Hi Helen,

Thank you for the photo of your Black Eyed Susan. We would love to help you out here but it would be helpful to gather a little bit more information first if thats alright.

First, can you please send several other photos of the affected plant. Perhaps a close up, another angle, the underside of some of the leaves, etc. The more photos the better! Second, can you please tell us a little bit more about how long this has been happening and what the progression of severity has been like? 

Thank you,

CSU Extension - Master Gardeners

Helen,

Thank you for the additional photos. A few additional questions that I forgot to ask last time and then I'll share what I'm thinking and what I've found.

How is this area being irrigated? By what method and how frequently?

Have your rudbeckias flowered this year or has there only been leafy growth? If they have flowered, did the flowers present as expected?

It is difficult to say what exactly is causing this, but I've got a few ideas. I've done a bit of digging on this and have a few thoughts but unfortunately no definitive answers.

The distribution of the damage makes me lean away from an insect pest but I can't rule it out completely. You said you saw a grasshopper, were you able to get a photo by chance? One of the ideas I found is actually a bacterial infection that is spread by a specific type of grasshopper that we have here. The aster grasshopper can spread the bacteria Aster Yellows, which causes leaf spot and necrosis along leaf margins in plants. I am leaning mostly towards this being a bacterial leaf spot disease based on the lack of fruiting bodies seen in the photos but again, it is hard to be definitive.

Another idea that I found is a type of fungal leaf spot, specifically septoria leaf spot as rudbeckia can be susceptible to this. Some of the signs your Black Eyed Susan is showing are very similar to this but it was difficult to say from the photos if there is any sort of fruiting bodies attached in the margins of the leaf holes.

There are many types of leaf spot and a lot of them present with similar signs/symptoms so it can be difficult to diagnose further from photos alone. 

My best suggestion at this point would be to carefully remove as much of the infected foliage as you can, being careful to dispose of it quickly/not rustle it up too much. If the pathogen is fungal, spores could be released and it may spread further. Also, make sure the plant is dry when you do this as bacterial pathogens can spread more easily when the plants are wet. Be sure to sanitize your gloves and tools before using them to work on anything else. Then instill a watch-wait-and-see approach. Perhaps a pest will definitively reveal itself and then we can find a good control method to recommend. Perhaps the spots continue and fruiting bodies present themselves and again we can look for a good control recommendation from there. If no fruiting bodies or pests present themselves, there are some recommended treatments for bacterial spot diseases such as copper-based fungicides that have shown success in combating bacterial infections. Depending on how things go, removing the plant may be a good option should other mitigation methods fail.

Here are some of the resources I found when trying to look this up for you:

https://ipm.ucanr.edu/agriculture/lettuce/bacterial-leaf-spot/#gsc.tab=0

https://agsci.colostate.edu/agbio/ipm-pests/aster-leafhopper-and-aster-yellows/

https://extension.umn.edu/plant-diseases/aster-yellows

https://content.ces.ncsu.edu/leaf-spotting-bacteria-on-ornamentals

https://ag.umass.edu/greenhouse-floriculture/photos/rudbeckia-septoria-leaf-spot

I'm sorry that this isn't a more cut-and-dry response with a clear solution.

Best of luck, please reach out and let us know if anything else develops or you've got further questions on this.

Thank you,

Ashley - Colorado Master Gardener Apprentice