This is cleanup time in the garden. And this year it’s especially important to think about garden hygiene.
The cool summer now disappearing in our collective rearview mirror left some icky-looking plants to remember it by. Powdery mildew and other plant diseases found the cool, humid conditions ideal and got footholds on many more garden plants than they do in a normal summer. By the end of the summer, lilacs and phlox were covered in milky splotches and shriveled leaves were everywhere.
To reduce these diseases’ chances of coming back even stronger next year, take care this fall to remove all the foliage of affected plants from the garden. That infected foliage carries disease spores that can survive a mild winter–such as we had last year–and return next spring, says Jennifer Brennan, the horticulture information specialist at Chalet Nursery & Garden Shops in Wilmette.
Don’t compost
If you compost yard waste–and for the health of your garden and your planet, you really should–don’t put leaves, stems or roots from any plant that showed signs of disease in the pile. Diseases can survive there and, next spring, you could spread them all over the yard with the finished compost.
Separate out the diseased foliage, including leaves from diseased trees, bag it and set it out to be carried away. Don’t worry that you’ll infect your municipality’s composting operation. Big compost operations get their piles much hotter than most homeowners do, says Tom Scheidt, operations manager for DK Recycling, which composts yard waste for Lake Bluff, Lake Forest and Winnetka. You can’t match that spore-killing capacity at home.
Bad guys
Here are some of the diseases that were especially bad this year:
Botrytis blight, a brownish-black fungal disease that kills buds, had a heyday this year because of excessive rains in the spring and early summer, Brennan says. Many peonies and zinnias were hit, with black spots nearly to the ground, she says. Prune the plants to the ground and properly dispose of the foliage.
Powdery mildew is familiar to most gardeners who have phlox, bee balm or lilacs, three of the most susceptible plants. It shows up as dull grayish-white patches on leaves, usually around mid-August when humidity is at its worst. But Brennan, says this year she started seeing it on customers’ plants a month early, thanks to cool temperatures and high humidity.
Powdery mildew is usually only a cosmetic problem, say both Brennan and Doris Taylor, the plant information specialist at The Morton Arboretum. Only in extreme cases does it damage the plant by blocking so much of the leaf surface that photosynthesis can’t happen.
Apple scab is a relentless problem that had stripped countless local crab apples of nearly all their foliage by late summer. Unlike some fungal diseases that infect a plant just once a year, apple scab “keeps re-infecting the tree–every time the tree tries to send out new leaves, apple scab infects it again,” Taylor says. By wiping out the leaves–the tree’s food source–the disease erodes the tree’s ability to survive.
Apple scab puts yellow spots on the tops and bottoms of leaves and on the fruit. The leaves sometimes curl up and drop off and the fruit can look deformed. Taylor notes that cleaning up repeatedly throughout the season helps slow the disease’s spread. But at a minimum, it is crucial to get rid of all the infected fruit and leaves in the fall.
Cedar apple rust also thrived this year, showing up as brown swellings on juniper needles and as orangey-yellow spots on the tops of leaves on hawthorns, apples and crab apples, Taylor says. The disease needs both hosts, traveling from the junipers to the deciduous trees and back again each year. Getting rid of all the leaves from the deciduous trees this fall is a way to interrupt the cycle.
Next year
Cleaning up diseased foliage will only reduce the future number of spores that can attack your plants. Control of fungus diseases also requires applications of fungicides, beginning in the early spring.
In the meantime, remember, while you’re planning the vegetable garden this winter, not to plant tomatoes or any other crops in the same place next year. That will interrupt the life cycle of soil-borne diseases and help prevent them from infecting next year’s plants.
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Information on controlling specific fungus diseases of trees and shrubs, including recommended fungicides, is available from The Morton Arboretum’s Plant Clinic. Call 630-719-2424 or see www.mortonarb.org. Advice on controlling diseases of garden plants is available from the Chicago Botanic Garden’s Plant Information Service. Call 847-835-0972 or see www.chicagobotanic.org.
