Apple & Crabapple Diseases

Prepared by Nancy Doubrava, HGIC Information Specialist, R. W. Miller, Jr., Extension Plant Pathologist and James H. Blake, Extension Plant Pathologist, Clemson University. (New 09/99. Images added 09/07.)

HGIC 2000

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Growing apples and crabapples in South Carolina can be both fun and rewarding. The success of your apple-growing enterprise will depend largely on the care and attention the trees are given throughout their lifetimes. Producing 'store quality ' apples requires attention to pruning, cultural practices and following a regular preventative spray program. Typically, 12 to 18 applications are required per season. The sprays are usually a combination of fungicide and insecticide applied at the same time. More information is available in HGIC 1007, Crabapple, and HGIC 1350, Apple.

Many diseases commonly occur on apple (Malus domestica) and flowering crabapple trees (Malus species), which can reduce flowering and the quality of the fruit in South Carolina. Planting resistant varieties is one of the best ways to reduce many of these disease problems.

In the home garden, planting the apple varieties 'Liberty' or 'Freedom' can help to reduce many disease problems. These are both resistant to apple scab and have some resistance to cedar apple rust, powdery mildew and fire blight. Both are susceptible to the summer diseases, which include black rot, bitter rot, white rot, sooty blotch, flyspeck, Brooks fruit spot and black pox. Summer diseases are particularly devastating in South Carolina and can result in 100-percent crop loss. The quality of both cultivars is better in the mountainous region.

  • 'Liberty' - Fruit is red striped to mostly red over yellow ground color. Flesh crisp, juicy, yellowish and subacid. Quality and fruit color poor in eastern part of the state. Ripens about September 10. High resistance to cedar apple rust, moderate resistance to powdery mildew and fire blight.
  • 'Freedom' - Fruit is predominately red, with bright red stripes on a yellow background. Flesh is cream colored, firm, juicy. Ripens around September 5. Susceptible to fire blight, somewhat resistant to powdery mildew, resistant to cedar apple rust.

In South Carolina, the recommended crabapple varieties that are resistant to many common diseases include 'Adams, ' 'Mary Potter,' 'Professor Sprenger,' 'Red Baron' and 'Indian Magic.'

Apple Scab

One of the most common diseases, apple scab, is caused by the fungus Venturia inaequalis. The disease begins in spring as dark, olive green leaf spots less than ½ inch in diameter. Severe infections can affect the entire leaf, causing it to turn brown and drop from the tree. Slightly raised, black spots deform the fruits.

Apple scab spots on a young fruit and leaf of apple
Apple scab on young fruit and leaf.
Clemson University - USDA Cooperative Extension Slide Series, www.ipmimages.org

Prevention & Treatment: Plant resistant varieties for best control. Rake and remove leaves to reduce early spring infection sources. If disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apple trees: thiophanate-methyl, bordeaux, sulfur, captan or lime sulfur. Apply all chemicals according to directions on the label.

Fungicides for crabapple trees are bordeaux 8-8-100, captan, captan and insecticide combination spray, chlorothalonil (do not eat fruit), propiconazole (do not eat fruit), ferbam, lime sulfur, mancozeb, or thiophanate-methyl. Apply all pesticides according to directions on the label.

Cedar-Apple Rust

This fungus disease of apple and crabapple is caused by Gymnosporangium species and requires another host plant, Eastern red cedar (Juniperus virginiana) or other ornamental junipers to complete its life cycle. The disease spreads from the cedar to the apple and then back to the cedar. It can be a severe problem wherever these two are grown together, and most ornamental crabapples and apples are susceptible.

Bright orange spots caused by cedar apple rust on upper and lower surfaces of apple leaves
Cedar apple rust leaf spots on apple.
Clemson University - USDA Cooperative Extension Slide Series, www.ipmimages.org

This disease has raised spots on leaves that are bright orange-yellow. Leaves and fruit can drop from the tree. Severe defoliation can lead to reduced bloom the next season. Spots develop primarily on the leaves in mid- to late spring.

Dry brown stage of a cedar apple rust gall on cedar
Cedar apple rust gall on cedar.
Karen Russ, 2007 HGIC, Clemson Extension

Infected fruit is often small and distorted. On the Eastern red cedar, hard brown galls up to 2-inches in diameter form near the ends of branches in the summer. In the spring following a rain, the galls produce large, orange, gelatin-like tendrils, full of spores, which can blow up to a ½ mile to infect nearby apple or crabapple trees.

A cedar apple rust gall after a spring rain, producing orange gelatinous tendrils
Cedar apple rust gall on cedar in spring, after rain.
Joseph O'Brien, USDA Forest Service, www.ipmimages.org

Prevention & Treatment: Plant resistant varieties. If possible, remove red cedars from the area or prune out galls on nearby cedars. If disease is severe enough to warrant chemical control, select from the chemicals below. For apples select one of the following: ferbam, sulfur or triadimefon. The fungicides for use on crabapple trees are chlorothalonil (do not eat fruit), ferbam, mancozeb, propiconazole (do not eat fruit) or triadimefon. Apply all chemicals according to directions on the label.

