How do I prevent powdery mildew from affecting my large Zinnias?

HighlanderNorthJuly 11, 2014

I am growing 2 types of very large Burpee zinnia plants that I grew from seed, just like last year and 2 years earlier. They get up to about 6 feet tall, even though advertised to grow to 36", and even though they get LOTS of sun all day. The first year I grew them in Pa, but this year I am growing them in Delaware like I did last year. No problems with them when I grew them in semi-rural Pa, but last year in suburban Delaware, they became covered with powdery mildew by August. It got to the point that all the leaves were basically whitish gray.

I read that they have less chance of becoming infected if grown in an open, airy place where they get lots of breeze and arent too tightly packed together. Well, that sort of cant be avoided, because there are large Hibiscus plants in front of them, and the zinnia get bigger than advertised, so they end up touching each other. But they are open from 3 sides, where there are no other tall plants against them.

I mixed a little of the Bayer all-in-one Rose feed, fungus and insect control, which I poured at the base of the plants per label instructions. That was about 2 weeks ago. But that will probably only last a few more weeks, then it will be August, and I'd rather not use that product due to the extra nitrogen which will make them grow even taller, but more importantly because of the Imidicloprid it contains, which may kill the praying mantis and beneficial insects as well as potentially cause problems for bees(per recent reports).

What else can I safely apply that will help prevent powdery mildew from forming without the insect control or extra fertilizer? After it started covering the plants last year, the fungus control didnt do much to clear it.

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Zinnias are one of those plants that is very susceptible to Powdery Mildew and understanding something about how and when and where PM can strike can help you control it.
I have found that two simple mixtures can help. 1. a 50/50 mix of fat free milk and water, and 2. a teaspoon of Baking Soda mixed into a quart of water, and both sprayed as needed.
Perhaps this link might be of some help[.

Here is a link that might be useful: Powdery Mildew

    Bookmark   July 12, 2014 at 6:37AM
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rhizo_1 (North AL) zone 7

You need to make use of products that do not contain all of those ingredients. There are other options! Avoiding systemic insectides on plants that are so attractive to bees and other insects that visit for the nectar and pollen should be important to all gardeners.

Imidacloprid is not likely to affect the mantids, but the bees!

    Bookmark   July 12, 2014 at 7:28AM
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ken_adrian Adrian MI cold Z5

6 feet tall zinnia??? .. who knew ..

there is a Z expert int he annual forum ... zenman ... but he is going to tell you the same thing ...

frankly.. we all have plants we quit growing ... simply because they arent worth the effort..

i gave up on Z long ago for PM ... as well as monarda ... etc ...

if you know its going to happen... why do you keep at it???

also.. i believe there are resistant varieties... but its still can be a problem ...

you went kinda nuclear on a remedy .... especially when there are thousands of alternatives ... but regardless.. the remedies will slow it down or stop PM.. but none that i am aware.. make it disappear.. meaning.. a leaf with PM is always going to have the remnants of PM ... how do you know the Bayer didnt work???

also.. pm is topical.. on the outside of the leaf... i can imagine.. how a systemic.. inside the plant.. can have any effect on it ...


    Bookmark   July 12, 2014 at 9:22AM
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rhizo_1 (North AL) zone 7

Ken....PM starts out on the surface of the plant tissue but very soon into the infection will send structures (haustoria) into the cells. That's how PM can result in the death of tissues. It's not merely a disease of the surface.

Highlander, I have not tried the baking soda or milk treatments so can't speak to their effectiveness from personal experience. I can share my very positive experiences with horticultural oils, including neem, applied to the foliage.

The oils do two things: kills the spores on the surface upon contact AND prevents the successful spore germination in the first place. Avoid spraying anything directly on the flowers. Read and follow the directions on the labels.

I assume that you know the importance of practicing good garden hygiene in terms of your maintenance practices regarding PM. The spores overwinter in plant debris, old mulch, and even on the soil.

I opt to avoid disease prone ornamental varieties in favor of resistant cultivars. That's my choice for all kinds of problem prone plants.

But the vegetable garden is different. That's where I have to resort to neem oil or frequent syringes of water. I don't have and can't recommend any particular commercial fungicide though I know that there are many which are labled for PM control. But without all of the cultural controls in place, no amount of chemicals will be of much good.

Hopefully, Zenman the zinnia sensei of the forum, will see this and share his expertise.

    Bookmark   July 12, 2014 at 12:09PM
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To ken_adrian: To your first point, I used the 3 in one bayer product because 1. I needed to fertilize, 2. I needed fungus control preventative, and 3. There are multiple insect pests nearby, and they were beginning to damage the zinnias. So I needed the benefits offered by that product, however I would rather use something else, which is why I posted this thread here.

To your 2nd point: You basically asked why I keep trying to grow something if I have failed, and if I know something bad is going to happen with them. Well, as I pointed out in the OP, I didnt have any major problems with them the first year I grew them, and really liked them! It wasnt til last year I had problems, and that didnt start til after they had been growing for over 3 months! I dont believe in giving up so easy just because of a problem 1 year. If everyone thought like that, nobody would grow grass lawns! There's always a risk, but you must decide is the risk worth the reward? I think so in the case of these larger zinnias..... These things produce boatloads of 4"+ wide blooms for 4 1/2 months here! Not many plants that do that. I also grow dahlias, and they do the same for about 3-4 months...

    Bookmark   July 15, 2014 at 1:40AM
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