One of the oddities that nature throws our way in late June is the rusty-orange dust that sometimes mysteriously shows up on cars and walkways under trees.
The powdery substance is usually from spores raining down from infected fruits of common trees such as flowering pears, serviceberries, apples, crabapples, hawthorns, and quince.
The culprit is a curious fungal disease called cedar-apple rust (or the closely related cedar-quince and cedar-hawthorn rusts).
These rust diseases are odd because they jump back and forth between two very different kinds of plants — and they need both to reproduce.
In one stage, the disease grows Jell-O-like orange blobs on juniper branches that mature into horned galls that look like something from an alien world.
Eastern red cedars are particular favorite hosts. Although these are called cedars (hence the disease name), they’re actually types of junipers, not true cedars.
Wind carries spores from these horned galls into the second host family, which is trees and shrubs in the rose family. The aforementioned trees are some of those family favorites for rust.
Infected trees develop circular rings on the leaves and orangish horned growths on the young fruits, which drop the rusty-orange powder onto surfaces below. Ohioans who reported raining infections last spring referred to it as “Cheetos dust.”
The powder is harmless to people, but it can be a nuisance when infections are bad enough.
Rust is becoming more common, especially on the many flowering pears that line streets and dot front-yard landscapes.
Pears were widely planted for decades for their bloom, fall foliage, and durability before they started developing gobs of tiny pears.
Besides making them invasive to the point where flowering pears are now
banned for sale and on Pennsylvania’s Noxious Weed List
, those plentiful pear fruits are highly susceptible to cedar-quince rust.
Rainy springs and warm, humid mid-summers are ideal conditions for rust growth.
Rust isn’t usually fatal to trees, although it can ruin fruit and look bad as the tree discolors and drops leaves prematurely.
Like most diseases, its severity from year to year varies by weather. So just because junipers and/or trees look bad one year doesn’t mean they’re doomed.
Junipers suffer potentially worse damage than flowering/fruiting trees because the growths can cause dieback on infected branches.
Penn State Extension recommends pruning out dead wood on junipers and, if you’re prone to chemical action, spraying a fungicide on susceptible trees starting when they flower and then two or three more times at 10-day intervals.
Penn State has a good
online fact sheet on rust diseases
.
Removing one of the two host plants also can help, although that’s not terribly effective if you’re in a populated area where neighbors have junipers, apples, crabapples, pears, and hawthorn.
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More when-to-do-what tips: George’s “
Pennsylvania Month-by-Month Gardening
” book
Gardening with George Weigel
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12 perennial flowers that can take the heat
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This late-spring trimming trick keeps the flowers from flopping
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7 key ways you can improve your front yard
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