What causes diminished excretion by the parotid and submandibular glands? I do not any autoimmune diseases.

Common Causes. gland stones, infection, tumors, systemic disease, meds that decrease saliva production, etc. Regardless the cause, for the best long-term dry mouth treatment, visit orofacial pain or oral medicine specialist.
A few causes. Radiation to salivary glands, drugs, idiopathic (no cause discernible), other medical conditions like diabetes, sarcoidosis, HIV, some viruses. To get to bottom of problem, see oral medicine expert if you are close to university dental school. This article will give you some ideas to cope with this difficult problem: https://en.wikipedia.org/wiki/Xerostomia.