Surgeons in the San Fernando Valley Thursday reported a surge in the number of inquiries about colon and rectal cancer, a direct result, they said, of President Reagan's widely publicized surgery.

Physicians stress the need for regular testing because the disease frequently exhibits few or no symptoms until it has reached an advanced state, when treatment is less often successful. People at particular risk are those with a family history of colon or rectal cancer or polyps. High-fat, low-fiber diets have also been linked to a higher incidence of the disease.

In particular, they argue that Reagan's bout with the illness may help overcome patient reluctance to investigate signs of colon or rectal cancer.