Pathogens
Several fungal pathogens are responsible for maple anthracnose in southern New England, including: Aureobasidium apocryptum (syn. Kabatiella apocrypta), Discula campestris and Colletotrichum gleosporoides.
Hosts
All native and non-native maples commonly planted as woody ornamentals are susceptible to infection. These include sugar maple (A. saccharum), red maple (A. rubrum), Norway maple (A. platanoides), silver maple (A. saccharinum) and Japanese maple (A. palmatum).
Symptoms & Signs
Maple anthracnose is often a minor disease that only reduces the aesthetic value of infected trees. Under ideal environmental conditions, the disease can be more serious, leading to premature defoliation and contributing to the decline of stressed and weakened trees. Healthy trees that are defoliated early in the growing season are often able to flush a new set of foliage and recover. Young trees that are recently transplanted are more susceptible to lasting damage while older, established trees typically suffer only minor growth losses. Symptoms can vary by host and fungal pathogen present, but in general the symptoms are characterized by dark-colored, irregularly-shaped, angular spots or blotches that occur primarily along the midrib, primary veins and leaf margins. Lesions on Norway maple tend to be narrow, purplish-black streaks along leaf veins. On sugar maple, lesions occur along primary veins and appear as large, brown blotches. On Japanese maple, lesions occur along primary veins and leaf margins and the lesions can appear both tan or black in color. Anthracnose fungi overwinter within senescent leaf tissue and in infected twigs and buds. In the spring, fruiting bodies are produced and spores are spread by wind and splashing rainwater. Anthracnose fungi produce asexual spores (conidia) within pads of tissue known as acervuli. These readily form within leaf spots and blotches and can be found on the upper or lower leaf surface and along veins or midribs. Spores are produced whenever environmental conditions allow (mild and wet) from spring through late-summer, but are typically most abundant in late spring and early summer. Many anthracnoe fungi have limited activity during mid-summer, when conditions are often drier and hotter. Symptoms on twigs and buds are less common on maples but under high disease pressure, tender shoots may be killed.
Management
Abundant rainfall in late April and early May often initiates maple anthracnose. While little can be done about natural rainfall, avoid overhead irrigation of ornamental maples to reduce free moisture on the foliage. Infected leaves are the primary source of inoculum and should be removed from the site. If the disease is severe, infected shoots should be pruned out, as they can also harbor the pathogen. This sanitation practice will reduce the amount of conidia (spores) present to initiate new infections in the spring as trees are leafing out. Maintaining tree vigor through adequate fertilization, watering, mulching and pruning will help lessen the impacts of the disease. 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 suffering from insect defoliation for consecutive years, and; trees are in decline due to environmental stress or attack by insect pests and disease. If necessary, labeled fungicides can be used to protect susceptible trees. The following products are labeled for use against anthracnose of maple: copper hydroxide + mancozeb, thiabendazole, 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 wet weather occur.