When was the last time you fertalized? Roses need to be fertalized 2-3 times a year (get a slow release formula specifically for roses). Your roses might also benefit from bonemeal to restore potassium to the soil. Lastly, is the yellowing because of rot/fungus? Cooler, humid climates cause numerous infections on roses, but are easily managed with garden sulphur.
Not sure if this will save this year's rose hip harvest, but it will help for next years.