I think it's probably DM -- it's a match for what we were told was DM, and L.O. suffered badly with it here. It's more of a problem in a climate like ours, where it never stays hot enough for 24 hours to suppress it. Avocados have been a huge crop here, but Avocado Root Rot (a closely related organism) is a big problem. They've found that drenches won't eliminate it, and that their best control is to mulch around affected trees, out to the drip line, with fresh horse manure. The resultant bacterial colony seems to be more effective than any chemical approach, over the long haul. Of course, many have given up, and sold the land for development. Jeri

Interesting article, thanks for the link! Article focuses more on Downy Mildew in Impatiens rather than roses, but still enlightening. The general Rose Rosette advice of "bag the infected rose, seal the bag and put it in the trash." is what I have seen on this forum more than once.

Hi Anne: There's a guy in dry Colorado that I exchanged info. in HMF. He wrote that "Green Cure" with potassium bicarbonate worked really well for his mildew. Potassium is needed to fight disease, and bicarbonate has high pH, so mildew can't spread. Horse manure is high in potassium, and alkaline ... works well with mildew. EarthCo. that tested my soil stated that 1/3 of soil tested is deficient in potassium. Both potassium and calcium is needed to regulate osmotic pressure of cell wall. My soil is tested most deficient in potassium due to my pH 7.7 heavy clay soil. Mildew has NOTHING to do with how much water is given. We have flash flood today in Chicagoland. Our rainfall is 40" per year, plus 23" melted snow. I have a bed mulched with wet leaves, everything in there get mildew ... weeds and all the perennials, except for 100% clean Lynnie rose among them. If your soil is deficient in potassium, plants will mildew no matter how much rain. Adding gypsum (calcium sulfate) will help to de-salt saline soil and water, which helps in mildew prevention. It took some research to understand why my soil high in limestone is also tested deficient in calcium. Limestone break down into calcium carbonate and chloride. Both are salts. chloride binds with potassium in the soil, resulting in potassium-chloride, which is the stuff we use to de-ice in our zone 5a winter. Some municipals add hydrated lime to the water so pipes won't corrode, raising the pH of water to 8 or beyond. Lime, be it from soil, or water, will drive down potassium. Potassium is needed to fight diseases, root growth, more blooms and bigger blooms. That's why "Green Cure" with potassium bicarbonate works better than plain baking soda for mildew. One bonus about applying Sulfate of potash NPK 0-0-50, low salt index is: you'll get deeper color on your blooms. Sulfate of Potash and calcium would be the best bet: both produce the biggest root growth in hydroponic-fertilizer experiment. Gypsum (calcium sulfate) is documented to correct deficiencies in alkaline soil: raising iron, manganese, and zinc levels. SteveinAustralia with a soil and climate similar to California was the one who recommended sulfate of potash to me. See the below Australian link on the role of potassium: Here is a link that might be useful: Potash and Diseases: rust, powdery mildew, BS This post was edited by Strawberryhill on Thu, Apr 18, 13 at 12:25

I am not sure if I posted on your question about Downy Mildew, but Mancozeb/Manzate will NOT handle this problem. I took advice from a CR and professor of horticulture and ordered the Aliette (overnight, with the extra expense). I lost 2 roses but saved the rest of the bed. This is nothing like Powdery Mildew although many posters here have confused them. This is a potential killer.