Anthracnose is a fungal disease that affects a wide variety of plants, including trees, shrubs, vegetables, and fruits. It is caused by multiple species of fungi that create characteristic dark, sunken lesions on the host plant tissue. While the damage is often primarily cosmetic, severe or repeated infections can lead to significant defoliation, branch dieback, and crop loss.
The Causative Agent and Environmental Triggers
Anthracnose is primarily caused by fungi belonging to the genus Colletotrichum, though other genera may also be responsible for similar symptoms in certain host plants. These fungi produce spores within tiny, saucer-shaped structures known as acervuli, which are the main source of new infections. Each fungal species often targets a specific range of host plants.
The fungi require specific environmental conditions to successfully infect plant tissue. Prolonged periods of leaf wetness are a significant factor for the fungal spores to germinate, often requiring 12 to 24 hours of continuous moisture on the plant surface. High humidity combined with moderate temperatures, typically between 60 and 80 degrees Fahrenheit, creates an ideal environment for the disease to develop rapidly. These conditions are most often met during cool, rainy spring weather or in tropical and subtropical regions experiencing heavy rainfall.
Recognizing the Signs of Infection
Identifying anthracnose relies on recognizing the specific visual changes it causes across different parts of the plant. On leaves, the disease initially appears as small, irregular spots that are often yellow or brown before developing into larger, dark, necrotic blotches. A characteristic sign on many tree species, such as oak and sycamore, is the appearance of lesions that follow the major leaf veins, causing the leaf tissue around the veins to die.
If the infection occurs early in the season on newly emerging leaves, the foliage may become distorted, curled, or scorched, often leading to premature leaf drop. The disease can also affect woody parts of the plant, manifesting as sunken, discolored areas called cankers on young twigs and branches. These cankers can girdle the stem, causing the tissue beyond the infection point to die (dieback).
On fruits and vegetables, the symptoms present as distinct, circular, sunken spots that may be water-soaked in appearance. As the fruit lesions mature, they often enlarge and darken, sometimes developing a dark rim around the affected area. Under humid conditions, a salmon or pink-colored mass of fungal spores may be visible in the center of the sunken spots. This damage often occurs on immature fruit, remaining dormant until the fruit begins to ripen, leading to significant post-harvest rot.
How the Disease Spreads and Survives
The life cycle of the anthracnose fungi is intrinsically linked to moisture, which facilitates both its survival and its transmission. The pathogen overwinters in infected plant debris, fallen leaves, and in dormant cankers on twigs and branches. The fungi can also persist on contaminated seeds, providing a direct source of infection for the next generation of plants.
When environmental conditions become favorable, fungal spores are released from the overwintering structures and dispersed to new, healthy plant tissue. The primary method of transmission is through splashing water, such as heavy rain or overhead irrigation, which can carry the spores short distances to nearby leaves and stems. Wind can also play a role in spreading the spores over longer distances.
The disease can also be unintentionally spread through human activity. Farm operations that involve working with wet plants can contribute to pathogen movement. Contaminated gardening tools, equipment, and even the hands of workers can pick up spores and transfer them to uninfected plants, allowing the disease to move rapidly through a garden or field.
Preventing and Managing Anthracnose
Management focuses on cultural practices that limit the fungi’s ability to infect and spread. Improving air circulation around plants is beneficial, achieved by ensuring adequate spacing and judicious pruning of dense canopies. Pruning also removes and destroys infected twigs and branches, reducing the amount of overwintering material.
Proper watering techniques are important for prevention, as minimizing the duration of leaf wetness is a primary goal. Avoiding overhead irrigation and using drip systems or soaker hoses keeps the foliage dry and prevents the splashing that spreads spores. Garden sanitation requires the immediate removal and disposal of all fallen leaves, infected fruit, and plant debris from the area.
In cases where the disease is severe or when growing highly susceptible crops, chemical control may be necessary to protect uninfected plant tissue. Fungicides, such as those containing copper or chlorothalonil, are applied preventatively, before or at the first sign of symptoms, to coat the plant surfaces. Planting varieties that are resistant or tolerant to anthracnose is a long-term strategy that minimizes the chance of future infection.