Fig. 1. Apple scab infection on crab apple.
Photo M. Grabowski
Fig. 2. Apple scab leaf infections
Photo: M. Grabowski.
Fig. 3. Apple scab on fruit
Photo: T. Sutton
The most troublesome disease for apple growers in all parts of Minnesota is apple scab. Scab is caused by a fungus that infects both leaves and fruit. Scabby fruit are often unfit for eating, and continued infection of leaves weakens the tree.
Scab infections on leaves start as olive green to brown spots with an irregular or feathered edge. As leaf infections grow, they may merge together and assume a dark brown velvety appearance. Severely infected leaves may turn yellow and drop prematurely. Scab infections on young fruit start out as olive green to brown spots. As the lesions enlarge, they harden, and eventually become black, corky, inedible areas on the fruit. Severely infected fruit may be deformed and often crack open.
Apple scab survives Minnesota’s winters on infected leaves that have fallen to the ground. In spring, spores are ejected from last year’s fallen leaves and create new infections on the young leaves and tiny fruitlets. Leaf spots quickly mature and produce new fungal spores. These spores spread to other leaves to create new infections throughout the growing season.
Abundant rainfall in May and early June is conducive to scab infection, and rainy summers allow the fungus to infect trees even more severely.
Many ornamental crab apple trees are susceptible to apple scab, so the disease can be spread to your fruit trees from nearby flowering crabs.
Depending on where you live in Minnesota, scab control may be very difficult or relatively easy. Growers in drier parts of the state may not have to struggle as hard against scab. An isolated apple planting with no other apple or crab apple trees nearby will also be easier to keep scab-free. Inspecting your trees for scab lesions and keeping records of each year’s infection severity is important in determining how intensively you need to manage scab.
Plant resistant varieties
The best way to deal with apple scab is to avoid it altogether by planting disease resistant varieties. Many varieties of apple trees are resistant or completely immune to apple scab. Susceptible and very susceptible trees will require fungicide sprays every year to control the apple scab fungus. Resistant trees will only require fungicide sprays in very wet years, where the apple scab fungus is nearby in other infected trees or in infected leaf litter. Good sanitation and cultural control practices will minimize the need for even these sprays. Immune trees do not require any fungicide sprays at any time, and will remain disease free all season long.
Susceptibility to Apple scab of apple varieties grown in Minnesota
|Zestar!TM||Paula Red||Pixie Crunch|
|Chestnut Crab||William’s Pride|
Rake all the fallen leaves from around your trees each autumn and remove them from the area. Infected leaves can be burned, buried or composted. Perfect sanitation in an apple planting could, in theory at least, control the disease. If there are other apple or crab apple trees in the vicinity of your planting, however, spores could become airborne and drift onto your property, starting the infection cycle again.
The apple scab fungus needs moisture on the leaves to start a new infection. A well pruned tree with an open canopy will allow air to move through the tree and dry the leaves quickly. This will create an environment less favorable to the fungi and can help reduce the severity of apple scab in a tree. For proper pruning of apples see Apples and pears in Minnesota home gardens.
What fungicides should you use?
Materials available to home growers for scab control include captan, lime-sulfur, and powdered or wettable sulfur. Applications of lime-sulfur closely following captan sprays can damage leaves and flower buds, so use caution when rotating these two materials.
All-purpose sprays, containing combinations of fungicides and insecticides, are also available. For scab sprays just after petal fall, when insecticide sprays may also be necessary, these chemical mixtures may be appropriate. If the goal of a spray is only to control apple scab, however, the insecticide portion of the spray is wasted. In addition, you may need to spray for scab while your trees are in bloom. Pollinators, including honeybees, bumblebees, solitary bees, and many other beneficial insects, will be killed by a spray that includes an insecticide, so never use an all-purpose fruit spray during bloom.
When should you start spraying?
To protect leaves and fruit from scab, most home apple growers need to spray fungicides in spring. Sprays should start at the growth stage known as "half-inch green tip," when the leaf buds have swollen and begun to open so that about half an inch of leaf tissue is visible.
How often should you spray?
Scheduling fungicide sprays for scab after the first application at half-inch green can involve a little guesswork. In warmer weather, leaves grow quickly, and newly-exposed tissue will be unprotected. In cooler weather, growth slows or stops
Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. (E.g. seven to ten days after spraying, you will need to spray again). In plantings where there was a severe scab infection the previous year, use the shortest interval. In plantings where scab has not been a problem, a longer interval will probably give adequate protection. In addition if the weather is dry the longer interval is acceptable.
When can you stop spraying for scab?
In mid-June, examine the leaves on your trees for scab lesions. Be very thorough, checking upper and lower leaf surfaces, leaves on the interior and exterior of the canopy, leaves close to the ground and those higher in the tree. If you find no or very few apple scab leaf spots, you need not spray fungicide again. If you find scab lesions, or if there are unsprayed trees in your neighborhood with scab lesions, you should continue to spray, because the lesions on the leaves will release more scab spores all summer long.
If scab has been a problem in your apple planting, it may take a year or two to get it under control. If you continue with appropriately timed sprays that cover all leaf and fruit tissue, and practice excellent sanitation of fallen leaves, and if outside sources of fungal spores are few or distant, you should find in the second or third year that you only need to spray from half-inch green tip to mid-June.