My Lilac has come down with something and I need to know what it is and how to fix it. Can anyone help?
The damage to these leaves does not seem to match any of the descriptions and photographs of typical leaf diseases of Syringa - bacterial blight/canker, powdery mildew, botrytis/gray mold, sooty mold. It is strange that the mid-rib on the reverse of the leaf is affected, which suggests a fungus to me, although I do not know. Can you take some samples to your local extension office? If there is any discoloration of the stems or trunk, that would be important information as well. It is also helpful to know what species you have there. Let us know what you find out.
Some microorganisms don't need a certain plant to grow in drops of moisture remaining from dew, rain or irrigation if the temperature is just right. The chemical changes they produce in the water can discolor leaves of many types of shrubs and trees. It's not identical, but if a dog urinates on a lawn, it would not matter whether the grass was bentgrass, ryegrass or bluegrass. Each could become damaged. The disease issue wouldn't be relevant. Don't abandon your search for a pathogen, but that came to mind from my outdoor adventures and pesticides class in college.