Heavy fruit infections by the Cedar-Quince Rust fungus (Gymnosporangium clavipes) are occurring on Callery pears (Pyrus calleryana, family Rosaceae) in southwest Ohio. The “rust” in the common name refers to the rust-colored spores.
In late June 2020, homeowners in southwest Ohio were surprised to find sidewalks, cars, and streets beneath Callery pears covered in a heavy sprinkling of carrot-colored dust. Some in the affected communities referred to the fine powder as “Cheetos dust.” The homeowners didn’t know the source of the orange dust and the unusual event spawned rampant speculation on social media that captured the attention of the local news media.
Here are two headlines from the news media:
“What’s that strange orange dust covering roads and cars across the Tri-State?”
“Streets of Sharonville painted orange by mystery powder.”
The “Cheetos dust” originated from short, tube-like fungal spore structures called aecia. The proper name for the orange spores is aeciospores. Aecia may arise on Callery pears from infected fruit and to a lesser extent, from cankers on small stems.
The cedar-quince rust fungus must alternate between hosts belonging to two widely divergent taxonomic groups of plants to complete both portions of its life cycle. One part of the cedar-quince rust fungus' life cycle involves a member of the Juniperus genus. The “cedar” in the disease name comes from Eastern Red Cedar (J. virginiana), which is a juniper, not a cedar. The fungus produces a different type of spore, called a teliospore, on junipers.
The other part of the cedar-quince rust fungus’ life cycle occurs on a member of the rose family (Rosaceae). Although many plants belonging to the rose family are susceptible, the “quince” in the disease name comes from Common Quince (Cydonia oblonga) and Flowering Quince (Chaenomeles spp.).
In their 2nd edition of Diseases of Trees and Shrubs, Sinclair and Lyon noted: "Rosaceous hosts include more than 480 species in 11 genera." Beyond Callery pear and quince, other notable hosts include Serviceberry (Amelanchier spp.) and Hawthorn (Crataegus spp.). Indeed, hawthorn haws and serviceberry fruit sprouting “orange hairs” were a common subject of BYGL postings in the past.
On rose hosts, the majority of the cedar-quince rust fungal infections occur on the fruit, thus there’s no impact on the overall health of the host. Although infections may occur on small stems to produce cankers, the impact is usually inconsequential.
On the other hand, the impact of the spores that drift from a rose host onto a juniper is nothing to sneeze at. Cedar-quince rust fungal infections produce unsightly stem cankers on junipers that may disrupt vascular flow causing twig dieback.
The bottom line is that the spores that drift from junipers must land on rose hosts to successfully initiate infections. Conversely, spores that drift from rose hosts must land on junipers for infections to occur. If the spores from a rose host land on another rose host, nothing happens. The same is true if spores from a juniper host land on another juniper.
The development of cedar-quince rust on Callery pear has been a recent phenomenon. There was a time when Callery pears rarely produced fruits. This valued trait meant Callery pears would not spread beyond the bounds of their plantings. Of course, that has changed dramatically in recent years with heavy fruit production making the trees pariahs (peariahs?).
I don’t know if we’ll see a repeat performance this season of the heavy spore production on Callery pears that occurred in 2020. However, I’m posting this Alert just in case Extension and Green Industry professionals receive questions about “Cheetos dust” covering streets, sidewalks, cars, and slow-moving gardeners.
Pear Postscript
In 1960, another Gymnosporangium fungus was reported on common pear in British Columbia and California. The fungus was identified as G. sabinae which is native to Eurasia and North Africa. In the intervening years, the fungus has spread to several states east of the Mississippi.
The disease associated with G. sabinae is called Pear Trellis Rust. As with the cedar-apple rust fungus, the trellis rust fungus must alternate between Juniperus hosts and rose hosts such as Callery pear.
However, unlike cedar-apple rust, the fungal infections produced by the trellis rust fungus are largely confined to the leaves. You can see this in the photo below with the Callery pear leaves showing rust symptoms while the fruit is free of infections.
There is a probability that a web search using the keywords, “pear rust,” will point to this rust rather than cedar-quince rust. However, I could find no records of trellis rust being associated with a widespread sprinkling of “Cheetos dust.”