Rose plant brown to dark spots
I've tried various pesticides and fungicide including: 1 - Bayer all 3-in-1 (last applied 2 wks ago) 2- Tristar 8.5 SL (applied yesterday) 3- Southern Agriculture liquid copper fungicide 4- spectracide immunox multi-purpose fungicide All with limited success. I'm also provide liquid and granular fertilizers to help them grow and fend off fungal and insect damage. But some breed of roses are vulnerable, no matter what. Please advise.
Your roses appear to have black spot, the common fungal disease of roses. We recommend starting with highly disease resistant varieties.
Because fungicides do not cure black spot, but can only prevent it, spraying needs to begin before infection starts. In the spring, watch your roses closely. Familiarize yourself with the stages of black spot infection beforehand The first time you see a single leaf infected with black spot, pull it off and put it in the trash. Then immediately spray with a fungicide listed for black spot. Rotate fungicides. If you have a highly susceptible rose, you may need to spray weekly.
Be sure your roses get enough sun (some afternoon shade is fine) and water during droughts. Do not overfertilize, as overly tender foliage is more susceptible to disease.
We do not recommend treating the shrubs with a systemic pesticide containing imidicloprid, as pollinators will be visiting the blooms and contacting the pesticide.
Here is our webpage on blackspot: https://extension.umd.edu/hgic/soils/soil-testing