Apple Scab

Apple scab is a fungal disease most common in areas of relatively high rainfall and relative humidity. However, scab outbreaks such as those that occurred in the early 1980's indicate that persons associated with the central Washington apple industry must constantly be alert to conditions that are conducive to the development of this disease, even in arid climates such as central Washington. If uncontrolled, the fungus may defoliate trees and blemish fruit to a point where it is unmarketable. In central Washington, the disease is particularly troublesome under overtree irrigation.

Figure 1. Apple Scab. Warty, necrotic lesions on leaves, infected by Venturia inaequalis. (Photo courtesy of Dr. M.A. Ellis, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, OH)





Symptoms
The apple scab fungus infects leaves and fruits. Initially, velvety olive-green spots appear on the leaves (Figure 1). Eventually the lesions enlarge, turn brown and assume a scabby appearance. Infection on or near the petiole may result in leaf drop.



Figure 2. Apple Scab. Circular, necrotic lesions on fruit infected early in development. (Photo courtesy of M.A. Ellis)








Figure 3. Apple Scab. Cracks in fruit infected early in development. (Photo courtesy of Dr. R.N. Campbell, Department of Plant Pathology, University of California, Davis, CA)

The scab fungus can also infect fruit during any stage of fruit development, but fruit becomes less susceptible with maturity. Infections occurring very early in development may result in a blossom blight and possibly fruit drop.

Symptoms on mature fruit infected early in development include olive-green or brownish warty lesions (Figure 2). Sometimes the scab lesions may develop deep cracks in the fruit (Figure 3). Small black spots resulting from infections occurring late in fruit development are termed "pinpoint" scab (Figure 4). Although infection occurs only in the orchard, pinpoint scab symptoms do not develop until the apples are in storage.

Pathogen and Disease Cycle
The fungus causing apple scab, Venturia inaequalis, over winters in colonized dead leaves (Figure 5). The fungus survives the winter in these leaves in the form of a structure called a pseudothecium or perithecium.

Figure 4. Apple Scab (Pinpoint Scab). Small black spots resulting from infections occurring late in fruit development.

Spores (known as ascospores) are produced within the pseudothecia and are discharged during rainy or wet periods in the spring. Ascospores landing on wet leaves or fruit germinate in the water film if temperatures are suitable.

Once the initial ascospore caused infections (called primary scab) are established, the fungus continues to grow in the infected tissue and eventually produces a second type of infectious spore (known as a conidium) in the scab lesions. Infections resulting from conidia are termed secondary infections. After leaf fall, pseudothecia form ensuring a means for the fungus to survive the upcoming winter.

All spores of the apple scab fungus require a film of free water for a given period of time to ensure germination and subsequent infection. The wetness periods required are temperature dependent and are presented in Table 1.

The optimum temperature for ascospore germination is 70°F (20°C). At this temperature a wetness period of nine hours is required to result in a light level of infection. As the wetness period lengthens at a given temperature, the level of infection becomes more severe.

As temperatures increase or decrease away from this optimum temperature, longer wetness periods are required for spore germination, e.g., 20 hours of wetness is required at 45°F to reach the level of infection expected after 9 hours of wetness at 70°F (20°C).

If two wetness periods are separated by a period of eight hours or less, the wetness periods should be added together. Recent research in the eastern U.S. indicates that the wetness periods required for secondary foliar infections are about 2.5 hours more than those required for primary infections.

Figure 5. Disease cycle of apple scab. (from G.N. Agrios, Plant Pathology, [Second Edition], copyright 1978 by Academic Press, Orlando, FL. Used with permission of G.N. Agrios and Academic Press)

Control
Apple cultivars vary in their susceptibility to apple scab. Scab resistance or susceptibility of various apple cultivars is presented in Table 2. Ornamental crabapples can also be severely impacted by scab. The most susceptible cultivars can be defoliated every year and also exhibit tip dieback if spray programs are not followed. Ornamental crabapple cultivars suggested for use in Washington are presented in Table 3. They will produce showy flowers, maintain their leaves throughout the summer, and produce showy fruit with minimal or no fungicide application.

Because the scab fungus overwinters in fallen leaves, the disease can be partially controlled by raking and burning. However, fungicide sprays usually provide the only practical means of scab control in commercial orchards.

