Apple Scab

Who wants scabby apples? Yuck!

If you see pale yellow spots on your apple leaves, fruit, and stems, that turn brown, your tree may be infected with apple scab.

Apple infected with scab (Margalob) CC BY-SA 4.0

Apple scab is a fungal disease that can defoliate your tree and destroy your crop. This disease is most common in areas with cool, moist weather in spring and early summer. This disease can also affect ornamental crabapple. Before losing your apples to the Venturia inaequalis fungus, let’s see what we’re up against.

Apple scab symptoms

Apple scab starts out as chlorotic (yellow) spots on leaves. Then dark, olive-colored spots appear on leaves and fruit. In severe cases, these spots may also show up on stems. If you look on the underside of infected leaves, the surface will look velvety. That ‘velvet’ is fungal growth. Affected leaves may also pucker or twist. Severe infestations cause leaves to turn yellow and fall from the tree. Infected stems can lead to blossom drop. Finally, later in the growing season, sooty or greasy grayish-black areas appear on the fruit. These lesions often have a red halo. As the fungi develop, the area becomes sunken, tan colored, and scabby. The fruit may also become distorted and crack, providing other pests and diseases with a point of entry.

Fruit and leaves infected with apple scab (Shuhrataxmedov) CC BY-SA 3.0

Apple scab lifecycle

When infected leaves fall from the tree, they take fungal spores with them. These spores then overwinter on these leaves. When rain or sprinkler water hits these leaves, the spores can be bounced back into your tree, where they will germinate and start the whole process over again.

Severe scab infect of leaf (James Lindsey, Ecology of Commanster) CC BY-SA 2.5

Controlling apple scab

One of the easiest ways to deal with apple scab is to plant resistant varieties. If it’s already too late for that technique, regularly monitoring your tree for signs of infection, especially in spring and early summer, can help you nip this disease in the proverbial bud. For this infection to occur, temperatures must be between 55° and 75°F and the surface of the tree must remain wet for 9 hours. While you cannot do anything about temperatures, you can avoid wetting your tree with sprinklers or overhead watering from a hose. If you live in an area prone to heavy dew, you can use fans to dry leaf surfaces. [I use a big fan on my roses in spring to reduce rust, another fungal disease.]

​For the backyard orchardist, the easiest way to break the apple scab disease triangle is to remove and dispose of infected leaves. [This disease can also overwinter on nectarine and peach leaves.] Commercial apple growers may apply zinc and fertilizer-grade urea to leaves in autumn to speed leaf drop, and apply fungicides during the rainy season, at budbreak and a month after petal fall. but these are probably not practices you want to be doing at home. 

Apple scab resistance by cultivar [Data compiled from multiple public sources]

If your apple tree(s) become severely infected, you can use fixed copper, Bordeaux mixture, sulfur, or neem oil from the time green tips appear, until the tree is in full bloom. Avoid using these treatments after that time as they can cause fruit russetting. And be sure to follow the directions for any of these treatments carefully. For example, applying sulfur within 3 weeks of using horticultural oils, or on days when temperatures are above 90°F can lead to phytotoxicity.

As you conduct a weekly inspection of your apple tree each spring, be sure to enjoy the emerging blossoms!