The first, best strategy is voice. Most dogs understand "bad dog!" and "no!" and that usually does it. I don't use spray, but I will wave a pump at them. I used to ride with a guy, and he began packing Halt. At a farmhouse on our ride, located right at the top of a long grade, a large dog gave chase. He dismounted and emptied the can in the dog's face. Fido was cured of chasing. Another riding companion who is mortally afraid of dogs did the same to one on his commute route. The dog eyed him from the porch for months before trying another chase. He simply held his hand out as if he was about to spray and the dog peeled off and ran for cover.

There is a product called the "dog dazer" that purports to use ultrasonic blasts to fend off dogs. I have no idea of its effectiveness.

In this part of Central California, farm dogs are often encouraged to be aggressive. I once had a youngster audibly sic his dog on me. Had I been able to stop, that young man would have learned a lesson in corporal punishment that he'd not soon forget.

I love dogs, and share our home with one, but dogs allowed to run loose chasing cyclists typically come to a bad end, and it's the fault of negligent owners. Aggressively discouraging the dogs from chasing is likely to prolong their lives significantly. Years ago, a friend and I were riding to play tennis. He had an old wooden racket with the frame clamped around its head. When a German Shepherd gave chase he vigorously thwacked said dog. Suffice it to say that the dog no longer chased cyclists. Good for the dog, good for the rider.

Happy trails.