Apple Scab – Chalet

Apple scab is a fungal disease that affects the leaves and fruit of apples and ornamental crabapples, especially during wet spring weather when foliage has no chance to dry out. Older varieties of crabapple are most vulnerable.

SYMPTOMS

Scab first appears on leaves as small, irregular spots (lesions) that are light brown to olive green. As the infection progresses, the lesions turn dark brown to metallic black and coalesce, causing the leaves to become dwarfed, curled and scorched at the margins. If left untreated, the leaves may drop prematurely, sometimes as early as June. The fruit will also show similar dark, scabby lesions. Although the symptoms can be quite alarming (especially if leaves start falling in the summer months), scab is not fatal, and otherwise healthy trees that defoliate will probably send out a new, albeit smaller and sparser, crop of leaves.

CONTROL & TIMING

It is extremely important to rake up and dispose of diseased leaves from a previously infected tree. This can be done in fall or early spring. Fungicides will control the fungus. Timing is critical, for once the fungus takes hold, it is difficult to control.

SPRING Spray at 7-10 day intervals, beginning when the leaf buds begin to open (bud break) and continuing until the flower buds open. Do not spray during flowering. Resume spraying after the flower petals have dropped and continue until the weather becomes warmer and drier. It is important to thoroughly cover the developing buds and young leaves with each spray. Apply spray on calm days when the temperature is above 40 degrees but below 80 degrees. Use Spectracide Immunox Multi-Purpose Fungicide or Infuse. We highly recommend Immunox due to its systemic nature. In addition, the use of TURBO spreader sticker will enhance any fungicide application.
* Note: The number of applications will depend entirely upon the variety of crab, weather conditions and past history (of an established tree). If the variety is considered quite resistant, it might not need to be sprayed at all in a dry spring or a few times in a wet spring. An older, highly susceptible variety may need 3-5 spray applications in a normal year or 6-8 in an unusually wet spring.

Keep your trees healthy by applying a 2-3" layer of mulch (shredded or chunk bark, coarse compost), watering during dry periods and pruning out dead wood. If your tree does become infected, always rake up and dispose of the diseased leaves. When choosing a new crabapple, refer to our Crabapple Article for one of the many newer varieties that display good to excellent scab resistance.