G1922

Apple Scab

This NebGuide discusses the causes and various treatments for apple scab, one of the most important diseases of apples and crabapples.

• Symptoms
• Host Range
• Disease Cycle
• Management

Apple scab, caused by the fungus Venturia inaequalis, occurs worldwide wherever apples are grown. Scab is one of the most important diseases of apples and crabapples in Nebraska. Disease development is favored when wet and humid weather prevails from late April through June. The disease decreases yield, reduces fruit quality, defoliates trees, and reduces tree vigor. Nearly every year, this disease defoliates susceptible varieties of ornamental crabapple. Repeated infection reduces their aesthetic values in landscape plantings and predisposes them to further injury from other stresses.

Symptoms

The first visible symptoms generally are small, discrete olive-to-greenish-black lesions on the underside of the leaves. Lesions are about one-fourth inch in diameter with an indefinite feathery margin. With time, the color darkens as the size of the lesions increases (Figure 1). Early infections tend to be on the underside of the leaf; later in the season the spots can be seen on either side. Leaves with a large number of lesions often become distorted, show dead or dying tissue, yellow, and drop prematurely from the tree. Similar lesions can appear on the leaf petiole. Lesions on the apples are superficial, and the enlarging tissues beneath them give the lesions a cracked or “scabby” appearance and misshape the fruit (Figure 2). Though the fruit may appear unsightly, it is safe to eat.

Figure 1. Apple scab symptoms on a leaf surface.		Figure 2. Apple scab on the fruit.

Host Range

V. inaequalis attacks members of the genus Malus. This includes varieties of apple and crabapples, including the common wild crabapples. Scab also has been reported on hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp.), and loquat (Eriobotrya japonica).

Disease Cycle

V. inaequalis survives the winter in fallen leaves that were infected the previous summer. In spring the fungus becomes active on the infected leaf debris in structures called pseudothecia and produces other structures called asci that contain ascospores (fungal “seeds”). The first dispersal of ascospores coincides with the first appearance of green tissue in the apple buds, commonly called “green tip.” Peak spore dispersal usually coincides with bloom. Rain helps increase spore production and dispersal. Spores that land on a susceptible leaf or fruit surface and come in contact with a droplet of water germinate and infect the host. Six to eight hours of free moisture on the leaf surface are necessary for germination and infection to occur. In 7-14 days the first disease symptom appears as a scab lesion. During late spring and early summer other spores called conidia are produced in the leaf and fruit lesions, causing secondary infections and resulting in more lesions and increased disease severity. As temperatures increase, the fungus becomes less active, resulting in few new infections during summer. A secondary fall infection cycle may occur under cool, moist conditions, but fall infections are less damaging than spring infections. However, these infections increase the production of the overwintering structures that are the main source of ascospore inoculum in spring.

Management

There are several approaches to managing scab. Each offers some degree of success when used individually, but the best long-term management involves integrating multiple tactics.

Resistant Varieties

Apples: Table I shows the disease reactions of apple varieties to scab. Although varieties vary in their susceptibility to V. inaequalis, no cultivated commercial variety has sufficient resistance to eliminate the occasional need for chemical spray.

Crabapples: The crabapple varieties listed in Table II are reported to be resistant or moderately resistant to scab.

Sanitation — Cultural Control

Rake and remove leaves in autumn or winter. Either destroy or thoroughly compost the leaves. This will reduce the number of spores that can start the disease cycle the next year. When using sprinklers to irrigate the turf in summer, water early in the morning so turf and trees stay wet for less than four hours. This helps reduce disease activity on both the turf and landscape plants.

Chemical Control

Regular, well-timed fungicide sprays have proven to be the most commercially practical means of controlling scab for susceptible trees, but the first line of defense is to consider scab-resistant apples and crabapples.

Preventing early infection of apple scab is the most critical step toward successful control of later leaf and fruit infections. For apples, fungicide applications can be made as early as the green tip growth stage (Table IV) and continued until petal drop. For crabapples, typically one to two fungicide applications starting at green tip are necessary. Sprays are only necessary if the weather is rainy. The list of fungicides in Table III is a guide. Observe all label precautions. Regardless of the fungicide used, effective control depends on the timeliness and repetition of applications and the degree of coverage obtained on both upper and lower leaf surfaces.

This publication has been peer reviewed.