Summertime: And Diagnosis is never easy. Even with something as dear to my pathological brain as apple scab and cedar-apple (or hawthorn) rust on crabapple. I was on a walkabout at a northeast Ohio commercial landscape two weeks ago and came upon side-by-side crabapples – and the different symptoms of these two diseases on crabapple.
On one crabapple, apple scab infections caused some affected leaves to turn yellow before dropping; on the other crabapple the leaves turned orangish-red (the cover photo). As for rust, the lesions on the upper leaf surfaces of one crabapple were reddish-orange; on the other crabapple they were the more typical orange-juice orange.
This brought to mind an image Joe Boggs used to show of lipstick-red rust lesions on crabapple. I claimed this was a photographic artifact. Joe insisted that this was what rust looked like; I said his brain was beginning to corrode. We were both right (not about the cranial corrosion), he was showing it like it is – but not for most crabapples with rust.
This is an interesting example of the Host component of the disease triangle: with both scab and rust the host expresses symptoms differently, reminding us that diagnostic symptom expression is variable and thus a challenge.
Further with rust diseases this is a reminder that the discoloration on top of the affected leaf is not actually the presence of the rust fungus (Gymnosporangium juniper-virginianae or J. globosum), the “sign” of the pathogen. Rather the presence of sporulation is seen on the underside of the crabapple leaves with the spores that return to junipers (Eastern red cedar) and with spore tendrils on the junipers. The orange (and less commonly red) lesions atop the leaves are the plant’s response to the infection (the “symptoms”) of the disease.
Other diseases induce variable symptoms as well, such as that of Guignardia leaf blotch of buckeyes and horsechestnuts, from brown to red, and with yellow edges. Just one of the dilemmas and delightful aspects of plant problem diagnostics. Speaking of which look for these upcoming diagnostic workshops and walkabouts. And, there are many more.
August 3: Tree Diagnostic Workshop, OSU-Mansfield, Kathy Smith (woodland stewards.osu.edu). See Upcoming Events.
September 7: 89th Ohio Plant Diagnostic Workshop, Secrest Arboretum (info coming soon)
August, September, and October: BYGLive Walkabouts (first Mondays) a la Joe Boggs, OSUE Hamilton County