Get Growing: Why crabapple trees are looking sad this year, and what to do about it


For the Gazette

Published: 10-18-2019 2:15 PM

Crabapple trees are one of my favorite ornamental spring trees. A mainstay of the New England landscape, they offer three seasons of interest, plus a handsome branching habit that looks good all winter long, especially when decked with freshly fallen snow or a sparkling coat of ice. Crabapples are typically low maintenance and drought tolerant. Their lovely dark pink or red buds open to a wide variety of shades, from white to magenta. They can have single, double, or semi-double blossoms as well as cupped or fringed blossoms. Some are deliciously fragrant. The fruits vary too, from yellow to orange and red. Birds love them. Crabapples also offer colorful fall foliage.

I’ve been concerned lately about the health of local crabapple trees. Over the summer, the crabapple tree in our backyard lost most of its leaves and looks terrible. And it’s not the only one having trouble. I’ve noticed that lots of crabapple trees are looking bare and forlorn. Fearing the arrival of a yet another dangerous tree blight, I contacted Nick Brazee, Ph.D. a UMass Extension plant pathologist. The bad news is that there are actually two fungi attacking crabapple trees. The good news is that although both are unsightly, neither is fatal to the trees.

According to Brazee, apple and crabapple trees have long been affected by a fungus called Venturia inaequalis, commonly known as apple scab. This fungus attacks the trees in the spring, and then continues to reproduce itself asexually continuously through the growing season, generating new spores that infect new leaves as they emerge. It appears on leaves as olive green spots with a velvety texture and causes the leaves to turn yellow and eventually to fall off. By late summer or early fall, affected trees have often shed most if not all of their leaves.

Unfortunately for commercial apple growers, the fungus also causes superficial blemishes on fruit. Although these blemishes do not affect the quality of the fruit, they do mar the picture-perfect appearance that buyers are looking for. Because of this, commercial growers have resorted to heavy use of fungicides to combat the scab and keep their apples looking pristine. Brazee said that this treatment process is laborious and involved. He advises homeowners whose apple and crabapple trees are afflicted with apple scab not to attempt to control the problem with fungicide.

The other fungus afflicting apple and crabapple trees is Marssonina coronaria, or leaf blotch. Unlike apple scab, it is a recent phenomenon. It behaves in much the same way as apple scab, arriving in the spring and continuing to reproduce and spread its spores throughout the summer. The blotch appears on leaves as interspersed patches of yellow and brown. It’s possible for trees to be affected by both apple scab and leaf blotch, suffering greater leaf loss as a result.

Brazee explained that apple and crabapple trees are particularly vulnerable to these fungal attacks because they leaf out early. The attacks are especially severe when spring weather is unusually wet, as it has been for the past several years. Fortunately, apple and crabapple trees tend not to suffer lasting harm from the fungi. “They’re tough” trees,” said Brazee. “They tolerate defoliation pretty well.” Despite having total leaf loss one year, the trees come back full of blossoms and fruit the following spring.

Controlling the fungi and their unsightly effects is difficult, because the fungus continues to reproduce and its spores are windborn and spread easily. But there are a several steps you can take to limit their effects. It helps to rake up and remove fallen leaves, where spores are produced. You can also limit the fungi by pruning the trees to thin out the interior canopy, allowing more sun and air to reach the inner leaves. Pruning vertical shoots or sucker branches that grow off the main, scaffold branches will open up the canopy. Brazee advises that if you are planning to plant a crabapple tree, make sure you choose a site that receives full sun, because sunlight is a powerful deterrent to fungal infections.

There’s more good news for crabapple enthusiasts. There are several new cultivars bred specifically to resist these fungi. These include Donald Wyman, a variety that grows to around 20 feet, with pink buds, white blossoms and brilliant red fruits. So, don’t let the threat of apple scab or leaf blotch discourage you from planting a crabapple tree. If you’re planning to plant a crabapple, seek out one of these.

Mickey Rathbun, an Amherst-based lawyer turned journalist, has written the Get Growing column since 2016.Pruning Woody Perennials

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