Correct spray timing during the primary scab cycle lessens the need for extensive fungicide applications during the later stages of disease development. The critical period for scab control is from the beginning of bud growth until the apples are 1/2 inch (1.27 cm) in diameter.

In order to properly control secondary scab, central Washington orchards should be closely monitored for primary scab. Fungicides should be applied if one or two scab lesions per tree are present. Irrigation sets 12 hours or longer should be avoided.

There are two approaches to control of apple scab using fungicides. Protective, post-infective (kickback or eradicant), or a combination of both types of programs can be followed. The protective schedule is the least complicated, but usually requires more applications.

Sprays are applied as soon as susceptible tissue is exposed in the spring and every 7-10 days throughout the season if scab is present on the leaves, or until all of the over\,vintering spores are gone. The interval between sprays is dependent on the rate of growth of the host, weather conditions, and stability of the fungicide. Fungicides such as captan and dodine (dodine can also be used in a post-infective program) are examples of Protective materials.

Table 1. Approximate number of hours of wetting required for primary apple scab infection at different air temperatures and the length of time required for the development of conidia.*

(a)Approx. no. days required for conidial development after primary scab infection.
(b)The infection period is considered to start at the beginning of the rain.
(c)Data are incomplete at low temperatures.
* From W.D. Mills, Cornell University. Mills, W.D. 1944.
Efficient use of sulfur dusts and sprays during rain to control apple scab. N.Y. Agr. Exp. Sta. Ithaca Ext. Bull. 630, 4 pp.

The post-infection approach to control requires accurate monitoring of orchard temperatures and length of time the leaves remain wet. It is imperative that the grower have access to accurate local weather information. When conditions are for scab development (when an infection period occurs), sprays are applied.

(E) Cultivars commonly grown east of the Cascade Mountains (W) Cultivars commonly grown west of the Cascade Mountains. (*) Resistant cultivars not usually requiring fungicide applications even in western Washington's wet climate.

Table 3. Scab tolerant flowering crab varieties recommended for use in western Washington

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Cultivar	Flower Color	Fruit Color	Growth Habit
Donald Wyman/	W	R	T
Sugar Tyme	W	R	T
Zumi (cv. calocarpa)	W	R-O	T
Jewelberry	W	R	S
Sargent's Flowering Crab (Malus sargentii)	W	R	S
Bob White	W	Y	T
Japanese Flowering Crab (Malus floribunda)	W-P	Y-P	T
Ormiston Roy	W-P	Y-P	T
Professor Sprengler	W-P	Y-O	T
Adams	R	R	T

Most fungicides having post infection (kickback, reachback, or eradicant) activity indicate on their labels the maximum time available to complete spraying after the beginning of the infection period.

Three items should be considered when interpreting these times:

1. they are maximum times available for eradicant fungicide activity;
2. times are determined under optimal infection conditions and can thus be extended under less favorable temperatures (the fungus requires longer wetness periods for infection at hotter or cooler extremes);
3. times specified are given under the assumption that a high level of control is desired (if the application is not complete when the time limit is reached, spraying should continue as some control will be achieved for a period after that listed on the fungicide label).

Some of the newer systemic fungicides have excellent "kickback" activity but limited protective capabilities. Be sure to follow label directions regarding spray intervals and information about tank mixtures of protectant and kickback type fungicides.

Homeowners not wanting to apply fungicides should plant scab-resistant cultivars If desired cultivars are susceptible, dwarf or semi-dwarf trees should be planted to allow good spray coverage. Rake and destroy leaves; do not put them in a compost. Prune to open the tree up allowing good air circulation and light penetration. Apply a registered fungicide at prebloom (prepink to pink stage), petal fall, and then every 7-10 days until dry weather.

(EB1044)

Warning. Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

The law requires that pesticides be used as label directs. Uses against pests not named on the label and low application rates are permissible exceptions. If there is any apparent conflict between label directions and the pesticide uses suggested in this publication, consult your county Extension agent.

Issued by Washington State WSU Extension, Larry G. James, Interim Director, and the U. S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. WSU Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, gender, age, disability, and gender preference. Trade names have been used to simplify information; no endorsement is intended. Slightly revised March 1992.

For more information contact your local WSU Extension Office.