By: Jean Ferdinandsen, Certified Arborist WI-0149ACrabapples are very beautiful in spring but can turn ugly by summer.
The beautiful flowers of crabapples herald spring in Wisconsin. Different shades of pink, white, red, and maroon offer a color spectacular that lasts four to ten days for each cultivar. The various forms, and small- to mediumsize stature of crabapples make them useful in a variety of sites.
However, shortly after the peak bloom time, many crabapples switch from “beauty” to “beast.” The leaves progressively turn a sickening yellow or brown and later fall off. The slowly thinning tree may look as though it is dying.
The culprit is a fungal disease called apple scab. Necrotic, irregular-shaped, olive brown spots on leaves are usually found along the mid-rid or in association with leaf veins. The scab fungus infects leaf surface, flower parts, fruit, and/or succulent twigs, and draws nutrients from the living tissue. Chlorosis (yellowing) and death of the leaves follows.
The tree is robbed of the foods produced by photosynthesis. Without proper food production, trees weaken and may become more susceptible to other insects and diseases. Most initial apple scab infections begin from airborne spores released from previously diseased leaves on the ground. The peak of this spore dispersal is often near the end of the bloom period.
The wetter the weather during this “infection time,” the more pronounced apple scab can be. How long the plant surfaces (leave, twigs, etc.) are wet, not how many inches of rain we receive, is the key. This wetting can be caused by high relative humidity, heavy dew or fog, and irrigation, as well as rain.
Temperatures between 45 and 80°F favor disease development – between 60 and 70°F is optimum. Symptoms appear eight to eighteen days after infection. Several “strains” of apple scab fungi have been identified, thus trees that were resistant years ago may no longer be.
Crabapples have beautiful foliage as well as flowers, an aspect often missed due to this disease. Keeping this foliage, which can be bright or dark green or even purplish, all season means a healthier tree and a more beautiful landscape.
Natural controls for apple scab, aside from resistance or dry weather, are few. Many species or cultivars show varying degrees of resistance or susceptibility. Planting a disease resistant crab apple is first line of defense.
For existing trees that are susceptible due to variety, age or location, natural decomposition of diseased leaves on the ground can be hastened to reduce initial infection. Raking, mowing with a mulching mower, composting leaves, and applying nitrogenous fertilizer to the compost will speed up degradation and reduce inoculum. A protective fungicide spray program may be recommended for certain trees.
Two to three prescribed sprays, starting early in the season will block the majority of infection. The goal is to retain about 80% or more of the leaves to maintain the health and appearance of the tree. You may see a minimal number of leaves affected, but this is no cause for alarm or to think program was ineffective.
Controlling apple scab is one of the more dramatic and effective treatments in arboriculture. Fungicide products are varied for greater efficacy and to prevent the disease from becoming resistant to a particular fungicide. Treatment can allow a crabapple to add beauty to your yard for an entire season. Call your Wachtel Certified Arborist for an accurate treatment prescription. Don’t miss the critical early timing needed to benefit your crabapples.
© Copyright 2008 – Wachtel Tree Science & Service, Inc.