What is Anthracnose?
Anthracnose is not one single disease, but a general name for a group of related fungal diseases that cause dark, sunken lesions on leaves, stems, flowers, and fruits. It's a very common problem, especially during cool, wet spring weather, and it can look a bit like a bad case of leaf spot or scorch.
The disease is caused by several species of fungi, most often from the genus Colletotrichum or Apiognomonia. These fungi thrive and spread in moist conditions. The key environmental factor for an anthracnose outbreak is prolonged periods of leaf wetness. A rainy, cool spring is the perfect storm for this disease to take hold and spread rapidly.
Anthracnose has a wide range of hosts, affecting many popular garden plants and trees. Some of the most commonly affected include:
- Shade Trees: Dogwood, sycamore, oak, maple, and ash are particularly susceptible.
- Fruits and Vegetables: Tomatoes, peppers, cucumbers, beans, grapes, and berries can all suffer from anthracnose, which often appears as a fruit rot.
- Lawn Grasses: It can also affect turf, causing irregular yellow or brown patches.
Life Cycle
Understanding how anthracnose survives and spreads is your best weapon against it. The fungi that cause this disease are excellent survivors, overwintering in infected plant debris. They can hide in fallen leaves, in dead twigs, or in sunken lesions, called cankers, on the branches of infected trees.
When the weather warms up and the spring rains begin, these dormant fungi produce millions of microscopic spores. Raindrops then splash these spores onto newly emerging, tender leaves and shoots. If the surface of a leaf stays wet for an extended period (typically several hours), the spore can germinate and infect the plant tissue. This infection creates the characteristic spots and lesions. These new lesions then produce another wave of spores, which can splash to other parts of the plant or to neighboring plants, continuing the cycle throughout the wet season.
Symptoms
Symptoms can vary depending on the host plant, but they generally follow a predictable pattern of progression.
Early Stage
- Irregular Spots: The first signs are often small, irregular spots on the leaves. They can be tan, brown, dark purple, or black.
- Vein Discoloration: On some trees, like sycamore and oak, the spots often form along the leaf veins, causing them to turn dark brown or black.
- Water-Soaked Lesions: On fruits and vegetables, the initial spots may look dark and water-soaked.
Middle Stage
- Enlarging Blotches: The small spots grow and merge, forming larger, dead patches on the leaves, giving them a scorched or blighted appearance.
- Leaf Distortion: Infected new leaves may become twisted, curled, and distorted as they grow.
- Sunken Cankers: On twigs and stems, the disease can create sunken, dead areas (cankers) that may girdle and kill the twig. On fruits, the lesions become sunken and dark.
Late Stage
- Premature Leaf Drop: The tree or plant may shed its infected leaves prematurely, often starting from the lower and inner branches and moving upward. A severe infection can cause significant defoliation by early summer.
- Twig Dieback: The cankers on twigs can cut off water and nutrients, causing the ends of the branches to die.
- Fruit Rot: On infected fruits, the sunken lesions may develop a characteristic pinkish or orange-colored gelatinous mass of spores in their center. The fruit will then rot and become inedible.
How to Control Disease Progression
Once you spot anthracnose, your goal is to manage its spread and minimize damage. The plant can often recover if you provide the right care.
- Sanitation is Number One: Immediately rake up and destroy all fallen leaves and fruit from around the infected plant. This removes a major source of spores. Do not add this material to your compost pile.
- Prune it Out: During dry weather, prune out and destroy any twigs and branches showing cankers or dieback. Sterilize your pruning tools between cuts with a 10% bleach solution or rubbing alcohol.
- Improve Air Circulation: Thinning the canopy of a dense tree or shrub allows for better air movement, which helps leaves dry faster and reduces the chance of infection.
- Adjust Your Watering: Avoid overhead sprinklers that wet the foliage. Use a soaker hose to water plants at the soil line, and do it in the morning so the sun can quickly dry any stray splashes.
Treatment Options
For anthracnose, cultural practices are the most effective treatment. Fungicides are primarily preventative and are most useful for high-value plants with a history of severe infections.
Organic Treatment Methods
- Copper-Based Fungicides: These are broad-spectrum fungicides that can be effective at protecting new growth from infection. They must be applied before the disease starts.
Chemical Treatment Options
- Protectant Fungicides: Products containing active ingredients like chlorothalonil, mancozeb, or propiconazole are effective. Always check the label to ensure the product is safe for your specific plant.
Step-by-Step Treatment Procedure
- Begin with thorough sanitation (raking and pruning) in late fall or winter.
- If using a fungicide, timing is critical. The first application should be in early spring, just as the buds begin to break open, to protect the new leaves.
- Follow the product label for mixing instructions and application rates.
- Spray to cover all new leaves and shoots thoroughly.
- Repeat applications every 7 to 14 days, as directed by the label, especially during prolonged cool, wet weather.
Expected Timeline for Recovery
You cannot cure existing spots on leaves. A successful treatment means the new leaves that emerge remain green and healthy. For a deciduous tree, it may look sparse after shedding its leaves, but it will often push out a new set of leaves in summer when the weather becomes warmer and drier. With good sanitation, the plant should be much healthier the following spring.
Prevention Strategies
Preventing anthracnose is far easier than trying to control it once it appears.
- Fall Cleanup is Essential: Your most powerful preventive action is to meticulously rake up and destroy all fallen leaves from susceptible plants in the autumn. This removes the overwintering fungal spores.
- Choose Resistant Varieties: When planting new trees and shrubs, look for cultivars bred for resistance to anthracnose. For example, some varieties of dogwood ('Kousa') and London planetree are much less susceptible than others.
- Maintain Plant Health: A healthy, vigorous plant is better able to withstand disease. Ensure your plants are properly watered during dry spells and fertilized appropriately to avoid stress.
- Promote Airflow: Space plants properly when you install them to ensure good air circulation from the start. Prune trees and shrubs regularly to keep their canopies open to light and air.
