Hello, bradster. Brown spots usually suggests a fungal infection but the type of fungus must first be identified before you can apply the correct fungicide. See the link below for some types of fungal infections and see which one seems to match yours.

I am concerned about how early in the year you have a fungal problem. It is normal to see some degree of leaf spots as the growing season progresses but this is way too early for that problem.

There are several things that you can do to prevent fungal infections. Separate plants so there is always good air movement between them. Pick up plant debris under the plants. Water the soil (not the leaves) early in the mornings: insert a finger into the soil to a depth of 4" and water the plant when the soil feels almost dry or dry. If the soil feels moist or wet, do not water. If the soil feels wet too often, try to determine why and take appropriate corrective action. Replace the old mulch -it is probably full of spores- with new mulch. In the Fall, dispose of blooms and leaves as trash, not on the compost pile. Additional steps that other people can take is to prune overhead shrubs or trees so the hydrangea gets an extra hour of sun; or use drip irrigation in this problem area. Note: the above mentioned suggestions can also be applied to other types of plants such as rhodies, azaleas, etc.

Here is a link that might be useful: Fungal Diseases of Hydrangeas by ACES