Phyllosticta minima (Phyllosticta leaf spot)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Order	Botryosphaeriales
Family	Botryosphaeriaceae
Genus	Phyllosticta

Scientific Name

Phyllosticta minima

Scientific Name Synonyms

Phyllostictina minima

Common Name

Phyllosticta leaf spot

Author: Garrett Ridge and Mike Munster, North Carolina State University

Pathogen

Phyllosticta minima is an asexual fungus in the phylum Ascomycota that causes leaf spots on Acer spp. The fungus produces dark pycnidia (up to 150 to 200 μm in diameter) which contain short, simple conidiophores that bear conidia (Figure 1). Conidia are hyaline, one-celled, and ellipsoid in shape (7.5 to 12 x 4.5 to 8 μm) with a single apical appendage (Figures 2-3). When mature, conidia are extruded in a slimy matrix through an ostiole at the top of the pycnidium. The teleomorph of P. minima is assumed to be an unknown Guignardia species.

Hosts, Signs, and Symptoms

Phyllosticta minima causes disease on a number of maple species, most prominently Amur, Japanese, red, and silver maple. Other maple species affected include mountain, sugar, sycamore, and Tatarian maples.

Tiny black pycnidia often form within lesions on the upper side of the leaf (Figures 4-5). Sometimes the pycnidia are arranged in a circle in the middle of lesions.

Symptoms of disease occur primarily on lower leaves. Infection by P. minima leads to the formation of roughly circular spots ≤ 5mm in diameter on maple leaves. Spots have tan to brown centers and distinct purple, red, or brown margins (Figures 6-8). On Japanese maples, discrete white to brown spots form on leaves (Figure 9). As spots mature, the centers may fall out, leaving circular to irregular holes (Figure 10). Occasionally, this disease becomes severe enough to cause partial defoliation of maple trees.

Ecology

P. minima overwinters in infected leaf litter, presumably as pycnidia, and produces spores in early spring. Spores of the fungus are dispersed from the pycnidia to newly developing maple leaves by splashing rain and irrigation. Infection occurs during these same wet periods and necrotic spots develop on leaves. Pycnidia found on infected leaves produce conidia that can cause new cycles of infection within the tree canopy and to nearby maple trees throughout the growing season. Wet weather in spring and early summer in successive years increases the severity of leaf spot caused by P. minima.

Geographic Distribution

Eye spot on maple is common throughout Eastern North America and most portions of the Great Plains. P. minima is uncommon outside of North America but has been reported in Europe and Japan.

Management

Cultural

Removing and destroying leaves from trees infected with eye spot will reduce the amount of P. minima that overwinters in fallen leaves. Young trees benefit from mulching around the base to prevent stress. Wet conditions favor spread and infection so wetting foliage during irrigation should be avoided. Air circulation may be improved by pruning and by preventing overcrowded plantings. Occasionally, eye spot may defoliate trees early in the growing season, but trees typically will produce new leaves within a few weeks. Defoliated trees should be watered and properly fertilized to promote regrowth. Avoid susceptible species and cultivars when planting in areas with a history of severe eye spot.

Chemical

Damage caused by P. minima almost always is minimal and localized, so fungicides are rarely necessary. Fungicide sprays may be used on trees with a history of severe disease or in nurseries where aesthetic quality is important. Fungicides must be applied before or during the early stages of infection, in early spring and summer to obtain the best results.

Diagnostic procedures

The symptoms of eye spot are similar to those of other foliar diseases of maple such as anthracnose and tar spot. P. minima can be distinguished from other fungal pathogens by spore characteristics, the presence of pycnidia, and the distinctive purple borders on lesions in some cultivars. The ocellate gall midge (Acericecis ocellaris) also causes leaf spots with purple borders. If the spots are caused by this insect, the translucent to whitish maggots can be found in cup-like depressions on the lower leaf surface. Conidia of P. minima are 10–25μm long, ellipsoidal, and bear a single apical appendage.

If signs of the pathogen are not present, incubate affected leaves in a moist chamber at room temperature for three days. Keep leaf tissue elevated on a screen to avoid excess moisture and development of secondary organisms. Phyllosticta species can be cultured on APDA, but culturing the fungus is not necessary for diagnosis.

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)