Adding picture:
{{gwi:644827}}

I see something on the top leaves that concern me. Check under the leaves. Do they have spider mites on them? They are barely visible with a magnifying glass, but leave fine webs. Particularly check near the ends of the leaves. If so, a pesticide spray, or a mixture of dish soap, rubbing alcohol, and water may be in order.

I see the white spots near the bottom. These white spots can be caused by sun hitting the water drops, which in turn act like a magnifying glass and end up burning the leaf. When watering try to water the base of the plant rather than the leaves, particularly if after 9:00 a.m., but better still try to water around 7:00 a.m. when the sun is not too strong.

This watering thing is complicated. I have a sprinker system and a big lawn. I need to water the lawn but don't want to get the water on the dahlia leaves. I water them by hand. We just can't control water getting on the leaves as if it rains so be it.