Anthracnose Diseases of Trees
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
Anthracnose diseases occur on important shade and tree
species throughout Connecticut every year, although the severity and
distribution of these problems vary with each season, site, and species. These
diseases are common on ash (Fraxinus), maple (Acer), oak (Quercus),
and sycamore (Platanus). Anthracnose is most noticeable on trees in the
landscape, but disease also occurs on trees growing in natural woodlots and
forests. The degree to which each tree or species is affected by disease is
influenced by genetic factors, microclimate, and predisposition by other
stresses (e.g., drought, excess water, winter injury).
The term "anthracnose" refers to diseases caused by fungi that produce conidia in structures called acervuli (Deuteromycotina, Coleomycetes). These fungi can infect leaves, flowers, fruit, and stem tissues. Infections are usually initiated in the spring when new growth is emerging, but can occur throughout the entire season when the weather is favorable. Environmental conditions that are most favorable for disease development include periods of extended cool, moist, or wet weather.
Symptoms of anthracnose diseases range from minor cosmetic spotting of leaves to blighting of leaves and tender shoots and dieback of twigs and branches. Symptoms also vary with the individual host and the causal fungus. Anthracnose symptoms are apparent from late spring to early summer, but additional cycles can result in damage that is visible later in the growing season. As leaves and shoots mature and approach full size, they become relatively resistant to infection.
Anthracnose diseases are generally considered aesthetic or nuisance problems. However, when infections are heavy, they can result in disconcerting levels of premature leaf drop and defoliation. Anthracnose diseases can also disfigure trees when infected twigs and branches die. This is most apparent after several successive years of disease. Most trees that have dropped leaves prematurely usually produce new shoots and leaves by midsummer. Trees that are otherwise healthy can fortunately withstand several years of defoliation without long-term implications.
Causal Agents: Gnomoniella fraxini (Discula fraxinea)
Hosts: Fraxinus (black and white ash; green ash is fairly resistant)
Symptoms: Symptoms develop on newly expanding shoots and leaves in spring. Tender shoots are blighted and killed during cool, wet weather. Infections on developing leaves first appear as water-soaked, irregular areas. These develop into brown, somewhat papery lesions. When infections are moderate, only portions of each leaflet will be affected. This can give the leaf a distorted appearance, but leaves usually remain attached to the tree. When infections are heavy, entire leaves will turn brown and drop prematurely. Branches that have dropped their leaves usually produce new shoots and leaves by midsummer.
Causal Agents: Discula sp., Kabatiella apocrypta
Hosts: Acer (Japanese, Norway, sycamore, red or swamp, silver, and sugar maple)
Symptoms: Narrow, purple to brown streaks develop along the veins of leaves of Norway maples whereas large, brown patches develop between the veins on sugar maple leaves. Symptoms on Japanese maples appear as light tan, papery areas at leaf margins. Although symptoms vary with the type of maple, symptoms that are common to most maples are the large, irregular, dead areas on the leaves. These are often V-shaped or delineated or defined by the veins. These areas can be tan and paper-thin. When infection is severe, the fungus enters the petiole and causes entire leaves to appear blighted, browned, and shriveled. These symptoms are often confused with drought and heat stress since they are very similar. Samaras can also develop necrotic or dead spots and drop. Significant leaf drop can occur in late spring, but trees usually re-foliate by midsummer.
Causal Agents: Apiognomonia veneta (Discula platani)
Hosts: Platanus (sycamore, London plane)
Symptoms: Sycamore anthracnose often occurs in three phases, each of which can result in different types of symptoms. The three phases are twig and branch cankers, shoot blight, and leaf blight. Weather patterns usually influence the severity of each phase. In the first phase, the presence of the fungus in twigs and buds over the winter results in cankers and bud death when the trees are dormant. During the shoot blight phase, new shoots are rapidly killed by the fungus as they expand. This symptom is particularly noticeable during or just after cool, wet periods in spring. These first two phases can be confused with frost damage. In the final leaf blight phase, newly expanding leaves are infected and killed as they emerge. Leaves are most susceptible during the first few weeks of growth. Symptoms include foliar lesions that extend along the veins and develop into large, brown areas on the leaves. Significant leaf drop can occur in early summer although, by midsummer, most trees will have re-foliated with a canopy of healthy leaves.
Causal Agents: Apiognomonia quercina (Discula quercina)
Hosts: Quercus (white, black, pin, burr, scarlet oak)
Symptoms: White oak is the most susceptible of all oak species. Although some twig and branch dieback can occur, the most common symptom appears as a blighting of newly expanding shoots and emerging leaves. Irregular, tan, papery, necrotic spots form on the leaves as they develop and are often concentrated along the veins or at the margins of the leaves. When the spots are numerous, they coalesce and give the leaves a blighted appearance. Heavily infected leaves become distorted and often drop prematurely by early summer. However, trees usually re-foliate by midsummer.
MANAGEMENT STRATEGIES FOR ANTHRACNOSE DISEASES:
1) Control of anthracnose diseases can be achieved using a multifaceted approach. These diseases are often effectively controlled by following good sanitary and cultural practices and are rarely serious enough to warrant chemical control.
2) Anthracnose fungi overwinter in cankers on twigs and branches and, to some extent, on fallen leaves. Because these serve as important sources of overwintering inoculum, symptomatic tissues should be pruned, raked, and/or removed as completely as practical. This practice reduces the number of spores available to infect emerging shoots and leaves in spring and during the growing season.
3) Tree vigor should also be maintained by following sound cultural practices such as proper watering, fertilizing, mulching, and pruning.
4) Although anthracnose diseases are usually considered to be more aesthetic than life-threatening, there are situations where they can be serious and cause permanent damage or even tree death. Newly transplanted trees or trees weakened by stress are particularly sensitive to repeated defoliation. In such cases, chemical control can be beneficial. Among the fungicides registered for homeowner use in Connecticut are thiophanate methyl, chlorothalonil, copper sulphate pentahydrate, and mancozeb. The pesticide label will contain information on dosage rates, application intervals, and safety precautions. Since most anthracnose fungi infect in spring as the buds are swelling and new leaves and shoots are expanding, the first fungicide spray is applied at or just prior to budbreak. Two or three additional sprays are subsequently applied at intervals specified by the label for the particular fungicide being used. Additional applications may also be necessary under unusually wet or prolonged spring conditions. Once symptoms of anthracnose are visible on the leaves, it is usually too late for effective chemical control.
Anthracnose diseases occur on important shade and tree species throughout Connecticut every year, although the severity and distribution vary with each season, site, and species. The most common hosts are ash, maple, oak, and sycamore. Symptoms range from minor cosmetic spotting of leaves to blighting of leaves and tender shoots and dieback of twigs and branches. Anthracnose diseases are generally considered aesthetic or nuisance problems despite disconcerting levels of premature leaf drop that can occur. This fact sheet discusses these diseases and methods to minimize their impact on tree health.