Defoliation of Sweet Box due to Wildfire Smoke in Oregon

Jay W. Pscheidt, Extension Plant Pathology Specialist

Coincidental with the heavy smoke that pushed air quality off the charts, we have seen unusual defoliation in plants. Much of it was normal old leaf drop in response to stress conditions. After a droughty winter (half our normal rainfall) and even doughtier summer we had those hot dry Santa Anna-like winds that resulted in a million acres of burned forest. Old (first to form) leaves that are shaded, diseased and inefficient for the plant turned yellow and were shed. Even evergreen plants (which aren’t) were casting those older leaves or needles. This is a normal response by many different plants and is not a concern. Most of these plants will survive and return in the spring for a new season.

What is new and concerning was the rapid defoliation of Sarcoccoca spp. (sweet box) and some Ilex spp. (various holly) and potentially some Euonymus sp. The Sarcoccoca experienced green leaf drop over a few days after the smoke cleared out. Reports of this symptom have been confirmed on plants grown in Eugene (classic smoke accumulation area in the southern Willamette Valley), Corvallis, Rickreall, Damascus, Milwaukee, Mollalla and somewhere between Oregon City and Canby. Washington County is not confirmed. This rapid defoliation was not seen in Seattle, Tacoma, La Conner, Bainbridge Island or Gig Harbor, WA. Plants in gardens and open air structures were affected but not in a plastic hoop house. There is less species information but it appears S. confusa and S. ruscifolia were affected but the more common S. hookeriana var humilis were not.

A report of a garden grown blue holly (Ilex x meserveae) and nursery grown Ilex sp. with the same rapid defoliation of green leaves occurred at the same time. These reports have been confirmed in Springfield, Coburg, Lebanon and Mt. Angel. Unusual defoliation of a large holly bush in Eagle Creek and a sky pencil holly (I. crenata) in Tigard was also reported.

Also, Euonymus sp. were reported with unusual defoliation where older leaves were being shed. In one case the Euonymus had this condition but the Sarcoccoca was fine. Although this symptom is more like the normal reactions described above there have been reports of this condition from Eugene, Molalla and Tigard.

Although the smoke was hazardous for humans and animals it is not generally considered a problem for plants. There was ash fall and blocked out sun for several days all of which contributed to these defoliation issues. It is important to realize the difference been a normal plant response (dropping of old leaves) vs something that is not normal (rapid dropping of green leaves). We suspect that most of these plants will survive pending favorable conditions this winter.

Notes: Sarcoccoca sp. are susceptible to a new fungal disease called boxwood blight but the symptoms are very different. Boxwood blight symptoms include leaf spots, stem lesions as well as defoliation. Rapid defoliation of green leaves is not a character of this disease.

Air Quality during this 2020 event was in the “unhealthy” range or higher (>150) for 7 to 10 days and in the “hazardous” range (>300) for a few days in much of the Willamette Valley. Average highest reading was 476 with a high in Salem of 563. Air quality during the 2017 Eagle Creek (Columbia River Gorge) fire by comparison was an average high of 158 with a high in Springfield of 332.

Jay W. Pscheidt, PhD, Professor

State Extension Plant Pathology Specialist