Apple & Crabapple Diseases

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. However, do not expect to produce ‘store quality ‘ apples unless you are willing to go on a spray-intense pesticide program. 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 disease resistant apple varieties, such as ‘Liberty’ and ‘Freedom’, is strongly recommended. You still will need to combat summer diseases. They 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 sub-acid. Quality and fruit color poor in eastern part of the state. Ripens about September 10. High resistance to cedar apple rust and scab, 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. High resistance to scab, resistant to cedar apple rust, and somewhat resistant to powdery mildew and fire blight.

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 on young fruit and leaf.
Clemson University – USDA Cooperative Extension Slide Series, www.ipmimages.org

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 that are 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.

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: captan, mancozeb, thiophanate-methyl or sulfur. See Table 1 for examples of brands and specific fungicide products. Apply all chemicals according to directions on the label. Apple varieties resistant to apple scab are ‘Goldrush’, ‘Enterprise’, ‘Pristine’, ‘Gala Supreme’, ‘Liberty’, ‘Freedom’, and ‘Jonafree’.

Fungicides for crabapple trees are captan, mancozeb or thiophanate-methyl. See Table 1 for examples of brands and specific fungicide products. Apply all pesticides according to directions on the label.

Cedar apple rust leaf spots on apple.
Clemson University – USDA Cooperative Extension Slide Series, www.ipmimages.org

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.

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.

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.

Cedar apple rust gall on cedar.
Karen Russ, ©2007 HGIC, Clemson Extension

Cedar apple rust gall on cedar in spring, after rain.
Joseph O’Brien, USDA Forest Service, www.ipmimages.org

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.

Prevention & Treatment: Plant resistant varieties. If possible, remove Eastern 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: mancozeb (do not apply after bloom), myclobutanil or sulfur. The fungicide for use on crabapple trees is mancozeb (do not spray after bloom) or myclobutanil. See Table 1 for examples of brands and specific fungicide products. 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 that contains bacteria can be seen on the stems and branches.

Early symptoms of fire blight on apple.
University – USDA Cooperative Extension Slide Series, ww.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). Make 3 applications with the first at the beginning of bloom. Make 2 more applications at 3 day intervals during bloom.

Additionally, if fire blight is expected to be severe on apples in the upcoming season, a copper fungicide spray (copper sulfate or copper ammonium complex) can be made during dormancy – just before buds open.

For crabapple trees, use either streptomycin or a copper fungicide (copper sulfate or copper ammonium complex) during bloom. See Table 1 for examples of brands and specific products for fire blight disease control. 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 on young growth of apple.
University – USDA Cooperative Extension Slide Series, www.ipmimages.org

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.

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, myclobutanil, a copper fungicide or sulfur. Some of these chemicals can injure the tree if applied at the wrong time. See Table 1 for examples of brands and specific fungicide products. 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.

Black rot damage to apple fruit.
Clemson University – USDA Cooperative Extension Slide Series, www.ipmimages.org

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.

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. See Table 1 for examples of brands and specific fungicide products.

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 (small dots) and sooty blotch on apple fruit.
Clemson University – USDA Cooperative Extension Slide Series, www.ipmimages.org

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.

Prevention & Treatment: Maintain good air circulation by pruning, to keep trees from becoming too dense. Thinning of fruit is also important. Apple varieties resistant to sooty blotch and fly speck are ‘Pristine’ and ‘Enterprise’.

If disease is severe enough to warrant chemical control, choose one of the following fungicides for use on apples: thiophanate-methyl, a copper fungicide, captan or 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. Consider apple trees on dwarfing rootstocks to reduce the height of the mature tree.

Clemson Fruit Bag developed for the home orchard and hobbyist fruit gardener will work on apples and peaches.
Guido Schnabel, ©2015, Clemson University.

Fruit Bagging

Though products are available at gardening stores for homeowners, many gardeners are not inclined to use pesticide applications for home fruit production. Instead, hobbyist gardeners may use bags to protect fruit from pests and diseases. Clemson University has tested and is promoting the use of specialty bags that, if used properly, allow for production of high quality fruit with very little pesticide input. The bags are recommended for use in a three step fashion: (i) properly take care of your trees to minimize tree stress; (ii) protect your fruit from pests and insects between bloom and the day of bagging; and (iii) enclose ½ to ¾ inch, green fruit (typically 3 weeks after bloom) with a specialty bag to be removed at harvest. If the apple fruit are bagged, the bags should be removed approximately 3 weeks prior to harvest to allow the fruits to color properly. For purchase information and use instructions please see: Clemson Fruit Bags or simply google this page using the key words “Clemson Fruit Bags”.

Table 1. Fungicides Labeled for Apple & Crabapple Disease Control.

Table 2. General Spray Guide for Apple & Crabapple Disease Control

Pesticides are updated annually. Last updates were done on 12/18 by Joey Williamson.

Originally published 9/99