Aphids.....whitefly, greenfly, and blackfly attack some plants, but these can be spotted more easily, and a gentle spray of strongly diluted washing-up liquid usually works. The Salvias which seem to me most affected are Indigo Spires, gesneriiflora, tubiflora, carduacea, and farinacea.

Outside, I sometimes get die-back on S. fulgens and S. patens, probably a virus. I just cut off the dieing stems.

Weather conditions play an important part. No two English summers are the same, and some Salvias can behave very differently each year.

Robin.

Ive not had any diseases but I have had aphids attack. One year aphids nearly destroyed a sal. subrotunda and I was about ready to yank it out , then I saw one lone ladybug on the plant getting busy gorging herself so I let it be for a few days. LB stayed there untill the plant was completely clean and that plant turned out to be the biggest and best plant in my garden. Since you brought it up I thought Id relate that good story.