Leaf spot is a broad term that refers to several diseases that attack the leaves of ornamental plants and shade trees. Most cases of leaf spot come from fungal infections, though bacteria can sometimes be the culprit. Additionally, certain insects cause injuries that closely resemble leaf spot symptoms. These spots are quite common on trees and usually do not need chemical treatment. While leaf spots can lead to some leaf drop, mature plants can withstand even heavy defoliation if it occurs late in the season or doesn’t happen repeatedly. However, young or recently planted trees that lose their leaves are more vulnerable and may suffer lasting harm until they are well established.
Symptoms and Diagnosis
The most noticeable sign of leaf spot disease is the appearance of spots on the leaves. Their size and color differ depending on the type of plant, the organism causing the infection, and how advanced the disease is. These spots are usually brown but can also show up as tan or black. Many develop dark borders or rings, and in some cases, tiny fungal structures appear as black specks within the spots, either grouped together or forming circular patterns. As the disease progresses, individual spots may merge or expand into larger blotches. When the affected areas look angular rather than round, the condition is often identified as anthracnose. Infected leaves can turn yellow and drop off earlier than normal.
Life Cycle
The pathogens responsible for leaf spots often survive in fallen, infected leaves and twigs. In some cases, they can persist in dead twigs still attached to the tree. The heaviest damage usually appears in spring. During damp conditions, spores are carried by wind or splashing rain onto young, tender leaves. Once they land, the moisture helps them germinate and invade the leaf tissue. Overhead irrigation can also create long periods of wetness, which makes it easier for leaf spot diseases to spread.
Common Leaf Spot Diseases
Here are some of the most common types of leaf spot diseases.
Septoria
Septoria leaf spots are usually round with distinct black edges. They often appear as if mold is spreading across the leaf surface. Over time, the affected leaves shrivel, turn black, and drop to the ground, where the spores remain dormant until the following winter.
Venturia
Venturia shows up in early spring with brown or black irregular spots on young leaves. Infected shoots darken, weaken, and often twist or become misshapen. Only fresh growth is vulnerable, while older tissue becomes resistant as the season progresses.
When new growth dies back, small trees may lose height or develop crooked stems. If left untreated, Venturia infections can gradually weaken trees and leave them open to other pests or diseases.
Cedar-Apple Rust
This aggressive fungus can quickly turn healthy ornamentals, apple trees, and berry plants into a troubling sight. Rust fungi create leaf spots shortly after leaves emerge in spring. By late summer, clusters of brownish threads form underneath the spots or on fruits and twigs. These threads release spores that infect the leaves of deciduous trees and shrubs. The disease also thrives on juniper twigs and needles during warm, rainy weather.
Anthracnose
Anthracnose describes a group of fungal diseases that produce dark spots on the foliage of many tree species. While it affects a wide range of trees, plant breeders are introducing more resistant varieties into the nursery trade.
Symptoms include small tan, brown, or black patches on leaves. On ash and maple, the dead tissue often looks irregular, while on sycamore and oak, the spots usually line up along the veins.
Ways to Control and Prevent Leaf Spot Diseases
If leaf spot becomes a serious issue on large trees, it’s best to consult a professional arborist. Still, there are several preventive steps and treatments to manage leaf spot diseases.
Fungicides and Fertilizer
Preventive fungicide sprays or soil treatments should be applied before symptoms appear. In most cases, they aren’t required unless a tree has been losing leaves for multiple seasons. Before adding fertilizer, test the soil around a tree with leaf spot to avoid unnecessary applications.
Moisture Management
Water at the bottom of the plant, soaking the soil and root zone rather than wetting the foliage. Allow the soil to dry a little bit before watering again. A soaker hose works best for watering trees and shrubs. Applying mulch once a year also helps conserve moisture, but avoid piling it high against the trunk as it can trap moisture and cause root rot, disease, and decay.
Spacing and Pruning
When planting, give trees and shrubs enough room to reach their full size without crowding. Regular pruning improves sunlight penetration and air circulation within the canopy, making conditions less favorable for leaf spot diseases.
Removing Debris
Quickly rake and destroy fallen, infected leaves and any pruned or broken branches. Always dispose of diseased material according to local rules, and never add infected leaves to your compost pile, as it can spread the disease further.
