Lindabel,

"I would think the PM would need wet conditions to spread this rapidly."

That's a common misconception. In fact, spores of most powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Do a Google search for powdery mildew.

"I have been spraying weekly with a fungicide for PM, but it doesn't seem to be slowing it down."

Just out of curiosity, what fungicide have you been using?

MM

MM what do you use? I have kept this insideous disease in check with Daconyl. but I missed last weeks spraying by just 2 days and ~*poof** the entire plants are covered, I think a total loss :(

Can you get systemics like benomyl anymore?

oxmyx,

"MM what do you use?"

I use GreenCure® to prevent Powdery Mildew on my zinnias. It is quite safe to use and, if you wish, you can give your zinnias a daily bath in it.

MM

Thanks MM, very informative link.
similar to Baking soda remedy, I see.
Does GreenCure® work effectively?

oxmyx,

"Does GreenCure® work effectively?"

Quite effectively, if you apply it frequently. However, even a light rain washes it right off. In fact, heavy dews can wash it off. But potassium bicarbonate is very "plant friendly" so you can give your plants frequent shower baths in it.

Frequent use of sodium bicarbonate could run the risk of sodium phytotoxicity. In contrast, the potassium bicarbonate that washes into the soil provides nutritious potassium ions for the plant, and the bicarbonate ions decompose into water and carbon dioxide, both also good for the plant. It's a definite win-win, whether it stays on the leaves or washes off and is sprayed on again.

The other advantage of GreenCure® is their excellent wetting agent/spreader-sticker. The stuff coats the foliage very well. Not too surprising, since they tried "more than 350 different additives" to find a formula that was "significantly more effective than any other combination of ingredients." I know from experience that it's better than dish-washing liquid, which can be phytotoxic.

This year I have been somewhat preoccupied with building garden infrastructure, as well as gardening. Next year I plan to do lots more spraying on my zinnias, aiming at controlling bacterial blight, Alternaria, and Cercospora leaf spot. If you look closely, you can see traces of all three of those in some of my photos. Next year I'll be experimenting with a copper soap spray, copper sulfate, Physan 20, and mono-potassium phosphate foliar sprays. There is a commercial product based on copper sulfate (Phyton 27) and others based on mono-potassium phosphate (Nutrol), but I'll start with the much less expensive simple chemicals. And I'll continue spraying the GreenCure®.

Incidentally, I think Benomyl has been discontinued.

MM

Powdery mildew isn't 'quite' so species specific. The same strain (Erysiphe cichoracearum) infects many members of the same family (Asteraceae) to which both Cosmos and Zinnia belong. It won't bother your lilacs or squash, just for example.

And the spores of PM will absolutely be found (physically) in saved seeds...as well as (probably) the genetic predisposition for being infected readily.

A good idea might be to begin with brand new seed, disease resistant cultivars, and even place your cosmos and zinnia (new cultivars) in a different location. Obviously, anyone who is breeding zinnias cannot do this, though.

PM cannot be treated by spraying the soil, sorry. Good housekeeping practices would include removing fallen leaves, etc. And, of course, not using seed harvested from infected plants. ;-(

Has anyone tried neem oil against PM? Oil applications, of course, are very good as a preventative measure for this disease, and neem has the added umph of having systemic qualities. It's recommended for PM and black spot, among others.

I love Zinnias. Here in CA I don't get PM on Zinnias until Fall. I thought it was caused by cooler and wetter conditions, mainly because it never appears here until the weather changes in the Fall. 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. Had no idea. Guess CA's climate is a preventative in itself.

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

DR,

And, I might add that my zinnias are also my "orchids". For example, I use premium grade orchid nutrients for my zinnias that are grown indoors under lights. I will probably be trying some additional orchid nutrients on my zinnias next year.

MM