Thursday, May 31, 2012By Jean Ferdinandsen
Certified Arborist WI-0149A
The beautiful flowers of crabapples herald spring in Wisconsin. Different shades of pinks, whites, reds, and maroon offer a color spectacular that lasts 4 to 10 days for each cultivar. The various forms and small to medium size stature of crabapples makes them useful in a variety of sites.
However, shortly after the peak bloom time, many crabapples switch from “beauty” to the “beast”. The leaves progressively turn a sickening yellow or brown and later fall off. The slowly thinning tree may look as if 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 either leaf surface, flower parts, fruit, and succulent twigs. It 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 insect and disease issues.
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 is the key, not how many inches of rain we receive. This wetting can be caused by high relative humidity, heavy dew or fog, and irrigation as well as rain.
Temperatures between 45 and 80 degrees F favor disease development, between 60 and 70 degrees is optimum. Symptoms appear 8 to 18 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 in addition to the flowers. This aspect is 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 on 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 not cause for alarm or to think program was not effective.
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. Do not miss the critical early timing needed to benefit your crabapples.