Homeowners in southwest Ohio were surprised yesterday to awake to find sidewalks, cars, and streets beneath Callery pears (Pyrus calleryana) covered in a fine sprinkling of orange dust. The unusual event spawned rampant speculation on social media and captured the attention of the local news media.
The source of the orange patina appears to be Gymnosporangium clavipes; the cedar-quince rust fungus. The "orange dust" is actually the spores of the fungus and the source are tube-like structures, called aecia, which are sprouting from the fruits and to a lesser extent, the stems of infected Callery pears.
Fruit infections cause no harm to the overall health of infected trees. Although the stem infections may cause minor tip dieback, the damage is usually inconsequential to tree health. The rain of orange is generally considered to be an aesthetic issue; however, affected homeowners may have a different perspective.
The Triangular View
Plant pathologists developed the Disease Triangle to graphically illustrate the three conditions that must be present at the same time for a plant disease to develop. Viewed from a management perspective, the Triangle is helpful with showing that by removing any one of the three components, disease development can be prevented.
So, what happened in southwest Ohio? How did all of the components of the Disease Triangle come together to spawn the perfect storm behind orange spores raining down on streets, sidewalks, and parked cars?
The cedar-quince rust fungus must alternate between hosts belonging to two widely divergent taxonomic groups of plants in order to complete both portions of its life cycle. One portion of the cedar-quince rust fungus' life cycle involves a member of the Juniperus genus. It's often Eastern Red Cedar (J. virginiana) which accounts for the "cedar" in the disease name.
If environmental conditions are supportive of infection, the fungal pathogen produces twig cankers and spores on the Juniperus host. The cankers are both noticeable and potentially damaging by disrupting vascular flow to cause twig dieback.
The rusty-orange colored fungal spores of the fungal pathogen that arise from the cankers will drift on the wind to initiate the other portion of this fungus' life cycle. If the spores land on another juniper, or on oaks, maples, pines, gardeners, etc. nothing happens except perhaps for sneezing in the case of gardeners. That's because these hosts are not susceptible to infections by the cedar-quince rust fungus; no disease can develop.
However, many host plants belonging to the rose family (Rosaceae) are susceptible to infections by the cedar-quince rust fungus. Common quince (Cydonia oblonga) and flowering quince (Chaenomeles spp.) are possible rosaceous hosts, thus the "quince" in the disease name. However, there are many other rose hosts with junipers serving as the middleman. In fact, according to Sinclair and Lyon in their 2nd edition of Diseases of Trees and Shrubs, "Rosaceous hosts include more than 480 species in 11 genera."
Callery pears belong to the rose family, so they can be susceptible hosts to the cedar-quince rust fungus. However, not all parts of rosaceous hosts are susceptible to infection by this fungus. The cedar-quince rust fungus doesn't invade the stems to wreak havoc on the vascular system nor does it infect the roots to produce root rots. It does infect the fruit meaning that the greater the number of fruits, the greater the level of infection.
There was a time when Callery pears rarely produced fruits. Indeed, 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?). The story is presented in the BYGL Alert title, "Callery Pear: the Jekyll and Hyde Tree" that was posted earlier this season.
You can read the Alert by clicking no this hotlink:
As you can see with the images in this Alert, the pears were loaded with fruit providing a massive target for infection. Consequently, all three sides of the Disease Triangle were satisfied this spring. Spores (= the pathogen) wafted from nearby junipers to land on a rosaceous plant (= susceptible host); more specifically the spores landed on developing Callery pear fruit. Presumably, environmental conditions conducive to infection were present. Had any one of these three conditions not been met, this Alert would not be necessary.
On a side note, hawthorns (Crataegus spp.) are also members of the rose family and are a common host for the cedar-quince rust fungus in Ohio. BYGL Alerts highlighting "rusty hawthorns or "hairy hawthorn haws" have been annual postings for years. Of course, hawthorns have not been used nearly as frequently in Ohio landscapes compared to Callery pears.
Finally, there is nothing you can do to limit this disease in 2020; the rust-colored die is already cast.