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Disease

Scab is one of the most common and serious diseases of apple and flowering crabapple in Kansas. The disease causes premature defoliation and a reduction in the number and quality of flowers the year following defoliation, and can predispose trees to winter injury and other diseases. Scab first appears in early spring as roughly circular, velvety, olive-green spots on both the upper and lower surfaces of leaves. The spots eventually turn dark-green to brown and develop a rough texture. Some leaf distortion may accompany infection of expanding foliage. Numerous leaf or petiole infections will cause leaf yellowing and premature defoliation.

The fungus also may attack the fruit at any stage of development. Early infection results in blossom blight and dropping of young fruit. Later infections produce dark-green to black, circular lesions on the fruit. These rough, scaly spots cause surface blemishes to the fruit, but do not extend deeply into the flesh.

Cause

The scab fungus (Venturia inaequalis) overwinters as immature fruiting structures in partially decayed leaves on the orchard floor. During winter, the black fruiting structures of the fungus mature and begin to release spores (ascospores) in the spring. In Kansas, primary spore release begins in early to mid-April and continues for a period of five to nine weeks. Windblown spores are deposited on expanding leaves where they germinate and penetrate the leaf surface. Germination and infection of the host require that the leaf surface remains wet for a period of time, the length of which varies according to the ambient temperature. Symptoms normally develop 9 to 17 days after infection; at this time the fungus begins to produce secondary spores (conidia) which can reinfect apple leaves or fruit throughout the summer during favorable weather.

Control

The best means of avoiding scab is to plant resistant cultivars. Several cultivars of flowering crab are available with good resistance to scab, fireblight, and cedar-apple rust. Disease resistance, aesthetic quality, and adaptability to Kansas conditions should all be considered before choosing a flowering crab for planting. A partial list of flowering crabs and their disease reactions is given in Table 1.

Most apple cultivars are moderately to highly susceptible to scab (Table 2). Recently, several new apple cultivars have been released with excellent resistance to scab and to some other diseases, and should be considered before planting to reduce the number of pesticide applications.

Since the fungus overwinters on fallen leaves, raking and removing leaf debris in the fall may partially control the disease. This will not give complete control, since even a few leaves missed during sanitation can start the infection process in the spring.

Several fungicide applications may be required for control of scab on susceptible flowering crabapples. The most critical spray period is in the spring when primary spores are being released from leaf debris. If this initial infection process is prevented, fewer sprays will be needed in late spring or early summer. In most cases, the first spray should be applied just before bloom. One to two additional applications of fenarimol, propiconazole, or myclobutanil at 10- to 21-day intervals are necessary for adequate control during most springs. See the table for a complete listing of fungicides for scab control.

Susceptible apples in home fruit plantings also must be protected against scab by fungicide sprays. The types of chemicals and timing of sprays are similar to those for flowering crab; however, several additional sprays are required during late spring and summer to protect fruit from infection. For scab control on apples, follow spray recommendations in the Fruit Pest Control Publication C-592.

benomyl

Homeowner: benomyl

chlorothalonil

Homeowner: Many trade names including Daconil

Coppers (many formulations)

fenarimol

ferbam

Homeowner: Ferbam

myclobutanil

Homeowner: Immunox

propiconazole

sulfur

Web updated 9/01/06
Web comments to jpierz@ksu.edu