Apple scab causes naked crabapple trees

Master Gardener

What causes attractive crabapple trees to drop their leaves in August?  A fungal disease called apple scab. Unfortunately many of the older cultivars of crabapple trees are susceptible to apple scab.

A fungus called Venturia inaequalis infects crabapple leaves early in the spring.  Infections occur during moist conditions (rain, dew or constant irrigation). The temperature affects the severity of infections. In order for infection to occur in cool weather, the plants must remain wet longer than in warm weather. Most people don’t notice the disease until the infected leaves turn yellow and fall off the tree. The crabapple that was spectacular while blooming in spring has become an eyesore by August.

Symptoms of apple scab usually start on the undersides of leaves. At first, spots are small, irregular lesions that are light brown to olive green in color. As infection continues, lesions become more circular and olive green to black in color. Leaves may curl and scorch at the margins. Leaves usually turn yellow and fall off. If the fruit stems become infected, fruits may drop early, develop scabby lesions, or fruit may be deformed and cracked. Very susceptible trees become defoliated. Several years of early leaf loss can result in decreased growth, reduced bloom and increased susceptibility to winter injury.

The apple scab fungus spends the winter in the fallen diseased leaves. In spring, the fungus forcibly ejects spores into the air from the fallen leaves. These spores are carried by wind to newly developing leaves, flowers, fruit or green twigs. Spores need several hours of moisture on the plant surface in order to start new infections. Infections grow into spots or blotches that are capable of producing new spores within 9-17 days. These spores are spread by wind and splashing rain or irrigation throughout the canopy, initiating new infections. This cycle can repeat many times throughout the growing season whenever leaves remain wet for a sufficient number of hours. Warm rainy weather in the spring and summer results in repeated disease cycles and increases the severity of the disease.

Why do some crabapple trees get hit with scab every year, while others seem to be unharmed most years? Crabapple cultivars vary greatly in their ability to fend off the apple scab fungus. Disease resistance is the most effective strategy for managing scab of flowering crabapples. Several crabapple cultivars have been developed over the years that are hardy in Minnesota and include not only scab resistance but also beautiful flower color, graceful form and bright fruit. Other cultivars are resistant, but even a resistant variety can get apple scab if the weather is very favorable for disease, but in most years it will be disease-free. To find a list of cultivars that show strong resistance and are tolerant of Minnesota’s low winter temperatures go to the University of Minnesota website: www.extension.umn.edu/garden type ‘crabapple trees’ in the search box.

With apple scab management you have two options:

With existing crabapple trees you can do nothing and let the tree defoliate each summer. Apple scab is generally not life threatening for the plant, but certainly lessens its ornamental appeal. As with other diseases, try to keep plants healthy by watering during drought and fertilizing periodically.

Second option is a fungicide program. There are several fungicides labeled for apple scab control. Be sure to read and follow all label directions and precautions. Because spores are released so early in the growing season, fungicide sprays must begin when the first green leaf tips emerge in spring. Sprays should be repeated until petal drop for crabapple. Remember the battle against scab is won or lost during late April through early June. If some spray intervals are missed, apple scab would still be lessened but complete control may be lost. Fungicide sprays are protectants against infection so new leaves have to be sprayed before infection occurs. Thorough and uniform covering of all leaves and developing fruits is required for control. In addition fungicide sprays have to be applied every year to protect the tree.

Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. Example: (7 to 10 days after spraying, you will need to spray again). Several factors affect what spray interval is most appropriate. In plantings where there was a severe scab infection the previous year, use the shortest interval. In plantings where scab has not been a problem, a longer interval will probably give adequate protection. In addition, if the weather is dry the longer interval is acceptable, whereas during rainy weather the shorter interval is needed. The name of the plant being treated MUST BE LISTED on the fungicide label or the product should not be used! Some products are registered for use on ornamental crabapples but are not safe to use on crabapple fruit intended for human consumption. Remember once leaves start to yellow and fall off the tree that it is too late to spray fungicide for control during the current growing season.

Following good sanitation practices may help.

Remove and destroy infected leaves, flowers, and fruit as soon as possible.

Rake up and destroy fallen leaves before the first snowfall to eliminate locations where the fungus can survive to re-infect the plant the following growing season.

Do not overcrowd plants – use size at maturity as a spacing guide.

Prune crabapple trees so that the branches are spaced well enough apart from each other so that air can move through the trees and dry the leaves quickly.

Remove vigorous upright suckers and water sprouts that have formed along the main trunk or within the canopy in order to improve air movement and sun penetration. This will help leaves to dry quickly after rain or dew.

The best way to avoid apple scab is to select scab resistant varieties of crabapples and hopefully your crabapple stays beautiful and leafy throughout the summer. After all, we plant crabapples to be pretty and not naked!

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