This year has been a rough one in the garden. I don't remember ever having a year with so much powdery mildew, and so early in the season. I grow about 75 dahlias on a small city plot and the mildew showed up early July this year, at least two months earlier than I've ever seen it on my dahlias. Because I was growing many rare, expensive varieties that I planned on selling in the spring, I knew I had to do something to save my plants. Powdery mildew rarely kills a plant outright, but it definitely affects the health of the plant and it's ability to photosynthesize, which is critical for tuber production and energy storage. I use my income from dahlia tuber sales to help support my family, so nipping this problem in the bud was a high priority.
Because I'm a master gardener and a friend to pollinators, I started at the bottom of the chain for treatment. I used neem oil and then copper fungicide to start, spraying diligently in the evenings, rubbing it in to leaves and cutting off any foliage that was too far gone. I stripped the bottom 12" of leaves from all of my plants in an effort to contain the moisture effects from watering. I did this for weeks, but eventually I came to the realization that the neem and copper wasn't working (every plant was getting worse) and I moved on to a product called Atticus Gravex on the recommendation of some professional growers. This product required full PPE and keeping my pets indoors while it dried on the leaves. I had high hopes and crossed my fingers hard, but after application I saw mild improvement, and then a week later the powdery mildew returned with a vengeance.
Meanwhile my dahlias were flowering well, in some cases not as vigorous as the year before, but not a disaster, but as I defoliated the plants more and more trying to contain the issue, I knew I needed a systemic treatment or I wouldn't get any tubers. I went back to a product I had used once before with success, something that is actually marketed for roses: Bio Advanced All-in-One. I had to play around with the concentrations and amounts because there weren't clear guidelines for dahlias. I ended up using 4oz per 2 gallons of water and used about 4 cups of water per plant. Please note this is also a fertilizer and contains alot of nitrogen, not something I really wanted to do in August, but considering how I had just defoliated almost all of my dahlias down to the bare minimum I figured it couldn't hurt. (for those that don't know, you generally don't want to give a dahlia much nitrogen after they have grown to their full mature height, this can cause them to stop making flowers and instead focus on foliage). FYI the active ingredient in this fungicide wise is Tebuconazole, and the concentrations I was using were on the weaker side, if you have a really bad fungus problem on a dahlia, I would use as much as 2oz of concentrate per plant diluted in 2 quarts of water.
Long story short, by the first of September my plants had bounced back well, the powdery mildew was under control, and I breathed a sigh of relief. I was still getting flowers and lots and lots of green foliage growth due to the fertilizer in the product. It hadn't been my plan to use a chemical, systemic fungicide to control my powdery mildew issues, but after exhausting my other options, it felt like the right move to save my plants. I remained concerned however about pollinators and the possibility that they could be affected by this product. I began dead-heading twice a day, anything that looked like it might be about to open it's center was discarded immediately (I did this because this product, because it's systemic, can become concentrated in a plant's pollen and affect the nervous system of bees). I found no dead pollinators at all, and deeply hope that my diligence was the reason, and not that they simply flew off elsewhere and succumbed.
It's always a dilemma in the garden -- trying to interfere as little as possible, but also balancing that with the reality of running a small business. I'm not happy about using a chemical fungicide, but I'm also grateful that the product existed and I could ultimately save a portion of our family's income from evaporating.
It's now October here and in the past two weeks the powdery mildew has returned, and I have decided not to treat it again. At this point the season is almost over, and while it's not an ideal situation, I'm not prepared to treat my plants again and risk any further eco-system effects. I also know that powdery mildew doesn't persist in soil, and it needs living tissue, so when the dahlias die back, the mildew will likely follow, and next year will hopefully be a different story.
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