Rainy weather this spring helps fungal diseases thrive in your garden. Here’s how to treat them.

A wet spring has done more than dampen graduations and douse picnics. In yards and gardens, it is also brewing up fungal diseases.

Ailments caused by fungi tend to be more common in a growing season that starts out wet, according to Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle.

“Wet weather allows fungi to grow,” she said. Every fungus reproduces from dormant reproductive bodies called spores, which can be in the soil or may be wafted through the air and land on plants. When conditions are wet in early spring, while leaf buds are opening, spores readily germinate and the fungi spread to the new, vulnerable leaves.

Wet springs can lead to many fungus-caused diseases such as verticillium wilt in maples; anthracnose in sycamores; apple scab in flowering crab apples; rust on roses; maple leaf tar spot; and powdery mildew, which affects lilacs, phlox and many other plants.

“The fungi that cause these diseases are always around in our gardens,” Yiesla said. “Even in dry years, susceptible plants may show an infection. But in years with a wet spring, the diseases become more severe and more obvious.”

By late May this year, some plants were already showing leaf damage. The Plant Clinic’s advice to homeowners is simple: Don’t panic.

“The majority of leaf diseases are not very damaging overall,” Yiesla said. “The plants will survive a bad year of powdery mildew or maple leaf tar spot. It’s just not nice to look at.”

By the time you see evidence of an infection, it’s already too late to spray plants with a fungicide. “You need to spray plants in early spring, just when the leaf buds are opening, in order for a fungicide to have any effect on leaf diseases,” she said. “By the time the leaves have opened, a fungicide won’t work.”

Some fungal diseases that develop inside the plant’s stems — such as verticillium and fusarium wilts — are not affected by fungicide sprays at all.

What can you do about these diseases?

Prune out diseased branches. If you see an obvious growth, such as the orange glob of rose rust, prune out that branch back to the main stem. Sterilize your pruners before you make another cut. Save one sample of an affected branch for diagnosis. Dispose of the rest in the landscape waste, not in your compost pile, so it will be removed from your property and reduce the chance of spreading the infection.

Have diseases diagnosed. Take photos of the disease symptoms, such as the affected leaves or stem growths, and of the entire plant. Photograph both the top and bottom of the leaves, as well as their arrangement along a branch, to help in identification. Send your photos to the Plant Clinic for help in diagnosing the problem and deciding what to do. If you bring samples in to the clinic, seal them in a plastic zipper bag to avoid spreading the infection.

Clear away infected foliage. Good hygiene can reduce the spread of disease. When distorted or spotted leaves fall, or when you remove diseased plants, dispose of them in the landscape waste. “It isn’t always possible to remove all diseased leaves, such as those from a big maple with maple leaf tar spot,” Yiesla said. “But whatever you can do will help.”

Air out your shrubs. In overgrown shrubs dense with stems, moisture lingers because air can’t circulate. To allow drying breezes to flow through an overgrown shrub, prune out one-third of the stems, choosing the oldest, thickest ones to remove.

Be ready to water. Yes, water! “There will be dry periods in every summer, even in a wet year,” she said. “Drought stresses plants, and plants that are stressed have a harder time fighting off diseases. So make sure your plants’ roots have water in the soil to draw on.”

Water roots, not leaves. “Getting the leaves wet can encourage fungal diseases,” she said. It’s also wasteful, since water on the leaves will simply evaporate without helping the plants. Instead, try to deliver water to the soil where plants’ roots can absorb it. Instead of using a sprinkler, you can water by hand, use soaker hoses, or water young trees with drip bags, a slowly trickling hose laid on the soil or a bucket.

“The best things to do when fungal diseases are bad,” Yiesla said, “are to help your trees and plants stay basically healthy so they can survive an infection, and wait for next year.”

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.