Minor Leaf Spots and Blights

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Contents 1 Importance 2 Identifying the Disease 3 Identifying the Fungi 4 Biology 5 Control 6 References

Importance

Leaf-spotting fungi are generally of little importance to tree health. Severe infections that trigger early leaf abscission may have some effect on growth, but the main consequence is to the tree’s appearance.

Identifying the Disease

Mycosphaerella platanifolia and M. stigminaplatani leaf spots appear as unevenly round, tan spots (0.1 to 1 cm) with redbrown halos that may coalesce into larger, irregularly shaped lesions (fig. 25). Leaves infected by P. multimaculans develop unevenly round or angular, brown to purple spots (1 to 3 mm). Septoria platanifolia produces brown, circular spots that later develop gray centers with dark halos. Spots typically develop on the upper surfaces of leaves. The lower surfaces of leaves infected by M. stigminaplatani may have a diffuse, sooty film.

Figure 25. - Mycosphaerella leaf spots

Identifying the Fungi

Ascospores of both Mycosphaerella species are twocelled. The conidia of the Cercospora or Stigmina stages are multiseptate. Pycnidia of P. multimaculans grow on the lower surfaces of leaves and produce colorless, cylindrical, slightly curved conidia with one to four septations.

Biology

Initial infections of Mycosphaerella, the sexual stage of the first two fungi, occur in early spring from spores produced in fruiting bodies on fallen leaves. Subsequent infections occur as asexual conidia from the Cercospora or Stigmina stages are moved by wind or rain splashing. Conidia of S. platanifolia and P. multimaculans are spread by wind and rain from pycnidia that form on the leaf spots.

Control

Leaf spots typically do not warrant control on ornamental or forest trees because they are of minor consequence to tree health. However, organic fungicides are sometimes used to control severe outbreaks.

References

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.