What are these spots on native Texas ash leaves?
Thank you to Shannon Viscardi for this great question about spots on her Texas ash tree. We consulted Dr. Kevin Ong, Extension plant pathologist and director of the Texas Plant Disease Diagnostic Lab, for this one, and he reminded us that disease development is always reliant upon environmental conditions.
When weather is rainier and/or cooler than “normal,” diseases take advantage of the situation. And when spring arrives earlier or later than normal, this also contributes to disease development, with some, perhaps normally less common, diseases becoming more prevalent due to abnormal weather patterns.
The weather also has an effect on our ability to treat for diseases, since many products should not be used once temperatures turn hot and the air becomes dry. And that is the case here.
The problem on this ash tree appears to be some sort of fungal leaf spot, so, while a fungicide would be in order, by the time Shannon noticed the problem, summer had settled into Central Texas in full force.
Also, fungal treatments for leaf issues are most effective when the leaves are developing, before their waxy cuticle develops. Once summer arrives, most plants take a break from growing, so no new, susceptible leaves are formed, thus, no need for treatment.
Another reason to avoid treatment in summer is that in our extreme heat products can easily burn and damage the plant. And so, our best recommendation is to let this problem run its course and see if it develops again next year, when you can get on top of the situation from the very start.