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Fungal growth on apples is harmless, but there are ways to prevent it

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Jessica Walliser
Flyspeck on an apple

Question: We’re having a great apple harvest this year from our three trees. But, we noticed that many of the apples have clusters of tiny, dark spots on them. These spots don’t wash off and seem to be a part of the skin. There also are larger splotches of gray, moldlike growth on them, but these wash off. What is this, and is there something we can do to keep it from happening next year? They don’t seem to affect the flavor of the apples, but they look weird.

Answer: The groups of tiny, dark spots you describe are called flyspeck. This is a common fungal disease of apples and pears. It often appears in conjunction with another fungal issue called sooty blotch, which is what’s causing those splotches of gray, moldlike growth.

Both are considered surface-blemish diseases and occur when the fruits are almost mature. Though the two are caused by different fungal organisms, they have a lot in common, including the conditions in which they thrive.

Flyspeck is a fungal disease that overwinters on the twigs of infected apple trees and other host plants, including some hardwood trees and brambles. In spring, the fungal spores of this organism spread during rainy weather and on wind currents. Flyspeck occurs only during very humid conditions. It lives on the wax layer of the apple’s skin.

Sooty blotch also overwinters on infected twigs of apples and other trees, as well as brambles, and spreads via water droplets during the late spring and early summer. Like flyspeck, this organism lives only on the outer surface of the apple’s skin. It also requires high humidity to survive.

Often, both of these fungal organism aren’t evident during the hot, dry summer, but come fall, when rains arrive and the harvest nears, the blemishes appear.

Though neither of these diseases affect the flavor or storage life of the apples (nor will they hurt you), they do affect the apple’s aesthetics. Many home gardeners choose not to battle these issues, simply washing off the sooty mold before eating the fruit and ignoring the flyspeck blotches. But, if you’re bothered by them enough to want to do something about it, here’s what you can do.

As with most fungal issues in the garden, prevention is the key.

Good air circulation is critical to preventing both issues. Prune apple trees to an open canopy form, and thin the fruits to one per 6 inches of twig length when they’re about the size of a nickel. This not only improves air circulation around the fruit but also allows it to have plenty of room to grow.

Keep brambles, such as raspberries and blackberries, out of the orchard. As I mentioned earlier, these plants can serve as a reservoir host for the organism, and their presence may increase the chances of infection.

If the aesthetics of the fruit are important to you, you also can use organic fungicides to help control both of these issues. I recommend a biofungicide based on the active ingredient Bacillus subtilis, a bacterial organism that battles many fungal issues, including sooty mold and flyspeck. One brand name found at most local garden centers is Serenade. Apply according to label instructions, starting soon after petal drop and continuing until harvest.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to [email protected] or The Good Earth, 503 Martindale St., Third Floor, D.L. Clark Building, Pittsburgh, PA 15212.

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