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