DR, "I thought it was caused by cooler and wetter conditions..." It's favored by cooler more humid conditions. But liquid water actually prevents PM from growing. We had a very rainy period for several weeks and the rain itself prevented powdery mildew. "In other parts of the country, you could have PM on Zinnias during the summer? And you need to control it by some method all during growing season? Bummer." I think you have gotten a somewhat exaggerated impression of the seriousness of the problems with zinnias. PM is also mainly a problem here in the fall. Fall comes earlier here in Maine than in California. There are areas of California that are very favorable for zinnia seed production. I have zinnias that are still blooming that are seven months old. I have already harvested seed heads from them, so they are senescent and have much reduced resistance to foliage diseases. Part of the problem with zinnias in the fall is that the plants are older and declining with fading natural immunity. Next year I am going to experiment with Messenger on my zinnias. My fall crop of younger zinnias haven't been sprayed yet, and only in the last few days have I seen any traces of powdery mildew. It's highly likely they will be felled by a killing frost before the PM becomes serious, so I probably won't even spray them. Zinnias, like roses, can be affected by a number of diseases. If you are motivated to protect your plants, as many rose growers are, you may very well spray them fairly frequently with a number of products. For most people, zinnias are a "plant and forget" ornamental. But for hobbyists like me, the zinnias are "my babies" and I am willing to spend some time caring for them. And that includes spraying perfectly healthy plants as a preventative measure. My zinnias are my "roses". I even get some of my gardening products from RoseMania. MM

There are biological products like Serenade you could try. The studies I have seen on it's effectiveness are all over the map though. Baking soda mixed with water or milk has also demonstrated some effectiveness when used regularly. Then there is sulfur. It's generally considered about as effective as the synthetic controls, but there are safety issues in using it both for the applicator and the plant so read the label carefully. Other products such as neem oil can be used preventatively. It is also an insecticide that requires ingestion to work on the insect so is considered 'soft on beneficials' If you use any oil based products, do not use sulfur on the plant. The combo will damage the plant. Part of the 'problem' may simply be the age of the plant. These are annuals and once they have produced a certain amount they age and die. It may just be that it's their time and so are weak to whatever is in the area.

I've used other brands of potassium bicarb fungicides. As others have said, it is pretty good for powdery mildew. However, over the years I have culled my roses of the worst offenders. These days I get in my dry summer climate with a single copper fungicide spray at pruning time. In a more humid climate I might have to reconsider the fungicide issue. Here I don't use them much. Folly

It appears that the reference that I gave about the problems of wood mulches and the advantage of compost could use some backing up by more primary scientific material. The link below is to an Ohio State University Extension Fact Sheet, Plant Pathology, 2021 Coffey Road, Columbus, Ohio 43210 Title: "Control of Nuisance and Detrimental Molds (Fungi) in Mulches and Composts" Please note the following and put in context of the full article (also note the appearance of statements such as "These bacteria compete with fungi to reduce the potential for the development of major mold problems.") "The best way to avoid all these problems and bring about beneficial effects by mulching is to add nitrogen to woody and hardwood bark products followed by composting to lower the carbon to nitrogen ratio. Blending of grass clippings with wood wastes before composting is one way to achieve this. Addition of poultry manure or urea to supply 1.2 lbs. available nitrogen per cubic yard of material satisfies the nitrogen need also. Some landscapers add 10-15% by volume composted sewage sludge to hardwood bark or wood wastes, and this makes an ideal product that has performed very well in landscapes. These amended products should be composted at least six weeks. This process kills plant pathogens, eggs of insect pests, and produces a nitrified product that releases plant nutrients rather than ties up nitrogen. As mentioned above, the microorganisms that have colonized these products reduce the potential for growth of nuisance fungi and provide control of many plant diseases" "What to Do Once the Problems Occur Sometimes very little can be done to control nuisance fungi other than to spade the mulch into the surface soil layer followed by soaking with water. Another option is to remove the mulch, place it in a heap after thorough wetting to allow for self-heating to occur (110-140 degrees F). This will kill nuisance fungi. If fresh dry mulch is placed on top of mulch colonized by nuisance fungi, the problems may occur again the following year or even earlier. The best control strategy for homeowners and landscapers is to purchase composted products low in wood content. Fresh, finely ground woody products should be avoided for many reasons unless composted first. Coarse fresh woody products are much less likely to cause problems unless applied too deep. It is important to soak all mulches immediately after they have been applied. Generally, mulches should not be applied to a depth greater than two inches. Mulches and composts applied in this manner provide many types of beneficial effects rather than nuisance problems, or worse, plant diseases. Sour mulches should be avoided altogether." Here is a link that might be useful: link for above