Apple scab on Plants

Symptoms

Apple scab first shows up as spots on the leaves and the fruit. The initial spots are velvety, and their color runs from dark olive to black. These first spots are superficial. You may see signs of infection on the leaves, on the stem, or at the blossom end of the fruit.

Affected tissue can also show dull, olive-green lesions or spots. On the fruit, the spots become scablike with age. The fruit tissue may also become misshapen. Over time, fruit that began with velvety dark spots shows scablike spots and may be misshapen.

Leaves carry the velvety dark spots as well, since the initial symptoms appear on fruits and leaves alike. Those velvety, dark-olive-to-black spots are the initial symptoms to look for. Check the leaves, the stem, and the blossom end of the fruit when you look for scab. A dull olive-green color on a spot is another sign to note. Apple is the plant listed as the host for this problem.

Causes

Apple scab is an infection by the fungus Venturia inaequalis. The same disease is also called black spot of apple or scab of apple. The fungus overwinters in dead leaves on the ground. Leaves left on the ground can carry the disease from one season to the next.

In spring, primary spores are discharged during rain. These spores infect young leaves and fruit during prolonged moisture. Prolonged moisture is the condition during which young leaves and fruits become infected. Wet periods, temperature, and relative humidity are important factors in how the disease develops.

Each of those three factors, wet periods, temperature, and relative humidity, plays a part in disease development. Dead leaves on the ground are where the fungus survives. Four things stand behind the problem: the fungus, the dead leaves, the spring spores, and the weather. Apple varieties named as susceptible include Bellflower, Blushing Gold, Fuji, and Gala.

Diagnosis

Begin with the host, because this page covers apple scab on apple. The spots to look for are velvety and dark olive to black. Those are the initial symptoms, and they appear on both fruit and leaves. Check the leaves, the stem, and the blossom end of the fruit for signs of infection.

On fruit, older spots become scablike and the tissue may be misshapen. Spots that do not match this description may point to a different problem. Think about the weather in your area this spring, because primary spores are discharged during spring rains. Infection happens when young leaves and fruit stay wet for a prolonged period.

Wet periods, temperature, and relative humidity all affect how the disease develops. Dead leaves on the ground are where the fungus survives, so check the ground under the tree. The sources used here name no look-alike problems to rule out. What they do describe is a fungal infection, with a specific set of spots and a specific link to wet weather.

Treatments

The treatments below are grouped as cultural, organic, and chemical.

Remove leaves soon after they have fallen, which reduces the carryover of disease from one season to the next. Never allow fruit to remain on the ground from one season to the next. Fallen fruit and fallen leaves are both worth clearing. A fall foliar fertilizer application of zinc sulfate and urea hastens leaf fall and speeds decomposition of fallen leaves. This lowers the overwintering inoculum. Zinc sulfate and urea are the active ingredients named for the fall fertilizer step.

Plant scab-resistant apple trees. Prune trees annually to improve air circulation and promote rapid drying. Monitor wetness periods throughout spring, for example with a wetness recorder, so trees are adequately protected before and during infection periods.

For an organic approach, treat with lime and sulfur, sulfur alone, or approved fixed copper products, which are organically acceptable. Lime sulfur, sulfur, and fixed copper are the active ingredients named for the organic step.

Chemical options include protectant fungicides, which protect plants against infection at the application site. Systemic fungicides protect plant parts away from the application site. Target the final fungicide sprays as close to harvest as possible to limit storage scab.

This page gives no dose, rate, dilution, or spray interval, because the product label is the legal instruction and must be followed for the crop and site being treated.

Prevention

Clear fallen leaves soon after they drop, since the fungus overwinters in dead leaves on the ground. Do not let fruit stay on the ground from one season to the next. Leaf litter left in place carries the disease into the next season. Choose scab-resistant apple trees when you plant.

Prune each year so air can move through the tree and the leaves dry quickly. Rapid drying after rain is the goal of pruning. Watch wetness periods through spring, because spores infect young leaves and fruit during prolonged moisture. Protect trees before and during infection periods.

Wet periods, temperature, and relative humidity are important factors in disease development. A fall foliar fertilizer of zinc sulfate and urea is another step that lowers overwintering inoculum.

Plants most often affected

The host named for apple scab is apple. Apple is the only host recorded on this page. Susceptible apple varieties include Bellflower, Blushing Gold, Fuji, and Gala. The disease is also known as black spot of apple. If you are choosing a tree, plant scab-resistant apple trees.

• European Crab Apple
• European Mountain Ash