I have a couple of roses that has severe black spot disease. I have been trying to treat with the copper fungicide and things from box stores like Walmart but it’s not helping at all. I have researched and found I need a systemic fungicide but do not know which is the strongest. I need the strongest one since my roses are almost bare of leaves since it’s rained so much in Ky lately. Any suggestions or recommendations is greatly appreciated.
You can read past issues of the ARS magazine and find several effective treatments. Personally, I use nothing but BioAdvanced All-In-One twice a year, around April 15 with granulated and around July 15 with liquid, and have had superb results. No more black leaf where I used to get it routinely. I never use granulated in the summer since it would still be in the soil when the roses are moving toward dormancy. Always pick up fallen black leaf leaves from the garden and dispose of them in the garbage–not the compost.
Copper oxanonate failed me for years. This year I switched to Bonide Infuse (propiconazole) applied to the leaves and it worked much, much better. Also be sure there is air circulating between leaves: don’t let the foliage get too dense. Timm’s post about picking up leaves is spot-on. [Sorry]. And presumably you know not to spray water on the leaves before nightfall. Good luck.