How to prevent Powdery Mildew on large Zinnias?

9 years ago

Last year I grew about 6-7 large Zinnia plants in the front bed just outside a client's front door, so they were the most obvious plants you'd see anytime you looked out that door or walked up to it. They were labeled to grow about 36-40" tall, but for the 2nd time I grew these large Burpee Zinnias, they actually grew to 6 feet tall(72").

But once August and September rolled around, they became affected by Powdery mildew, which quickly covered almost every leaf on every plant, and made them look gray-ish. I tried treating them with fungicide, but it didnt really work after they were already affected, and I tried spraying off the fungus with water, but that didnt work.

So I replanted more of these same Zinnias in the same area, only I planted more of them. Its now 6-14-14, and they are between 14" to 24" tall right now, and I am wanting to find a reliable preventative measure to stop them from becoming covered with Powdery mildew again.

Previously the only issues I had with these type of Zinnia were Tree Hoppers that would literally chew all around an area on each stem, causing the stems to break off precisely where they chewed. I have sprayed with insect control already.

What can I do to prevent powdery mildew? Thanks....

Comments (5)

Kimmsr
9 years ago
You cannot prevent Powdery Mildew from happening, but you can takes steps to minimize it by understanding when and how it will appear and which plants are most (Zinnias) susceptible.

Here is a link that might be useful: About Powdery Mildew
rhizo_1 (North AL) zone 7
9 years ago
I very much disagree with Kimmsr about not being able prevent PM. I consider it quite preventable in most cases.

The housekeeping tasks mentioned in the fact sheet linked above are essential as are cultural changes you can make. But daily rinsing is truly helpful, as is occasional mists of neem or other horticultural oil.

It's very important to understand that zinnias are very susceptible. Very. You might consider researching resistant hybrids or even using something else, entirely.
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ken_adrian Adrian MI cold Z5
9 years ago
the number one thing to avoid ... is wetting your plants in the evening...

mildew thrives in dark and damp ... especially during the warm/hot nights of summer ...

and if you are spraying them down in the evening.. you are making matters worse ... a water wand and watering only the soil also helps ...

increasing air movement also helps keep them dry .... planting too close usually ends up being the problem ... so next year.. space them better ..

if you cant find CULTURAL information on PM .. you arent trying very hard ...

plus everything they said ...

including moving on to a new plant ... its not too late to rip them out.. and plant something else ... it took me a few years ... to learn that beating my head on a wall with problematic plants... doesnt usually work out well.. for the wall.. nor my head .. lol

ken
highlandernorth
Original Author
9 years ago
To Ken_adrian: Thanks for the advice, but I am not one to give up on a plant because I had a problem with it for a little while during 1 season. I have dealt with issues with dahlias as well from time to time, like slugs, spider mites and tree hoppers, but I stiIl grow them. I think some things are worth a little extra effort, and its really not that much more effort anyway to be honest.

I grew the same variety of zinnias about 2 years prior to last year's powdery mildew, and at that time I had more zinnias in a more shaded area that tended to hold moisture/humidity moreso than this new area does, yet I didnt have any problem with powdery mildew at that time.

I dont mind having to take 10-15 minutes 2-3 times a season mixing up some spray solution and coating the plants with a safe, relatively non-toxic spray that helps prevent fungus if it means having 5-6 foot tall flowering plants that continuously flower from June til early November.

I will also try to keep them more ventilated. Maybe they were too close last year. But we also had a generally wet first half of summer.
Kimmsr
9 years ago
As the plant pathologists from the University of California indicate in the article linked above water on plants leaves can kill off the PM spores or keep them from gaining a foothold. However, they do not recommend allowing those leaves to remain wet overnight.
Spending some time with that article may prove worthwhile.