Fire Blight

Fire blight is a devastating disease caused by the bacterium Erwinia amylovora and is very difficult to control. The disease develops rapidly in early spring during rainy weather when temperatures are above 60 °F and the tree is in bloom. Blossoms and young leafy twigs show the first symptoms, appearing wilted or shriveled and turning brown to black. The tips of infected young twigs wilt and die, forming a shepherd's crook as the disease moves down the branch. Dead leaves often remain attached to the branch. During wet weather, a milky-like, sticky liquid containing bacteria can be seen on the stems and branches.

Early symptoms of fire blight on a young apple shoot. The end of the twig, young leaves and a newly formed fruit are blackening and withering.
Early symptoms of fire blight on apple.
University - USDA Cooperative Extension Slide Series, www.ipmimages.org

Prevention & Treatment: Remove all infection sources, such as blighted twigs and cankers, before growth starts in the spring. Pruning cuts should be made 12 to 18 inches below any sign of infected tissue. Disinfect all pruning tools between each cut, using a solution of 1 part household bleach to 9 parts water. Succulent new growth is easily infected, if injured by insects, hail or wind. Avoid high nitrogen fertilization, which increases succulent growth.

Chemical control is difficult. If disease is severe enough to warrant chemical control on apple trees, use streptomycin during bloom when temperatures are above 60 °F and rain is possible (wait 50 days to harvest). For crabapple trees, use either streptomycin, copper hydroxide or basic copper sulfate. Apply all chemicals according to directions on the label. More information about fire blight is available in HGIC 2208, Fire Blight of Fruit Trees.

Powdery Mildew

This disease is caused by the fungi Sphaerotheca species or Podosphaera species and is most prevalent during dry, hot periods. The fungus causes gray-white powdery patches on leaves and new shoots. New growth is often stunted, curled and distorted. Fruit may turn russet-colored and develop poorly.

The grayish-white powdery coating of powdery mildew on young apple leaves
Powdery mildew on young growth of apple.
University - USDA Cooperative Extension Slide Series , www.ipmimages.org

Prevention & Treatment: Prune out branches or infected twigs early in the season. If disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apple trees and crabapple trees: thiophanate-methyl, basic copper sulfate, lime sulfur, wettable sulfur (minimum 80 percent), propiconazole (do not eat fruit) or triadimefon. Some of these chemicals can injure the tree if applied at the wrong time. Apply all chemicals according to directions on the label.

Black Rot (Frogeye Leaf Spot)

The fungus, Physalospora obtusa (Botryosphaeia obtusa), causes black rot. Highly susceptible crabapple varieties may lose most of their leaves, which weakens the tree and reduces flowering the next year. The disease begins on the leaf as a purple speck that enlarges to have a brown or tan center, which looks like a frog's eye. Heavily infected leaves drop from the tree. Limbs may have slightly sunken, reddish brown areas called cankers.

Infected fruits begin with tiny red or purple spots occurring opposite the stem end. After a few weeks the spots enlarge and have alternating zones of black and brown. The rot eventually affects the entire fruit, which wrinkles, mummifies and often remains attached to the tree.

Black rot on an apple fruit, gradually engulfing the entire fruit
Black rot damage to apple fruit.
Clemson University - USDA Cooperative Extension Slide Series, www.ipmimages.org

Prevention & Treatment: Remove and discard dead branches and diseased fruit, called mummies, where the fungus overwinters. The fungicides captan and thiophanate-methyl are effective if applied early and at regular intervals throughout the season.

Flyspeck & Sooty Blotch

These two diseases caused by the fungi Schizothyrium pomi and Gloeodes pomigena, respectively, infect the surface of the fruit and are mainly cosmetic problems. They often occur together, even though they are each a distinctive disease. Although unsightly, the fruit is still edible. The sooty blotch will wipe off of the fruit and fly speck will not.

Flyspeck's name describes it well, since this disease looks like groups of very small superficial black dots on the surface of the fruit. The dots are slightly elevated and occur in groups of six to 50. Sooty blotch looks like a brown or black blotch (¼-inch in diameter) on the fruit. Spots may coalesce to cover the entire fruit. During the summer these diseases develop during cool rainy weather, particularly in dense, unpruned trees with poor air circulation.

Tiny spots of fly speck, and larger, blurred spots of sooty blotch on an apple fruit
Flyspeck (small dots) and sooty blotch on apple fruit.
Clemson University - USDA Cooperative Extension Slide Series, www.ipmimages.org

Prevention & Treatment: Maintain good air circulation by pruning, to keep trees from becoming too dense. Thinning of fruit is also important. If disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apples: thiophanate-methyl, basic copper sulfate captan (must wait 14 days to harvest), ferbam, sulfur or lime sulfur. Apply all chemicals according to directions on the label.

Note: Control of diseases and insects on large trees is usually not feasible, since adequate coverage of the foliage with a pesticide cannot be achieved.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.