Crabapple losing its leaves? Apple scab could be to blame

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The problem with many plant diseases is that once you notice them, it’s often too late. This is especially true with our trees. Often we don’t look to the tops of our trees for trouble.

The typical homeowner will not notice disease or insect damage until the visible symptoms exceed 10% of the canopy.

One tree disease that seems to sneak up on us every year is apple scab.

Many who grow apples or crabapples (Malus spp.) have probably seen this disease before and may or may not know about it. Apple scab is so common that plant pathologists expect to see it every year. In 2023, our spring started mild, turned warm and then cooled for several weeks. Extended cool weather is a good indicator of increased apple scab pressure on our crabapple and apple trees.

Apple scab (Venturia inaequalis) is a fungus that infects apples, crabapples, mountain ash, pear and Cotoneaster. Symptoms usually begin on the underside of leaves, forming irregularly shaped light brown to olive green lesions. As the infection progresses, the lesions become more circular and the spots eventually turn black or brown.

The infected tissue will thicken creating a bulge on the upper surface of the leaf. The leaves can curl and burn at the edges. Apple scab can progress to leaf petioles and fruit pedicels, causing early leaf and fruit drop. As the canopy becomes sparse in mid to late summer, this is often when homeowners notice it.

Unfortunately, at that time it is too late to treat the tree.

Although usually not fatal to an infected tree, there are some options for controlling apple scab.

Plant resistant varieties. This is the most desirable way to manage apple scab in the landscape. Keep in mind that in excessively wet years even resistant varieties can show signs of this disease.

Keep the area under the tree free of infected leaf residue. Removing dead leaves can be beneficial. Note that apple scab is so common and spreads so easily that the infection is likely to recur. In apple orchards, simply mowing off fallen leaves has been shown to reduce the severity of apple scab in subsequent years.

Promote a healthy tree. Mulch with arborist wood shavings and water the tree during dry spells.

Use preventative fungicide sprays. Fungicides labeled to control apple scab include mancozeb, chlorothalonil, calcium polysulfide, copper sulfate, or potassium bicarbonate. Read and follow all label instructions. Most of these sprays are preventive and not curative. You will therefore have to spray before the fungus appears. Don’t waste money spraying infected leaves.

Once the leaves start to turn yellow and fall off, it’s too late to protect the tree. If all that fungicide spraying sounds like a chore, but a defoliated tree in August is an eyesore in your landscape, you may need to prune horizontally to the soil line, thus removing the tree and replanting with a resistant variety. . There are several hardy crabapples, including the popular Prairiefire.

Fungicide applications for apple scab begin when leaves emerge from their buds and continue for up to two weeks after flower petal fall. Follow fungicide label instructions for reapplication intervals.

Avoid all-purpose fruit tree sprays as they contain insecticides that will kill pollinators that visit your crabapple blossoms.

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