Mycosphaerella Leaf Spots
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Importance
Leaf spots cause premature defoliation of ash seedlings, and forest and shade trees in North America. Mycosphaerella leaf spot, heretofore called "Piggotia leaf spot," is most common in nurseries. Infection by Mycosphaerella fraxinicola, previously called "Phyllostica leaf spot," is a problem in large trees.
Identifying the Disease
Leaf spots caused by M. effigurata appear in June as flecklike, yellow spots, 1 to 3 mm in diameter, on upper leaf surfaces. Hundreds of flecks may occur on a single leaflet. By late summer, dark asexual stromata give lower leaf surfaces a sooty appearance. Seedlings may defoliate prematurely near the end of the growing season. Initial leaf spots caused by M. fraxinicola are pale-green, irregular blotches, 5 to 15 mm in diameter. Spots sometimes coalesce and entire leaflets may die. Trees may appear scorched due to necrotic blotches on foliage. Severe infections can result in premature defoliation.
Identifying the Fungus
Both M. effigurata and M. fraxinicola have two asexual fruiting stages. Those of M. effigurata develop earlier in the growing season than those of M. fraxinicola. Mycosphaerella fraxinicola produces cylindrical, colorless conidia in contrast to the two-celled, colorless ascospores in pseudothecia that mature and overwinter in fallen leaves. Ascospores of M. effigurata are 1.5 to 2.0 times longer than those of M. fraxinicola.
Biology
Ascospores dispersed by wind from fallen leaves initiate primary infections on new leaves in the spring. Wet weather increases the severity of the disease.
Control
There is no practical control available for these leaf diseases.
References
Solomon, J.D.; Leininger, T.D.; Wilson, A.D.; Anderson, R.L.; Thompson, L.C.; McCracken, F.I. 1993. Ash pests: A guide to major insects, diseases, air pollution injury and chemical injury. Gen. Tech. Rep. SO-96. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station. 45 p.
