My dahlia buds all seem to have brown spots and when they open the brown spots appear large and not pretty . Plants are 3 feet tall . Healthy stems and leaves. Full 8 hours of Sun I was watering frequently due to a month of no rain then we had heavy rains. Well drained soil
Thank you for contacting the Toronto Master Gardeners about the brown spots on your Dahlias.
In your photo, I noticed the water droplets on all the foliage. Absent of any visible insect pests on your dahlias, I suspect the brown spots are related to watering.
Water your plants thoroughly twice a week if there is insufficient rain. Mulching around the dahlias will conserve moisture and keep the soil uniformly moist.
Water in the morning so any residual moisture on the leaves and buds has a chance to dry off during the day. Water the soil rather than the plant, try to avoid watering the foliage/buds. Excess water on the foliage and buds/flowers could result in fungal diseases taking hold, a possible cause of the brown spots you see.
Here is a link to a website where you will find some watering recommendations from the American Dahlia Society.
Dahlias are beautiful flowers. Good luck resolving this problems with them.