The rain has been beneficial in many respects. We’ve gotten a good workout mowing lawns and bailing out basements, and rainfall deficits largely have been erased. With every silver lining comes a dark cloud, unfortunately.
Ample rain — in excess of 10 to 15 inches since the first of April — has accelerated leaf spots on trees and plants and also led to banner seedhead production on Michigan’s cool-season turf grasses.
Keeping the lawn mowed, not scalped, will keep seedheads in check, said Kevin Frank, professor of crop and soil sciences at Michigan State University.
“Seedhead production requires energy from the plant, so it is likely the turf will not only look ‘stemmy’ due to the seed stalks, but the turf may even thin out,” Frank said.
Applying lawn fertilizer once the seedhead flush has run its course will help turf recover, “especially if you haven’t fertilized yet this spring or fertilized back in April,” Frank said.
Trees also are producing many more seeds this year, including maple seed helicopters, thanks to heat and drought last summer. Thinking they’re facing impending doom, trees channel more energy into reproduction than into their own survival. If you collect pine cones for Christmas decor, you’re in for a record year.
Grass pollen, meanwhile, is in production overdrive, as are molds. Both rank high on the pollen index, according to the National Allergy Bureau, while pine and oak pollen is winding down.
Late May rainfall spurred more fungal growth, such as mold, and the release of spores, which doesn’t bode well for the more than 50 million Americans who suffer from seasonal allergies.
Fungus among us
Leaf spots, be they fungal or bacterial, thrive in wet weather. Splashing rains and wind spread them around. “Infections will occur when the plants remain wet for extended time periods,” said Tom Dudek, Michigan State University Extension horticulture educator based in Ottawa County.
Check plant leaves for early signs of leaf spots and treat with fungicide as soon as the first symptoms occur, he recommends. Fungal leaf spot symptoms include irregular lesions on leaves and new shoots or stems that are tan, gray or black in color.
“Sometimes, dark black, ‘pepper grain-like’ structures are observed inside the lesions,” Dudek said. “These are pycnidia and are the structures that contain spores that spread the fungus to other leaves.”
Dogwoods, hydrangea, lilac and roses are among plants often hit with fungal infections. A preventative shot of fungicide is warranted, especially if the plant has a history of leaf spots.
Common diseases include tar spot on maple leaves, black spot on roses, Alternaria leaf spot on hibiscus and Phyllosticta on maples and dogwoods.
Good plant spacing, avoiding excessive shade, keeping plants fertilized and treated with a fungicide when symptoms appear will help combat these fungal foes.
Dudek advises against overhead watering, especially if leaves will not have a chance to dry off before nightfall. Giving plants adequate space also is a good preventative measure; it improves air circulation. Removing and destroying infected leaves helps stem the spread of bacteria leaf spot.
While we’re grappling with water-related woes, the ripple effect of last summer’s record heat and drought continues to reverberate.
“A prolonged drought throughout most of Michigan in 2012 weakened many landscape trees, making them more susceptible to borers, bark beetles and some disease problems,” said Dave Smitley, an entomologist with MSU.
“Drought stress last year apparently increased activity from Zimmerman pine moth and Diplodia disease. Some of the needlecast pathogens also appear to be playing a role.”
Drought-stressed trees are more vulnerable to disease and insect problems, with ash trees being a good example, Smitley said.
At MSU’s Hancock Research Farm in East Lansing, ash trees with adequate sunlight and water grew rapidly while shaded, and drought-stressed trees grew slowly and were much more susceptible to attacks from emerald ash borers — even though both sets of trees were treated with imidacloprid to protect them from ash borer infestations.
“Many types of trees are susceptible to borers when they are drought-stressed,” Smitley said. “I would put some of our imported trees at the top of that list: Austrian pine, Scots pine, honeylocust and blue spruce, for starters. However, all of our trees become more susceptible after severe drought stress.”
Products containing imidacloprid, such as Bayer Advanced Tree & Shrub Care, commonly come as a concentrate. Mix them with water and pour at the base of the tree. As they are absorbed and carried up through the tree, they protect it from new borer damage.
“It will not erase the damage already done from borers last fall and this spring,” Smitley said. “Even protected trees may need two to three years to recover from a severe borer attack.”