Leaf spot diseases of shade trees and ornamentals

Leaf spot is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of the leaf spots are caused by a variety of fungal pathogens but some are caused by bacteria. Anthracnose is a term for a group of loosely related fungal diseases that often cause irregular blotches on leaves.

Primary hosts in Missouri

All species of trees of shrub can potentially develop leaf spots, but some species are more susceptible than others.

Closeup photo of the fruiting structures of the Cercospora sp. fungus sporulating in the center of the leaf spot.
Figure 2
A closeup of the fruiting structures of the Cercospora sp. fungus sporulating in the center of the leaf spot.

Symptoms and diagnosis

The primary symptom of a leaf spot disease is spots on foliage. See figure 1. The spots will vary in size and color depending on the plant affected the specific organism involved, and the stage of development. Spots are most often brownish, but may be tan or black. Concentric rings or a dark margin around the spot may be present. Fungal bodies may appear as black dots in the center of the spots. See figure 2. Over time the spots may combine to enlarge to form blotches. Spots or blotches that are angular and located around the veins are generally referred to as anthracnose. Leaves may yellow and drop prematurely.

Life cycle

The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the tree. During wet weather, spores are released which may splashed or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering late in the day or during the night, heavy dews and close spacing of plants prolong wetting of the leaf surface and provide more opportunities for fungal or bacterial infections.

Figure 3
Crabapple leaf infected with apple scab.

Integrated management strategies

Live with the disease
Leaf spots are largely an aesthetic problem as few leaf spots seriously damage the host. Also control is seldom achieved after the infection has started.
Remove infected leaves and dead twigs
Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can reinfect the new leaves. This will not cure the problem but it can help minimize infections.
Keep foliage dry
Avoid overhead watering. Use soaker hoses or water early in the day so the foliage can dry before night. Watering can also spread the disease by splashing. Prune plants and space plants to allow for good air circulation that promotes rapid drying of foliage.
Keep plants healthy
Since most plants can tolerate some defoliation, keep them in good health so they can rebound quickly. Avoid over fertilization as it promotes a flush of young leaves that are more susceptible to attack by insects and disease.
Use fungicides wisely
In rare cases of severe infection where the size and value of plants make it practical, applications of fungicides may be helpful. Generally fungicidal control is warranted if:
- Repeated defoliations occur in one year or subsequent years
- The plant is under stress
- The plant is in decline
- The plant is a needled evergreen
- The disease is black spot of roses.
Sprays will not cure the infection but protect leaves from becoming infected. To be effective fungicidal sprays must be begun at bud break before symptoms are noted and be continued at intervals specified by the label (usually 10 to 14 days) through the period of spring rains. Spraying after infection is present will provide little benefit. Recommendations will vary with the disease and fungicide used. It is always good practice to have the disease identified before purchasing a control product. See table 1 for a general listing of active ingredients listed for control of various leaf spotting fungi.
Replace the plant. For plants that chronically are plagued by leaf spots, gardeners find it more convenient to replace a plant with a different species or a variety that is more resistant or tolerant of disease. A nursery can help you in your selection.

Updated 3/31/05