Downy mildew has been a big problem on impatiens plants in the eastern part of the U.S., and it can also get on plants in the cucumber/squash family. Moisture/high humidity encourages its spread. If it has infected your impatiens, there's not much you can do except get rid of the infected plants to keep it from spreading, and be sure the remaining ones aren't spaces close together. The fungus spores can survive the winter in some parts of the country, so depending where you are, next season you should plant something in that spot other than impatiens.

I battle powdery mildew, but this is first year for pumpkins
I am in Richmond Va. The humid south.

Here's some more information about both: http://www.caes.uga.edu/extension/cobb/anr/Documents/Powdery.pdf

which has a great facebook page.

just shift the pH on the little bastards-
1 gallon of water
1 tbsp baking soda
1-2 drops of dishsoap

spray

did it the other day. appears to have worked really well. will do one more spray today or tomorrow.
have a sneaking suspicion this would work on a lot of things.

I was thinking of a foliar spray this weekend with kelp to give them a boost.

and am still amazed that something so simple can be so darn effective.

Downy Mildew Milk spray
a weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water)
significantly reduced the severity of powdery mildew infection on the plants by 90%.

While some gardeners may be tempted to increase the concentration of milk for more control,
Bettiol found that once concentrations rose above 30%, an innoccuous fungus began to grow
on the plants.

low fat milk works fine.
The 1 to 10 ratio is perfect.

What you might want to try...spray half the plants with milk, half with the soda mixture and see what happens.
I would definetly spray plants every couple days until it looks like it is working, then spray less often.

There is probably a lot of good info online about the milk mix and how/why it works.

i don't know what the pH of milk is, but i would think the calcium in the milk would neutralize any acidity. that's the point of the baking soda spray. mildew needs acidity.