• For powdery mildew we use food grade potassium bicarbonate from a brewing supply company or millstop. 
• We had corn borers 1 year. We can use dipel and gemstar. If you see wilting, start looking for drill holes right away in the stem.
• I find organza bags to be the magic bullet for all petal chewing bugs. If you have a variety that sunburns or bleaches, darker bags seem to provide protection for that also. I do not grow many large varieties so I use 5x7 and 6x9 sizes. I just remove the bags in the field and put it on the next one up in the same motion. I can grow any color, and if careful, get 2 years out of a bag.
• www.cgdahlias.com has some nice info and links on their website about insects and diseases.
• We HIGHLY recommend rotating crops, and using cover crops.