Removal of lower leaves to help with air circulation is the only thing we do to control mildew. A friend sprays with skim milk & swears by it.
How do you trail an upright plant like a dahlia??

I had not one speck of mildew last year even though I planted quite a few plants closer together than usual. Remove the bottom 9-12 inches of leaves- great advice, and plant 2-3 ft apart if you can spare the space.

I have heard of corn gluten for weed suppression, but never dry molasses. What is the purpose of that?

As far as disease in plants, organisms are already present in the soil, in the plant, and on the plant. These organisms generally exist in a pretty much balanced relationship with the plants. If the plant gods shine down upon us, our plants flourish and we have beautiful plants and flowers. However, sometimes this just does not happen and we have terrible growing conditions such as a drought or days and days of rain with no sun. This drastically shifts that balance, and most likely your going to have a disease epidemic situation on your hands. I've heard from many growers in Pennsylvania. They've talked about the difficult 2006 growing season due mostly to all the rain.

The microorganisms which cause plant disease are: viruses, bacteria, or fungi. The most common diseases found in flowering crops are caused by fungi. Botrytis, rusts, mildews and damping-off organisms are examples of this. Preventative measures such as good air circulation, proper drainage and good sanitation practices are a primary management practice. Spraying of the crop is considered a secondary measure.

Circulation: Here in the Upper Midwest in the Great Lake Region, we also suffer from high humidity during the growing season. Stagnant air does nothing but promote the spread of fungi. I realize that you're not a grower; however, it is worth mentioning for your benefit as well as the fact that some of my fellow cutflower growers do actually read the posts on the Dahlia Forum. As growers, our crops are planted with the prevailing winds. Our winds generally come off Lake Michigan; therefore, our dahlia plants are planted east to west. Our crops are planted tight - meaning one foot apart. In your house garden, you'll not want to plant them so close. Two feet apart is a good rule of thumb to follow. If they're planted closer, wounds on the plants caused by the friction of plants constantly brushing against one another exacebates the problem, and sets up conditions for colonization of fungi. Good air circulation can also be improved by pruning. This practice not only helps with the control of disease, it also increases the vigor of the plant and produces more shoots. Pruning also helps promote better light penetration and air circulation.

Drainage: Amending heavy soil with organic matter is essential. Water does not drain well in heavy soil which in turn produces not very healty plants; and, thus setting them up for disease. Good drainage also promotes nice, healthy roots and the production of healthy tubers as well.

Sanitation: It is important to keep your garden free of weeds. And, when rain and cloudy days are in the forecast, remove the weed piles from the area. If you're working in one area of your garden where disease may be present, be sure to hose down your boots and sterilize your garden tools with a bleach solution in order to prevent the spread of disease to another area in your garden. We've found that keeping our field free of weeds before the dahlia shoots emerge is so vital. Some commercial growers mound up their rows and cover the newly emerged shoots, and spray with Round-Up which is considered a post-emergence herbicide. Corn gluten is being tested to be used as a pre-emergence means of controlling weeds instead of the widely used commercial herbicides.

Again, spraying is secondary. Disease doesn't happen overnight. If you're out in the garden and doing maintenance on a daily basis, you're going to be aware of the warning signs. Generally, you will have a few days up to a couple of weeks advance warning. There are fungicides available; however, it might be worth noting that with the use of the fungicides you will also be affecting the beneficial fungi found in the soil. It has been our experience that spraying to control plant disease has proven to be futile. We have also heard of homemade milk concoctions. Our dahlias are never sprayed for disease. And, yes, there have been some years where we've seen some mildew. However, it's usually been really late in the season. And, a certain amount of disease is just to be expected and tolerated.

Finally, if you have followed the above practices of good air circulation, drainage, and sanitation, then you will have the very best chance of being able to survive some bad weather without the worry of any devastating disease.

Molasses is the best sugar for horticultural use because of its trace minerals. Blackstrap is hard to find but is the best molasses because of the sulfur and iron, but any kind will work. Molasses is a carbon source and feeds the beneficial microbes creating greater natural plant fertility. I will be using this in the pumpkin/squash areas.
Trialed the corn gluten meal last year with excellent results for a pre emergent and added bonus of 10% nitrogen. Also worked exceptionally well in the compost heap. Don't get the bag wet though, smells worse than any dead animal I ever smelled.

Round up, yes for post emergent. That was done in the fall, second application will be in the spring for the winter seeded weeds.
I have been using the 1 foot per plant measure,It's windy enough on this ridge for a reverse grid windmill. So, I'll strip the lower leaves.
I did purchase the flats from a local greenhouse, perhaps that or our lovely humidity of 97% in July or August.
I have noticed the plants I grow from seed preform better.
I redesigned my propagation bed for this year to increase plug/plant production. I think I am going to build a low tunnel as soon as some rebar can be pounded in the ground.
Space isn't an issue, 33 acres here...just have to have less field corn. I wish it was flat land and not Weikert soil.
Thanks for the info.

Powdery Mildew is an annual event here in my dahlia garden as my backyard has a lot of protection from tall pines so the wind is less than if the land was open. I don't get enough sun for my dahlias as a result but I would have to move so far North to find open land that moving is not an option.
I use a product called Rubigan which I start spraying in late July and add it to my spray every two weeks. If I am consistent with my spraying I will never see powdery mildew but if I don't use it, all of my dahlias get it. I do the lower leaf thing but mainly to rid the plants of the unsightly yellow leaves the heat causes. If you are an organic gardener, sorry for wasting your time. If you use chemicals this product really works. Rubigan is not cheap and can only be found at pesticide companies. I get it from Southern AG in Florida/North Carolina. You can Google it for more information.

I agree with the suggestions above....remove the lower 9-12
inches of leaves, practice good sanitation and plant 2-3 ft
apart. In addition, I also use RUBIGAN. I start spraying at the end of July/beginning of August at 2 week intervals and spray a total of 3 times. Powdery mildew is not an issue after that. Period.
DB

Thanks for the Rubigan info!
The end of July spray time sounds just about right for my area also Dahliaboy
I hope my 25 gal sprayer is up and running this year, I REALLY missed it when I needed last year.
Backpackin it Ok, but drives my tendonitis nuts.

Baking soda, 3-4 table spoons of baking soda to a gallon of water, dissolve and spray every two weeks ---> no mildew.

ACK! Are you sure that's not 3-4 TEAspoons to a gallon of water? (Which is the mix I use on my roses with great success.)

Krista