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PLANT PANDEMIC! COMMON LANDSCAPE DISEASES OF CHICAGO

So you’ve valiantly battled the pests ravaging your garden…

…but your plants still aren’t looking too hot. You’ve tried it all—fixing your watering regimen and checking for insects that might be feasting upon your foliage. So maybe your plant is in the right spot but there’s a disease rolling through town, like when Dutch Elm Disease wiped out almost every vulnerable elm tree in the Chicagoland area during the 1970s.

Some diseases are thankfully shallow enough to only be cosmetic, while others may mean the death of your plant. Keeping an eye on your landscape during the straining summer heat might not stop disease in its tracks, but it can be the difference between catching a problem in time for mitigation or removing dead trees from your garden.


Credit: Liza Lanum via Flickr [CC BY-NC-ND 2.0].
#1 Tar spots

Towards late summer, 1/3″ to 1″ black spots begin to appear on maple leaves affected by one of three Rhytisma fungi. Tar spots actually develop in early summer, though it is hard to tell as the spots are only light green at this point. After these spots fully die-off, the color matures to black or dark brown and becomes bumpy or raised, and if the leaf is covered enough it may even drop early.

Yet there is little cause for concern. It is all cosmetic! Since the fungi can survive our harsh winters, the best mitigation plan is to rake all fallen leaves and destroy immediately. Composting the leaves does destroy some, but not all of the fungi spores as compost piles rarely heat up enough to fully kill the fungus, so it is highly recommended that you burn, baby, burn.


Credit: Oregon State University via Flickr [CC BY-SA 2.0].
#2 Fireblight

Those of the delicate rose family beware: not only are Japanese Beetles ready to gnaw leaf from woody limb, but hard-hitting diseases like fireblight have you in their crosshairs. The disease is caused by the bacterium Erwinia amylovora entering an open wound. Temperature and humidity can increase the severity of infection as the right amount could mean an explosive breeding ground for the bacteria, while insect infestation can add undue stress to a plant and progress the infection at an accelerated rate.

If you see black twigs or gaping open cankers on an apple tree, crabapple, hawthorn, pear, Pyracantha, Cotoneaster, spirea, flowering quince, or mountain ash, you might have fireblight. However once the woody plant’s terminal bud has developed and new growth hardened, the disease stops progressing. In late summer to winter, prune out all diseased wood while dipping the blade in a 10% bleach solution between cuts to stop the spread of bacteria. Clean fallen fruit and twigs from the ground and dispose of all diseased material. Don’t compost and reuse!


Those of the delicate rose family beware: Japanese Beetles aren’t the only nuisance after you!


Credit: Jeff Kubina via Flickr [CC BY-SA 2.0].
#3 Powdery Mildew

Another of the more cosmetic afflictions is powdery mildew. It affects a whole range of trees, shrubs, annuals, and perennials as there are over 1,000 fungal species that develop the white fuzz in late summer to early autumn. It can make quick work covering a plant’s leaves, stems, flowers, and fruit. In severe infections it can affect the bud and cause fruit and leaves to drop prematurely. The fungi overwinters in leaf litter, stems, and dormant buds before releasing spores in early spring.

As this affliction is more harmful to a plants appearance rather than health, you can wait until infected leaves, fruit, and flowers drop before collecting to dispose. Properly sanitize any tool that comes in contact with the powdery mildew to prevent spreading. Dispose of all diseased material, and don’t compost and reuse this stuff either!


Credit: SEMA FoodSecurity via Flickr [CC BY-SA 2.0], left; NatureNerd (probably outside) via Flickr [CC BY-NC 2.0], right.
#4 Cedar-Apple Rust

One of the more unsightly problems in the landscape are the many cedar-rust diseases that require both an Eastern red cedar or juniper and a member of the rose family as hosts to survive. These diseases start their life cycle on junipers where the gals develop for a year before jumping to a rose family host the second spring. Wet conditions during this time engorges the gals to produce tendrils that release spores when dry. Any hawthorns, apple, or other susceptible rose family plants within 5-miles may succumb, where within 2 weeks of infection they will develop yellow/orange/black spots on leaves, fruit, and/or young twigs. Soon these spots develop on the underside of the leaves where they will finally release matured spores to infect local junipers in late summer.

Mitigation is a matter of keeping these two host plant types separated in the landscape. Keeping them no closer than a mile in known infected areas usually does an adequate job of reigning in the disease. There are also disease-resistant cultivars of many popular varieties that can be used in the Chicagoland area.


Credit: Oregon State University via Flickr [CC BY-SA 2.0].
#5 Boxwood Blight

Boxwood Blight is tricky as infected plants can pass this fungal disease before showing signs (#BoxwoodCoronavirus). It can even be transported in boxwood twigs used to decorate holiday wreathes. It’s currently speculated the Boxwood Massacre of 2019 was so bad because of rampant disease and insect problems that were exacerbated by a brutally cold winter.

First signs of this disease appear as darker-than-normal foliage that eventually develops into brown splotches, with the underside of leaves showing tiny white fungal spores. Avoid sheering boxwoods when wet as this will help the disease spread rapidly. Collect all debris from clippings and destroy. The cultivar ‘Green Velvet’ showed particular Chicago hardiness, with ‘Winter Gem’ almost completely wiped out in the Chicagoland area.

Kemora Landscapes is a full-service design, build, and maintenance company based in Chicago.

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