Apple canker is just one common apple tree disease.
The various diseases that affect apple trees can lead to cosmetic damage, reduced tree productivity, or even death. Fortunately, most diseases can be avoided by selecting disease resistant tree varieties. Here are a few of the most common apple diseases:
Apple Scab
Apple scab is one of the most prevalent diseases impacting apple trees, and can be one of the most dangerous as well. It normally appears in early to mid spring, especially during wet weather. The disease is caused by the fungus Venturia inqequalis.
This fungus survives winters in infected leaf litter. It releases spores in the spring that blow onto vulnerable, new leaves during windy, wet weather. The simplest method of prevention is removing fallen leaves and other debris from the ground after autumn.
Apple scab initially manifests on the leaves as tiny olive-colored lesions. As the associated fungus continues to develop, the lesions on the upper sides of the leaves also start to form, turning black or mottled with distinct borders. Severely affected trees may shed leaves by mid-summer, which predisposes the tree to additional infections.
The fruit also develops scabs that are black or brown and soft to touch. The scabs may be brittle and fissured on the surface but the interior remains unblemished.
Some cultivars, which include Jonafree, Liberty, Enterprise, Pristine, and Williams Pride, demonstrate complete resistance to apple scab. If this disease is common in your region, prioritize scab resistant varieties.
Avoid red delicious, Cortland, McIntosh and Rome beauty which are all susceptible to the disease. In the fall, promptly rake and remove leaves and other debris to reduce sources of potential infections, and in early spring, apply fungicide lime-sulfur, sulfur, or Captan on susceptible apple cultivars.
Fire Blight
Fire blight is an endemic bacterial disease of several regions of the United States that is hard to manage. Infested trees may exhibit the symptoms of water-soaked, brown flowers and abscission of brown leaves resembling apples. The limbs and branches of the plant can become chlorotic and necrotic, developing lesions filled with thick, brown exudates.
Additionally, the twigs may display a downward curl at the tips, resembling a shepherd’s crook. This disease is known to overwinter in diseased wood and is disseminated by rain and insect activity in the spring.
Avoid planting highly susceptible varieties such as Beacon, Granny Smith, Jonathan, Gala, and Fuji and instead choose resistant ones such as Jonafree, Liberty, Pristine and Williams Pride. Fertilize in early spring before growth begins, but restrain from overfertilization to prevent rampant lush growth, as this will leave the tree vulnerable to infections.
To mitigate fire blight, infected branches must be removed during late winter while the tree is still dormant. Active pruning during warmer months of growth poses a risk of aggravating the spread of the disease.
Burning the infected branches, if feasible, helps eliminate overwintering bacteria. These branches should not be left on your property. Currently, no sprays are recommended for home orchards, as per the Ohio State University Extension.
Cork Spot
Cork spot may be mistaken for hail or insect damage, but it is caused by a deficiency in calcium due to low soil pH. Cork spot manifests itself with small dimples on the surface of the fruit. Over time, these dimples expand to a half-inch and may take on a corky or soft appearance. Although damaged, the fruit remains edible.
If your soil pH falls below 6.0, add lime to the soil per the recommendations of a soil test analysis. Additionally, spray the trees with calcium chloride at the rate of 1.5 tablespoons calcium chloride per gallon of water per tree. Apply calcium chloride four times starting right after full bloom. Ten-day reapplication helps control cork spot.
Powdery Mildew
Powdery mildew starts as a white, felt-like fungal growth on the undersides of leaves and is caused by the fungus Podosphaera leucotricha. As the disease worsens, it leads to withered leaves, stunted growth, and the appearance of black pinpoint specks on leaves and twigs.
Avoid planting susceptible varieties like Cortland, Rome, Granny Smith, and Jonathan. For best results, plant the trees in full sun with ample spacing for good air circulation and air movement between the trees. Early spring treatments with Myclobutanil, lime sulfur or sulfur can be applied.
Rust
Rust is an interesting disease as it requires host plants like cedar, hawthorn, or quince for development. The fungus develops in large galls or growths found on the host plant. In spring, the galls dry and release airborne spores which can be carried to apple trees. The disease causes yellow or orange spots on leaves, and the fruit can also be distorted or mottled.
For control, grow resistant apple cultivars and remove remaining host plants, as the spores can be transported up to two miles from the source. Spray apple trees with sulfur, Myclobutanil, or lime sulfur for treatment.
Black Rot and Frog Eye Leaf Spot
These names relate to the same disease in its different cycles. Botryosphaeria obtuse, the fungus responsible for the disease, starts by causing a small brown spot on the ends of the fruit. Concentric circles form around these spots which leads to the fruit blackening and decaying. Small brown spots and holes may also appear on leaves. Eventually, the disease progresses to the lower tree limbs forming cankers that may lead to death of the tree.
To prevent further damage, all infected tree parts should be pruned and burned or disposed off immediately. Trees infected with fire blight disease may become weakened making them more susceptible to Black Rot. Use Captan or sulfur sprays on the trees during the early stages of the disease.
Phytophthora Rot
The moist and heavy soils that foster this rot are often misdiagnosed as winter damage, injury from wet soil, or flooding. Phytophthora, the fungus responsible for this rot, thrives in wet conditions. Infected trees demonstrate declining vigor and growth with yellow leaves turning purple during fall.
Fungicides to treat the disease are not effective and not suggested for hobbyist growers. Instead, grow apple trees in loamy, light soils enriched with compost. Construct berms or employ raised beds if your soil is too dense or is poorly draining.
Crown Rot
Crown rot is caused by the same fungus responsible for phytophthora rot but causes decay at the root or crown region of a tree. Signs consist of delayed bud break, leaf and bark color changes, and twig dieback. To confirm if a tree has crown rot, the bark can be scraped revealing reddish-brown, water-soaked wood which suggests rot. The tree is then infected with crown rot.
There is no treatment for crown rot. In most cases, a tree burdened with crown rot will eventually die. As mentioned earlier, plant trees in regions with good drainage.
Due to the many diseases and insect pests that affect apples, commercially grown apples undergo several chemical treatments. However, home growers can avoid excessive chemicals by following a few strategies such as planting disease resistant varieties suited to the area, full sun and well-drained soil, prompt debris removal, and accepting minor imperfections on homegrown apples.