4 Common Apple Tree Diseases

Signs, Causes, and Prevention of Fungal Attacks

Two red apples on a branch.
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Apple trees (Malus spp.) and other plants in the rose family, like hawthorns (Crataegus spp.), are susceptible to many diseases. The good news is that these diseases are often preventable and, even when they aren't, often cause damage mainly on an aesthetic level. Large-scale growers can't tolerate this damage because their fruit must look good to be marketable; tolerance on the part of small-scale growers is often greater.

Learning to identify the most common apple tree diseases is the first step in handling worst-case scenarios. But you can avoid such scenarios altogether by buying the right cultivars and/or practicing sound horticultural hygiene.

Fungi are at the heart of some of the most common apple tree diseases. In each case, for small-scale growers, prevention is preferable to treating trees after they have become diseased via fungicidal sprays. Since fungi spread from infected plants to healthy ones via airborne spores and thrive in wet conditions, prevention involves improving soil drainage, providing proper spacing, and removing diseased plant parts as soon as you find them.

But large-scale growers often must resort to fungicides, which vary in terms of effectiveness and toxicity. If you do decide to apply a fungicidal treatment, check first with your local county extension because spraying schedules can be complicated. In addition to practicing good garden hygiene, such as by removing fallen leaves in autumn, you can prevent some of these fungal diseases through proper plant selection.

Apple Scab

In the case of apple scab, the fungus is Venturia inaequalis. You'll see the first sign of apple scab in the form of a lesion on the tree's new leaves, in early spring or mid-spring. The lesion will be darker than the leaf color; on the leaf's underside (which is light green), lesions will be olive-colored, and on top of the leaf (which is a darker green), lesions will be black.

Infected leaves may fall off altogether in summer. If the tree still manages to produce fruit, the apples will also have dark, scabby lesions. Luckily, the apples are usually still edible: Just peel the skin off before eating.

It's easy for the small-scale grower to prevent apple scab because the cause is simply lack of observation and poor hygiene. An infestation starts out small, perhaps even going unnoticed. The real problem begins when you allow the infected leaves that fall to the ground at the end of the growing season to remain there all winter.

Venturia inaequalis overwinters in this fallen, infected foliage and uses it as a launching pad for a spring invasion. Rainy weather provides ideal conditions for this invasion. Fungal spores blow up onto the new leaves, infecting them.

Cultivars resistant to the disease include:

  • Crimson Crisp
  • Gold Rush
  • Mac-Free

Powdery Mildew

Podosphaera leucotricha is the fungus responsible for this common apple tree disease. Even if you've never grown apples, you probably know about this disease because powdery mildew infects popular ornamental plants, including garden phlox (Phlox paniculata). It's unlikely to kill your plant, but it will sap its strength. It's easily recognized: True to its name, powdery mildew is that whitish powder that coats the leaves of many of your garden plants.

apple leaves infected and damaged by fungus disease powdery mildew
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If you're experiencing powdery mildew, the cause can be traced back to last year's garden (even if you didn't see it). The fungus overwinters in fallen, infected leaves. Spores blow up onto healthy leaves to infect them or are brought up by insects. Even a heavy storm can be the culprit because a pounding rain can send the spores flying up to your tree's leaves.

For prevention, besides cleaning up fallen leaves in autumn, follow the spacing requirements indicated on your plant labels to ensure good air circulation. Also, avoid overhead watering.

Types resistant to powdery mildew include:

  • Liberty
  • Gold Rush

Cedar-Quince Rust

Gymnosporangium clavipes is the scientific name for this strange fungus, which needs a host plant to attack your apple trees. For example, if you grow flowering quince shrubs (Chaenomeles speciosa, another member of the rose family) in your landscape, they can serve as hosts. It will spread from them to your apple trees.

The sign of cedar-quince rust is the presence of rusty spots on your tree's leaves; the apples themselves may also be misshapen and/or suffer from mottling. If you grow a type of plant that can serve as a host, you can also look for the sign that the host is carrying the disease: rust galls, which sprout orangey-rusty "horns" in spring that send out the spores that will attack your apple trees.

To prevent rust, get rid of the host plants. Plus grow the following rust-resistant apple cultivars:

  • Redfree
  • William’s Pride
  • Freedom

Phytophthora Rots

Phytophthora is a fungus-like disease that saps a tree's strength. It can attack various parts of the tree, including the trunk or roots.

If you suspect your apple tree may be infested with a Phytophthora disease, perform the same sort of test you would to see if an arborvitae shrub (Thuja) is dead or alive. Take a sharp knife and remove a small strip of the outer bark of the trunk to check on the color underneath. Healthy wood is green here; diseased wood will be orange or brown.

The cause of this disease is often contamination, which can come from soil you've brought onto the property, irrigation water, or even the plant itself (if you haven't bought from a reputable nursery).

For prevention, in addition to being careful to avoid contamination, take moisture-related precautions as you would for fungus prevention (since Phytophthora, too, thrives in moist conditions). For example, plant on landscape berms or in raised beds instead of at ground level to improve drainage. Also, when buying, ask for a tree with a Geneva series rootstock; it will have superior resistance.