Ask Mr. Smarty Plants

Our first thought would be too much sunshine, as the hydrangea thrives best as an understory plant with dappled shade. However, in Portland that doesn't seem like a big danger. Hydrangeas are susceptible to sunscald, chlorosis in alkaline soils, and winter dieback. Many weak, brittle canes are easily broken in wind and ice. Forms colonies from a shallow root system. Canes can be cut to the ground every two or three years to keep the shrub smaller, but if the canes are allowed to grow, the naturally peeling bark is attractive. Pest free. Prune immediately after flowering. There are three hydrangeas native to North America, but they tend to be more in the southern United States and none are native to Oregon. If it appears that the plant is getting too much sun, give it extra water, making sure it is not standing on the roots, and try to arrange a little more shelter for the plants next summer.

If that doesn't appear to be the problem, look at this Alabama Cooperative Extension article on Diseases of Hydrangeas. There are good illustrations of each of the symptoms of the various diseases, as well as suggested treatments. And you can go to this Oregon State University Extension page and try to find out if other similar plants in your area are being similarly affected.