Snails and slugs eat the tender shoots as they try to break through the ground in spring. USE SLUG BAIT to prevent these pests from eating up the tender new shoots. To control other pests a good spraying program is recommended. Be sure to spray Questions and undersides of plant leaves. Most spray programs should begin before you see a problem, preferably the end of June thru August.
Dahlias are also susceptible to powdery mildew.
This is not caused by overhead watering. It is caused by weather conditions. Dahlias should be sprayed in July and August for powdery mildew. The key is to spray before the problem appears. See Figure 1.