Hello,

Rhamnus spp is an alternate host of oat crown rust. Here is an excerpt of an article about it, with details

“Oats are susceptible to a variety of plant diseases including fungal, viral and bacterial pathogens. The most common disease with economic significance is crown rust, also known as leaf rust. This disease is caused by Puccinia coronata avenae.

Spores are air-borne and can originate in southern states, or come from nearby buckthorn, an alternate host that allows the disease to overwinter. Wheat, barley and rye are not susceptible. Symptoms include small, oval, orange pustules on leaves. Pustules may also appear on leaf sheaths, stems and panicles.

The disease spreads from leaf to leaf as pustules release spores. Under conditions ideal for the disease, new pustules can form in 7 to 10 days. Damage to the oat plant is due to leaf damage. 

 A properly timed fungicide application can provide protection against crown rust. Spraying should take place at flag leaf emergence. If pustules have already formed on the flag leaf, it is too late. Fungicides labeled for crown rust in oats are protective, that is, they can prevent the disease from entering the plant. However, once the pathogen is inside, a protective fungicide won’t help.”

I could find no control options for ornamentals, all the interest is in oat crops. You could try a fungicide labeled for ornamentals and for rust, but I would prune out the affected leaves and destroy them. Keep the foliage of the shrub as dry as possible, re-directing sprinklers away from these plants. Water in morning so leaves dry quickly. Keep the shrubs watered during droughts since a stressed plant is more susceptible to all diseases. As the cool humid weather conditions for rust subside, and the new leaves harden off for the summer, you should see less of this disease.

References-

http://woodyplants.cals.cornell.edu/plant/print/212

https://www.ncbi.nlm.nih.gov/pubmed/28846186

. Thank you for using our service.

Also, if you would like positive ID, please send samples to MSU Plant Diagnostic Services-   https://pestid.msu.edu