Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001666
01/01/98
Symptoms: Leaf spot, tar spot, leaf blotch, leaf blister and shot hole are symptoms of leaf diseases. Shot hole occurs when dead leaf tissue falls out, leaving a hole.
Cause: A number of leaf diseases that occasionally damage ornamental plants are caused by fungi and bacteria. Many species of fungi cause leaf diseases. Leaf diseases cause leaves to fall prematurely, leaving trees and shrubs without the green factories that produce food for them. Repeated defoliation of trees or shrubs makes them progressively weaker. Most deciduous plants can withstand several defoliations without serious damage, but broad- leaved and narrow-leaved evergreens usually do not recover from a defoliation.
Most fungi and bacteria that cause leaf diseases require a wet leaf surface for an extended time, usually about 24 hours. The wet leaf surface allows the fungal spores to swell, germinate and penetrate the plant and the bacteria to swim to a natural opening in the leaf surface, such as a stomate.
Leaf-infecting diseases are more severe if the fungus or bacterium is present at bud break, when the leaves are tender and new. If the weather is dry during bud break, infection occurs later in wet weather after leaves are expanded. Late infections may be unsightly but seldom harm the plant.
Bacterial Leaf Spots
Symptoms: Bacterial leaf spots may appear similar to fungal leaf spots, so it may be difficult to distinguish the two. Leaf spots caused by bacteria are often initially light green and look "water-soaked". Later, these leaf spots usually turn brown or black and may have definite margins.
Cause: Many species of bacteria may cause bacterial leaf spots of landscape plants. These bacteria are often splashed from the soil onto wet foliage, where they enter a leaf through its breathing holes (stomates) or wounds. Thereafter they spread from leaf to leaf when plants are watered or during rainy periods. Bacterial leaf spot of English Ivy is a good example of a common bacterial leaf spot.
Control: Avoid using high nitrogen fertilizers on susceptible cultivars. Only fungicides containing copper may be effective against bacterial blights and bacterial disease, but copper may burn foliage in some cultivars. The antibiotic streptomycin is used in commercial agriculture, but is not available to the homeowner.
Fungal Leaf Spots
Symptoms: Leaf spots are characterized by brown or black spots randomly scattered across the leaf. The spots may or may not have the appearance of concentric rings. The margins of the spot may be a different color than the center of the spot.
Cause: Leaf spots are caused by many different fungi that attack landscape ornamentals. Rainy periods favor their sporulation and spread to unaffected foliage. Infection may occur at any time, but the disease is usually most severe on new, emerging foliage during rainy periods.
Control: Fungi that cause leaf spots form spores on the leaves. The new spring infections are commonly caused by spores from last fall's leaves. Therefore, rake and burn or compost last season's leaves before buds break in the spring. Help foliage dry quickly after rain, heavy dew, or sprinkler watering. In the case of deciduous trees and shrubs, prune out overthick growth for better penetration of solar radiation and better air circulation to aid in drying foliage. In the case of conifers, it is most important to control weeds that surround the lowest branches. Weeds inhibit air circulation and increase humidity, favoring fungal infection of needles.