For a few years now, I’ve been saying that black spots on black eyed susans (rudbeckia species) were not a fungus. I even had seen insects on my black-eyed susans that I was sure were causing the damage–the four-lined plant bug.
I am not necessarily backing down from that diagnosis–but in researching some diseases for another post I came across some information about some diseases of rudbeckia. And I now think it’s possible that mine have at least a combination of insect damage and a leaf spot disease known as septoria.
Prior to this, I only thought of that disease as affecting tomatoes.
Of course, every extension service Bulletin I’ve read suggests spraying some strong chemical fungicide or copper to control this problem. I am willing to do neither, just as I am unwilling to do anything to attempt to treat for the four-lined plant bug.
This fact sheet from the University of Minnesota (one of the few with relatively good photos) has a decent discussion of fungal diseases affecting Black-eyed susans. They note that for their climate, septoria leaf spot is relatively common and the others are not.
So how do I plan to proceed? Basically the same way as always–with very good sanitation in the fall to prevent any over-wintering of “whatever:” fungal spores or insect eggs.
I’ll thin my black eyed susans a bit to increase the air circulation–that can never hurt.
But I don’t do any overhead watering. So I can’t stop that. What nature does is a different story! And since this June she’s dished out a lot of “overhead watering” (4th wettest June on record) I suspect that this year, at least, my black spots could indeed be some sort of fungal issue!