Alternaria leaf spot is a plant disease caused by fungi from the Alternaria genus. These common fungi affect a wide range of plants, from vegetables to ornamental flowers, and specific fungal species often target particular plant families. The disease manifests as leaf spots and can impact plant health. In edible crops, it may lead to reduced yields and poor storage quality.
Identifying Alternaria Leaf Spot
Signs of Alternaria leaf spot often begin on the lower, older leaves of the plant. Initial symptoms appear as small, dark brown to black spots, sometimes encircled by a yellow halo. As the disease progresses, these spots enlarge and form a “target” or “bullseye” pattern of dark, concentric rings. The centers of these spots can dry out and fall away, leaving a “shot-hole” appearance. Multiple spots may merge into larger blotches, causing the entire leaf to yellow, wither, and drop.
This disease affects many common garden plants, including:
- Tomatoes
- Potatoes
- Cucumbers and melons
- Cabbage and broccoli
- Zinnias and marigolds
Environmental Factors for Fungal Growth
The Alternaria fungus thrives in warmth and humidity. Prolonged leaf wetness from rain, dew, or overhead irrigation allows spores to germinate and infect plant tissue. High humidity further encourages fungal growth.
Spores are the primary means of dispersal and can be carried by wind or splashed by water from infected leaves to healthy ones. Contaminated tools, hands, or clothing can also move spores between plants. The fungus overwinters on infected plant debris in the soil, serving as an infection source for the next year.
Management and Prevention Techniques
Effective control of Alternaria leaf spot integrates cultural practices designed to create an environment less favorable to the fungus. Proper plant spacing promotes good air circulation, allowing foliage to dry quickly. When watering, apply water directly to the base of the plant to keep the leaves as dry as possible.
If signs of infection appear, promptly remove and destroy affected leaves to prevent the spread of spores. At the end of the growing season, a thorough cleanup of all plant debris is important to remove overwintering fungi. For vegetable gardens, practicing crop rotation by not planting susceptible crops in the same location for a few years can help break the disease cycle.
In situations where the disease is severe, fungicides can be used. Products containing copper or chlorothalonil are often applied to manage Alternaria leaf spot. These treatments are most effective when used preventatively, before the disease becomes widespread. It is important to read and follow all instructions on the product label for safe and effective application.