by leaf blotch disease.
Symptoms generally consist of tan to gray leaf lesions that develop on the first few leaves of the growing transplant. These affected areas tend to grow fairly large; they can expand and cover from one-fourth to one-half of the leaflet surface. Leaf infections commonly grow from the margin or edge of leaflets, and can be surrounded by a purple red border. Because these lesions are irregular in shape, the disease has been given the name of leaf blotch.
An important sign of leaf blotch is the presence of tiny, brown to black, fungal fruiting bodies in the gray blotches. Brown to black petiole lesions can also occur. The leaf blotch pathogen can cause a brown decay on the calyx end of strawberry fruit. This fruit phase of disease, known as stem-end rot, appears to be fairly rare in California.
Leaf blotch symptoms may be similar to those caused by another pathogen, Phomopsis obscurans (causal agent of Phomopsis leaf blight). Note that the blotches are not small and round as in the case of leaf spot caused by Mycosphaerella fragariae (this disease is also called Ramularia leaf spot). Leaf blotch superficially might resemble damage caused by anthracnose disease (caused by Colletotrichum acutatum) or abiotic factors (chemical burn, etc.).
Leaf blotch disease is caused by the fungus Gnomonia comari, which is an ascomycete that produces dark, spherical fruiting bodies (perithecia) and airborne ascospores. Like many pathogenic fungi, this organism also produces a second asexual form that has spores called conidia.
The conidial stage is named Zythia fragariae. The Zythia form also makes dark, spherical fruiting bodies that in this case release tiny spores. Both Gnomonia and Zythia forms can be found on infected strawberry tissues.
Only Zythia seen
Thus far in 2006, only the Zythia form has been observed on symptomatic strawberry leaves. Zythia fragariae survives on strawberry crop residues. The fungus does not appear to be a true soilborne fungus, so it will not likely persist in soil unless strawberry crop debris is present. The Zythia form is especially dependent on splashing water for spore dispersal and infection. This accounts for the typical appearance of leaf blotch during winter and early spring seasons when there are rains.
Leaf blotch is usually considered a minor problem and fungicide treatment programs have not been developed nor are they currently recommended. The strawberry plants usually grow out of the problem.
Leaf blotch spread and development is dependent on rains and splashing water, so once the winter rains cease, leaf blotch usually becomes a non-issue for growers. It is useful for growers to have leaf blotch identified so that other factors (such as chemical burn and other diseases like anthracnose) can be eliminated as causes of the brown, damaged leaves.