Anthracnose of maple is often a minor disease that only reduces the aesthetic value of an infected tree. Under ideal conditions, however, the disease can be severe, leading to premature defoliation and contributing to decline when other diseases and/or insect pests are present. Healthy trees may undergo defoliation in spring shortly after leaf out but are able to flush a new set of foliage and recover. Young trees are more susceptible to lasting damage while older, established trees can usually survive multiple years of defoliation.
Anthracnose of maple is caused by several fungi including Aureobasidium apocryptum (syn. Gloeosporium apocryptum and Kabatiella apocrypta), Discula campestris (syn. Gloeosporium campestre), D. umbrinella (syn. Gloeosporium umbrinellum) and Colletotrichum spp. (syn. Glomerella spp.)
Maple (Acer) species including sugar (A. saccharum), red (A. rubrum), Norway (A. platanoides), silver (A. saccharinum) and Japanese (A. palmatum) maples.
Symptoms & Disease Cycle
Symptoms vary by host and by the fungal pathogen present but are characterized by irregularly-shaped, angular spots or blotches that occur primarily along leaf veins or margins (Figure 1). Lesions on Norway maple tend to be narrow, purplish-black streaks along leaf veins. On sugar maple, lesions occur between leaf veins and appear as large, brown blotches. On Japanese maple, lesions occur between veins or at leaf margins as light-brown to tan, papery spots. Anthracnose fungi overwinter within senescent leaf tissue on the ground and in infected twigs and buds. Spores produced on this infected tissue in the spring are spread by wind and splashing rainwater. Anthracnose fungi produce asexual spores (conidia) in acervuli which appear as dark-brown to black spots within lesions. Acervuli can be found on the upper or lower leaf surface and along veins or midribs (Figure 2). Spores are produced whenever environmental conditions allow (mild and wet) from spring through late-summer. Severe infections cause premature defoliation and may cause distortion of young leaves. Symptoms on twigs and buds are less common on maples but under high disease pressure, young shoots may be killed.
Infected leaves shed by the tree are the primary source of inoculum and should be removed from the site. If the disease is severe, infected shoots can be pruned out, as they harbor overwintering spores. This sanitation practice will reduce the amount of spores present to re-infect trees during the spring and throughout the next growing season. Maintaining tree vigor through adequate fertilization, watering, mulching and pruning will help lessen the impacts of disease on tree health. Sugar maple prefers calcareous soils and many respond well to lime applications if soil pH is too low. Anthracnose is often an aesthetic issue on maple, but there are situations where the disease can cause permanent damage and chemical control may be warranted. These situations include when: young trees are heavily infected; trees are severely defoliated for consecutive years, and; trees are in decline due to environmental stress or attack by insect pests or other disease-causing organisms. If necessary, labeled fungicides can be used to protect at-risk trees. The following products are labeled for use against anthracnose of maple: chlorothalonil, chlorothalonil + thiophanate-methyl, copper hydroxide + mancozeb, mancozeb and propiconazole. The first application should be made at or just before bud break to protect new growth from initial infection in spring. Additional applications may be required at the labeled interval, especially if prolonged periods of wetness occur.