Apples are subject to a variety of diseases that can cause minor cosmetic damage or more significant damage, such as reduced yields and even tree death. Fortunately, home growers can avoid most diseases by planting disease resistant varieties. Below are a few of the most common apple diseases:
Apple scab is one of the most common and most serious diseases that afflict apple trees. It usually appears in early to mid-spring and is more prevalent during rainy weather. The disease is caused by the fungus Venturia inqequalis, which overwinters in infected leaves left on the ground. The fungus spores are released in the spring during wet weather and are blown by the wind onto vulnerable, newly emerging leaves.
Apple scab first appears as small, olive-colored lesions on the undersides of the leaves. As the fungus spreads, the top sides of the leaves develop lesions, as well, that may become black or mottled with defined edges. Severely infected trees may become defoliated by mid-summer, making the tree vulnerable to other diseases. The fruit develop black or brown scabs or soft areas. The scabs may appear hardened and cracked, but don’t usually affect the inside of the fruit.
Several apple varieties, including Jonafree, Liberty, Enterprise, Pristine and Williams Pride are completely resistant to apple scab. Choose scab resistant varieties, especially if apple scab is a frequent problem in your area. Avoid Red Delicious, Cortland, McIntosh and Rome Beauty, which are all susceptible to the disease. Promptly rake up and remove leaves and debris in the fall and spray susceptible trees with fungicide in early spring, such as lime sulfur, sulfur or Captan.
Fire blight is a bacterial disease that runs rampant in many parts of the U.S. and is difficult to control. Trees infected with fire blight may have water stained, brown blossoms and brown leaves. The twigs and the branches of the tree may turn brown or black and have open cankers that ooze a thick, brown liquid. The twigs may also turn downward at the tips to resemble a shepherd’s crook. The disease overwinters in infected wood and is spread in the spring through rain and insects.
Plant resistant varieties, such as Jonafree, Liberty, Pristine and Williams Pride and avoid susceptible varieties, including Beacon, Granny Smith, Jonathan, Gala and Fuji. Fertilize the tree in early spring before growth starts and avoid applying excessive fertilizer, which will promote rapid, lush growth that is most susceptible to infections.
Remove infected branches in late winter, while the tree is dormant. Pruning the tree while it is actively growing will likely spread the disease. Burn the branches if possible to kill overwintering bacteria. Do not leave them on your property. Currently, no sprays are recommended for home orchards, according to Ohio State University Extension.
Cork spot may resemble hail or insect damage, but is caused by low soil pH and subsequent calcium deficiency. Cork spot appears as small dimples on the skin of developing apples. The dimples spread to ½-inch wide and may appear corky or soft. The fruit is edible, but the spots reduce its aesthetic appeal.
Add lime to the soil, according to the recommendations of a soil test analysis, if the pH of your soil falls below 6.0. Spray the trees with calcium chloride at a rate of 1.5 tablespoons calcium chloride per gallon of water per tree. Make four applications, beginning immediately after full bloom. Reapply the solution every ten days to help control cork spot.
Powdery mildew is caused by the fungus Podosphaera leucotricha and develops first as white, felt-like growths on the undersides of leaves. As the disease spreads, it causes wilted leaves, stunted growth and black pinpoint specks on the leaves and twigs.
Avoid susceptible varieties, including Granny Smith, Jonathan, Rome and Cortland. Plant the trees in full sun and allow plenty of space between them for good air circulation. Spray the trees in early spring with Myclobutanil, lime sulfur or sulfur.
Rust is an interesting disease because it requires a host plant, such as cedar, quince or hawthorn to develop. The fungus develop in large galls or growths found on the host plant. In spring, the galls dry, releasing the spores into the air where they are carried to apple trees. Rust causes yellow or orange spots on the leaves and distorted or mottled fruit.
To control rust, grow resistant apple varieties and remove any nearby host plants. The spores can travel up to two miles, though, so any neighboring plants may infect your trees. Spray apple trees with sulfur, Myclobutanil or lime sulfur.
Black Rot and Frog Eye Leaf Spot
Black rot and Frog Eye leaf spot refer to the same disease at different points in the disease cycle. The disease, caused by Botryosphaeria obtuse, first manifests as brown spots on the ends of the fruit. The spots enlarge in concentric circles and eventually turn black, rotting the fruit. Leaves may become covered with small brown spots or holes. Later, the disease spreads to the tree limbs, causing cankers which can eventually kill the tree.
To combat this disease, prune out all infected tree materials and burn or discard immediately. Trees infected with fire blight disease may become weakened making them more susceptible to Black Rot. Spray the trees with Captan or sulfur while the disease is in the early stages.
This disease is sometimes confused with winter damage or injury from wet soils because the fungus that causes it, Phytophthora, thrives in wet, heavy soils. Trees infected with this disease show a decline in vigor and growth and may have yellowish leaves that turn purple in the fall.
Fungicides to treat the disease are seldom effective and not recommended for home growers. Instead, plant apple trees in loamy, lightweight soils amended with compost. Build berms or use raised beds if your soil is very heavy or poorly draining.
Crown rot is caused by the same fungus that causes phytophthora rot, but causes decay at the trees’ roots or crown. Symptoms include delayed bud break, leaf and bark discoloration, and twig dieback. Carefully remove some of the soil from the roots of the plant, taking care not to injure the tree. Scrape away the outer bark to reveal interior wood that is reddish-brown and water-soaked, confirming that the tree is infected with crown rot.
There is no cure for crown rot and the tree will likely eventually die. Plant trees in areas with good drainage, as recommended previously.
Commercially grown apples are subject to a variety of chemical treatments because apples are prone to so many disease and insect pests. But home growers can successfully grow apples without a deluge of chemicals by following a few tips: Plant disease resistant varieties suited to your area, grow them in full sun and well-drained soil, pick up debris promptly and learn to live with a few imperfections on the fruits.
More information about common apple tree diseases:
North Dakota State University has a great page of questions and answers about specific apple tree diseases and how to handle them.
New Mexico State University also has a great guide to apple tree diseases that are prevalent in that state, as well as elsewhere.
West Virginia University has an entire set of photographs of the different diseases to help you more easily identify what you might be dealing with.