Anthracnose: Causes, Symptoms, Disease Cycle, and Prevention
Anthracnose is not a single disease but a collective term used to describe a group of common and widespread fungal infections that affect a broad range of plants, including many deciduous and evergreen trees (such as sycamore, maple, oak, and ash), shrubs, vegetables (like tomatoes, cucumbers, and beans), fruits (including grapes and strawberries), and ornamental plants. The term often refers to the characteristic dark, sunken lesions that appear on leaves, stems, flowers, and fruits. While generally considered a cosmetic or nuisance problem for established shade trees, its severity can be significant in younger plants or high-value crops, and its cumulative effect can lead to branch dieback and reduced yields if infections are chronic.
Causes and Favorable Conditions
Anthracnose is caused by several different species of fungi belonging to genera such as *Colletotrichum*, *Discula*, *Apiognomonia*, and *Gloeosporium*. Crucially, the fungi are often host-specific; for example, the species causing anthracnose on ash is typically different from the one that affects maple. The development and severity of the disease are profoundly influenced by environmental factors. The fungi thrive and spread best in conditions of extended cool, wet weather, high humidity, or excessive water on plant surfaces. Mean daily temperatures between 50°F and 57°F during the critical period of bud break and early leaf development are particularly conducive to spore production and subsequent infection. Therefore, anthracnose is often most problematic during prolonged rainy seasons or periods of frequent rain, dew, or fog, which are essential for spore dispersal and germination.
Symptoms and Identification Across Hosts
The visual symptoms of anthracnose are highly variable depending on the specific fungal species and the plant host. The infection typically targets newly expanding shoots and young leaves, as mature foliage is generally more resistant. The most common sign is the development of tan, brown, or black spots or blotches on the leaves. On certain trees like oak and sycamore, these lesions frequently develop along the major leaf veins, while on others like ash or maple, the spots can be more irregular in shape. Infected young leaves often become distorted, curled, or cupped before turning brown and dying. Severe spring infections often result in premature defoliation, a process that starts on the lower and inner branches and progresses upward. Fortunately, most trees, excluding Chinese elm, will produce a new flush of leaves by mid-summer, minimizing permanent harm.
Beyond foliage, the disease can manifest in more severe forms. On twigs and branches, the infection can cause sunken areas with swollen edges known as cankers. On sycamore, for instance, these cankers can girdle and kill twigs and branches, resulting in dieback and giving the branch a gnarled or crooked appearance due to repeated regrowth from lateral buds. On fruits, such as grapes, tomatoes, or papaya, the lesions appear as dark, water-soaked, sunken spots that often have a purplish-brown edge and a lighter, gray center, creating a distinct “bird’s-eye” appearance. This fruit damage significantly reduces commercial quality and marketability. On crops, symptoms can also include stalk rot, leading to plant lodging, and reduced photosynthesis, which inhibits overall growth.
The Anthracnose Disease Cycle
The life cycle of the anthracnose fungi begins with overwintering. The pathogens survive the cold months in various plant tissues, including fallen leaves, infected buds, cankered twigs and branches, and mummified fruit on the ground or still attached to the plant. As the weather warms in early spring and new growth emerges, the primary inoculum, consisting of fungal spores (conidia and ascospores), is released from these overwintering sites. Rain and wind are the key dispersal agents, carrying the spores to the newly forming leaves and shoots. If a film of water is present on the plant tissue, the spores germinate and initiate new infections.
Once an infection is established, the resulting lesions produce new generations of spores, which are then splashed short distances by rain to healthy plant parts, leading to secondary infections. This cycle of infection continues as long as cool, wet, and humid weather persists. When the weather becomes dry and leaves mature and fully expand, the tissue typically becomes resistant, and the spread of the disease slows down or stops. On shade trees, this is often the point where the tree begins to push out a second, healthy crop of leaves. However, on certain susceptible hosts like walnut, the disease may continue to progress throughout the summer months.
Prevention and Cultural Management Strategies
For most ornamental trees, the most effective and practical management strategy for anthracnose centers on cultural practices aimed at reducing disease pressure and promoting tree health, as the disease rarely causes permanent damage. The primary goal is to eliminate overwintering sites for the fungus and reduce the duration of leaf wetness. Field sanitation is crucial: fallen leaves, mummified fruits, and infected debris must be promptly raked up and destroyed (burned, buried, or hot-composted) before the first snowfall to prevent spring reinfection.
Pruning is another vital cultural control. Infected twigs and branches, particularly those with cankers, should be pruned back to healthy wood. Furthermore, strategic pruning to remove interior or lower growth can significantly improve light penetration and air circulation throughout the tree canopy, which helps leaves dry faster and creates an environment less favorable for fungal growth. For crops, root-zone or drip irrigation should be implemented instead of overhead watering to keep foliage dry. Ensuring proper plant spacing also maximizes air flow. Finally, the selection of resistant plant varieties is the most recommended long-term prevention measure, as this eliminates the host’s susceptibility altogether.
Chemical Control and Treatment
Fungicide application is typically not recommended or warranted for established shade trees unless defoliation or branch dieback occurs for several consecutive years, or if the tree is a highly susceptible species like sycamore. However, for high-value crops, vineyards, or fruit trees where the disease can severely impact yield and quality, chemical treatment is often a necessary component of the management plan. The timing of fungicide application is critical and must be preventative, as fungicides cannot cure already infected tissue.
Effective control usually requires a series of applications beginning at bud break, followed by a second treatment when leaves are about half-expanded, and a third when leaves are fully expanded. Common active ingredients registered for anthracnose control include Copper-containing fungicides, Chlorothalonil, Mancozeb, Thiophanate methyl, and Propiconazole. For large trees, high-pressure spraying equipment is necessary for complete coverage, necessitating the hiring of a professional arborist. Growers must also be diligent in rotating the active ingredients used to minimize the risk of developing fungicide-resistant strains of the pathogen.