For the second time in five months, Republican Gov. Bruce Rauner signaled his distaste for broadening the state’s new medical marijuana program, declining to add chronic pain and seven other health conditions to the list of diseases that can be treated with the drug.

The Department of Public Health announced the decision, spurning recommendations of an expert advisory board largely appointed by the governor’s predecessor, Democrat Pat Quinn.

The expert panel reviewed medical evidence and listened to patient testimony before recommending the eight conditions, including post-traumatic stress disorder, which affects many military veterans. Other conditions recommended but rejected Friday were autism, irritable bowel syndrome, osteoarthritis and four pain syndromes.

New conditions would have meant adding more customers to a faltering industry: Regulated medical marijuana sales began Nov. 9 in Illinois, but just 4,000 patients have state approval to buy the drug at licensed dispensaries.

Supporters hoped that after nearly three...