Why is my crabapple tree losing leaves?
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EmailApple scab fungus on backyard apple trees can leave plants with few leaves, but the tree is not dying.
Why are my apples and crabapples looking so bad? This is a perennial question both on Ask Extension and Michigan State University Extension’s Lawn and Garden Hotline, especially during wet seasons. The fungus, Venturia inaequalis, known as apple scab, is a leaf spot disease that can cause serious leaf drop on susceptible apple trees, including crabapples.
Wet springs provide conditions for apple scab spores to develop. Scab-infected leaves from the previous season left on the ground around the tree are the source of fungal spores that can infect the crabapple the next spring. Spores from these old leaves are carried on air currents to new developing leaves. This primary infection produces olive-colored spots on leaves.
As the fungus grows on the leaves, new spores are produced, starting a secondary infection of leaves and fruit. Damaged leaves become curled and yellowish with lesions, eventually turning brown. Heavy disease pressure leads to premature leaf drop. Fruit infected by apple scab develops raised scab-like lesions and severely infected fruit will drop from the tree.
Crabapple varieties can vary greatly in their susceptibility to apple scab (Venturia inaequalis). Susceptible trees in a high-infection year may be defoliated by August, which reduces the plant’s ability to produce and store energy for future growth. This weakens the tree and reduces its vigor and ability to overcome environmental stresses. A colleague of mine has a 40–50-year-old crabapple tree in Flint, Michigan, that flowers every spring, if inconsistently, and loses all its leaves by summer. Trees may lose all their leaves each summer but generate enough energy before defoliation that they will still flower every spring.
What to do about apple scab
A few questions need to be considered when deciding how to manage this leaf spot disease. Does it occur every year? If it does occur every year, how serious is the impact? Does the tree retain most of its leaves?
One way to manage apple scab is to do nothing. This works well if you have a crabapple where the loss of leaves does not outweigh the beauty of the spring flowers. The tree will continue to get infected with apple scab, especially in wet years, and may over time experience decline. If your tree is close to other apple trees or if you would like to harvest fruit from the tree, you’ll need to be more involved.
For trees that are susceptible to apple scab and retention of leaves is the goal, management requires applying fungicides at the correct time. Fungicides work best as preventative treatments with a spray program that starts at the first sign of leaves in spring and continues through the humid weather and moderate temperatures of spring into early summer. University of Minnesota Extension provides detailed information on managing apple scab on both ornamental and fruiting trees at “Apple scab of apples and crabapples.”
Whether you decide to use a fungicide or not, there are a few steps that may help reduce the spread of the fungus. Trim trees in late winter or early spring to allow good air circulation, which reduces the time plants remain wet after a rain or from dew. Evaluate the surrounding landscape for overgrown trees and shrubs that are shading, growing into or are crowding the susceptible tree. Some plants may need to be trimmed back away from the tree to increase light and air circulation. Also, make sure sprinklers are not wetting the crabapple leaves, creating conditions where apple scab thrives.
Sanitation can help reduce the source of the fungus. Pick up and dispose of fallen leaves and fruit infected by the fungus, but diseased tissue should not go into compost. This will not eliminate all sources of the fungus since spores can be carried by the wind from apple and crabapple leaves in neighboring yards, but it may reduce the early impact of the disease.
Another choice would be to remove a very susceptible crabapple and replace it with a resistant variety. With over 100 varieties available in the nursery industry, you can pick and choose the size and shape of the plant, flower and fruit color, fruit size and also resistance to the apple scab pathogen. For a list of apple scab resistance crabapples, see the Purdue Extension publication, “Crabapples Resistant to Apple Scab and Japanese Beetle in Indiana,” or the previously mentioned University of Minnesota article.
