Apples and Leaf Spot Disease

Apples thrive in well-drained soil with at least six hours of sunlight.

Apple and crabapple trees (Malus spp.) are grown domestically and commercially throughout North America. Identifying and managing leaf spot diseases and other ailments is critical to establishing a healthy orchard and producing a quality crop of fruit. While leaf spot diseases rarely deal serious damage to their host, they do cause unsightly growths and deformities that can significantly reduce total fruit yield. Fungal leaf spot diseases are more common in areas with excessive heat or moisture and poor air circulation.

Types

There are many fungal pathogens capable of causing leaf spot disease in apple and crabapple trees in the United States. Fungi from the genera Cercospora, Coniothyrium and Monochaetia are a few of the possible agents behind this condition. While the fungus responsible for an outbreak of leaf spot disease varies, the symptoms and treatment measures are usually similar. However, some types of leaf spot disease are more serious and can spread beyond the foliage of their host plant. Frogeye leaf spot, caused by the Botryosphaeria obtusa fungus, is a serious threat to apple trees compared to many other leaf spot fungi.

Symptoms

The exact symptoms of leaf spot diseases varies depending on the precise fungus responsible. Nearly all leaf spot diseases begin as small specks or freckles on the surface of infested leaves. These tiny spots may be yellow or brown initially, but nearly all of them eventually turn to dark brown or black as the tissues decay. As the rotting legions expand, the infected leaf begins to wilt and eventually falls from the tree. The fungus responsible for frogeye leaf spot also causes a black rot of branches and stems of its host. It destroys woody growth, fruit, flowers and buds as well as leaves, making it one of the most dangerous leaf spot diseases that attack apple trees.

Impact

Leaf spot diseases rarely pose a significant threat to the long-term health of their host. However, they do weaken the tree's immune system, leaving it vulnerable to infestations of more serious fungal pathogens that can infiltrate the plant's internal tissues through damaged tissue and cankers. The combination of a leaf spot disease and one or more insect pests may prove fatal for young or newly established apple trees. Damage caused by leaf spot fungi can significantly reduce the total fruit yield of apple trees by preventing leaves from conducting photosynthesis properly, which reduces the total amount of energy available to the plant for growing reproductive bodies.

Management Techniques

The first line of defense against fungal infection is planting apple trees in a healthy growing location. Individual trees should be spaced far enough apart to prevent them from competing with one another and to allow air circulation between their branches. Gardening tools should be cleaned with a sterilizing solution regularly to prevent mechanical transmission of pathogenic fungi. Local outbreaks of leaf spot fungi can be treated with fungicide and by pruning dead or dying branches, leaves and stems. Most of these fungi survive in plant debris on the ground over winter, so removing this debris helps prevent an outbreak the following year.

References

• University of Illinois Extension; Fungal Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest; July 1998
• University of Missouri Extension; Home Fruit Production: Apples; Michele Warmund; May 2002
• Ohio State Univeristy Extension; Leaf Diseases of Ornamental Trees and Shrubs; Stephen Nameth, et al.
• Ohio State University Extension; Black Rot and Frogeye Leaf Spot of Apple; Michael A. Ellis; 2008
• North Dakota State University; Diseases of Apples and Other Pome Fruits; H. Arthur Lamey, et al.; August 1993