Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
Apple scab, caused by the fungus Venturia inaequalis, has occurred infrequently throughout Utah apple orchards since it was first observed in Utah County in 1984. It has since spread to other areas and should be considered a potential problem in most Northern Utah counties. This disease can cause serious economic losses to fruit in commercial apple orchards if left untreated. Apple scab is prevalent in the Eastern U.S. and has more chemicals applied for its control annually than for any other disease. There are both primary and secondary infections possible with this disease. Control of primary infection is critical in managing the disease severity within an orchard.
This fungus can infect leaves, petioles, stems, blossoms and fruit. The lesions on leaves and fruit are initially velvety brown to olive-brown in color, with indeterminate margins. The bottom surface of young leaves show the first lesions followed by lesions on the upper leaf surface. The lesions may eventually turn nearly black in color and merge with other lesions. Leaves typically show some distortion due to the infections. Fruit lesions initially appear as velvety brown to olive-brown spots, which enlarge and ultimately develop a scabby, dry, cracked appearance. In severe cases many lesions can form on leaves and/or fruit encompassing a majority of the tissue creating "sheet scab". Fruit may be infected before picking but not show symptoms until after being in storage resulting in small scab lesions known as "pinpoint" scab.
Venturia inaequalis overwinters in fallen infected leaves on the ground. Throughout the winter the fungus remains as immature pseudothecia (fungal reproductive structure). As the conditions become favorable in the spring, the pseudothecia mature and produces ascospores (analogous to seeds). These ascospores are the source of the primary infection. Once the ascospores are mature, a period of moderate temperatures and rainfall will trigger the ascospores to be forcefully ejected into the air currents where they may be carried to susceptible tissue. With optimal temperature and leaf wetness the spores will infect the tissues if the leaves are unprotected.
The fungus grows in the tissue and produces secondary spores termed conidia (analogous to seeds). These conidia infect the tissue and produce brown to olive-brown lesions covered with more conidia. When the conidia are mature they can be blown or knocked from the infected tissue surface and start a new secondary infection cycle. The fungus overwinters in the fallen leaves and is activated again in the spring to produce pseudothecia and ascospores which initiate the cycle again.
1. Since the apple scab fungus relies on early season maturation of the pseudothecia and leaf wetness for infection, many of the cultural controls are targeted to stop maturation of spores and decrease periods of leaf wetness.
2. Some cultivars are resistant and will influence the severity of disease. Varieties known to be resistant include: Akane, Liberty, and Prima. Those with some resistance include: Jonagold, Macoun, Red Delicious, Rome Beauty, Jonathan, and Granny Smith.
3. Apply 5% Urea to fallen foliage to increase the speed of microbial decomposition of the leaves.
Flail-mow fallen foliage after leaf drop or early in the spring as soon after the snow melts as possible.
4. Avoid wetting the foliage when irrigating lawn or groundcover. Irrigate in the morning so the sun will reduce the amount of time the leaves are wet.
5. Prune trees to open up canopy to allow good air circulation and reduce amount of time leaves are wet.
Fungicides used for control of Apple Scab are either preventative and/or curative (kickback). Preventative fungicides must be applied before an infection period, whereas those fungicides that are curative should be applied after an infection period but must be within the time period (kick back) suggested by the manufacturer. A tank mixture of preventative and curative fungicides should be used after an infection period. Determining infection periods is essential for deciding on a spray program. It is often more effective and economical to use preventive fungicides. Leaf wetness and temperature relationships have been used to determine these infection periods for many years and are a major component of current IPM techniques for applying pesticides only when necessary. Table 1 has the leaf wetness and temperature infection period values.
1. Captan 50 WP -- at 4 to 8 lb/A in 200-400 gallons of water. Do not apply more than 64 lb/A/year. May be applied up to the day of harvest. Do not use with oils, lime, or alkaline materials. For home use, Captan (5%) at 1 lb/10 gal water. 4-day re-entry.
2. Mancozeb -- 6lb/A prebloom; 3lb/A bloom through postbloom. Do not apply more than 21lb/A/year.
3. Wettable Sulfur (92%)-- 6-8 lbs/100 gal water. Do not use with oils, may cause damage. Consult label before application.
4. Lime sulfur (29%) -- at 1 to 6 lb/100 gal water. May injure fruit on 'Delicious' apples. Polysul and Sulf-R-Spray (both 29% lime sulfur) are registered for home use. 48 hr re-entry.
5. Vangard WG -- Prebloom 5 oz/A. Begin at green tip and continue on a 7-10 day schedule when the product is used alone. If product is used in a tank mix, apply 3 oz/A. Pink, bloom, and post bloom, apply as a tank mix at 3 oz/A with a recommended protectant fungicide. Vangard offers protective as well as 48 hours of post infection (curative) control. Do not apply more than 22 oz /A of Vangard per growing season. Do not apply within 72 days of harvest.
1. Procure -- 2-4 oz/100 gal water. Begin spraying at 1/2" green with a 7-10 day spray schedule. As an eradicant, apply 3-4 oz/100 gals of water within 72 hrs of the infection period.
2. Rally 40 W --1.25-2 oz/100gal water beginning at pink and through postbloom. Apply 2.0 oz/100 gal water within 96 hours of infection period for kick-back effect.
3. Rubigan EC -- 3-4 oz/100 gal water on a 7-10 day spray schedule. Apply 3-4 oz/100 gal water within 96 hours post infection period and again 7 days later for kick-back effect.
|Temperature (F)||Minimum hours
leaf wetness for
Information in Table 1 courtesy of D.M. Gadoury & R.C. Seem.
Precautionary Statement: All pesticides have benefits and risks, however following the label will maximize the benefits and reduce risks. Pay attention to the directions for use and follow precautionary statements. Pesticide labels are considered legal documents containing instructions and limitations. Inconsistent use of the product or disregarding the label is a violation of both federal and state laws. The pesticide applicator is legally responsible for proper use. This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Vice President for Extension and Agriculture, Utah State University.