I have tried before mixing 1 tbsp. of baking soda with 1 tbsp. of salt and 2 tbsp. of olive oil in a liter of water and apply the mixture to the plants with a spray bottle. It looks like it works, but mostly as prevention measure. You could add cinnamon to the mixture to strengthen.
although the literature states that the seeds may be contaminated, as Shawn said inoculating with beneficial fungi could help. You can coat the seed before planting and hopefully the good will outgrow the bad. I recommend culturing from one of your uninfected beds, if you go with a store bought blend maybe try something with more Trichoderma in it.
Another consideration if one is contemplating purchasing one of the myco-mixes on the market, is if it contains Trichoderma spores. Because Trichoderma is so much cheaper, the spore count for it in these mixes usually eclipses all the other organisms put together. Unlike endomycorrhizal fungi, Trichoderma requires no root contact to sprout and grow. In addition to this, its favorite food is…..wait for it…..wait for it….other fungi! So you guess what happens if you inoculate your roots with a mix that contains 10,000 spores per gram of Trichoderma and 100 spores per gram total of other fungal species which are slow to sprout.
http://forum.grasscity.com/organic-growing/976433-mycorrhizal-fungi%3B-myths-truths.html There are quite a few Myco-related links sorry about posting a link to grasscity, but I am giving sources for information's sake
Also try neem meal. to apply as a spray put 1/3-1 cup into a five gallon bucket of water, use an aquarium bubbler and an air stone, bubble for 4-6 hours to release the oils from the meal, strain, then apply this mix at roughly a 1 to 10 ratio of water(may vary I've used a 1 to 5 ratio), you can spray plants the next day but is only necessary when hot, repeat weekly for safety's sake. The effects on squash heavily infected with powdery mildew was a death of all sporing bodies, heavily infected leaves and stem died, minimally infected tissue died, but a majority of each plant survived.