Sage is usualy not afftect by any fungus problems. Sometimes the whitish coloration is normal. You didn't say wther the plant is indoors, outside, under lights, in a greenhouse. I grew sage from seeds in the past and have never seen any problems. Avoid water on the leaves, and if the spots are few, remove the infected leaves. Direct sunlight is very necessary for good plant health. Any leaf sprays are done carefully and at very weak concentrations. A copper sulfate spray might help, but again, the mixture is misted on and and is at a very weak solution, per the label instructions. Also, Neem oil has been shown to have some help with mild cases of diseases. Suggest that you avoid wetting the leaves and get the plant out into the strong sunlight all day.

I'd not spray anything on your sage until you have a good diagnosis of the situation. Would it be possible for you to take a picture and attach it for us to gawk at?

I have had exactly the same problem and the plant looks like it is now dying. I am growing mine in a pot on windowsill inside. It gets plenty of direct sunglight and I try not to water it too much. I have taken photos if I can send them to someone as can't attach it here.

Is there any way you could have the plant spend some time in a sheltered spot outdoors? I have found that putting a plant beset some some pestilence outdoors can help the plant over come the problem. Whether it is fresh air, more light, wind, beneficial insects, all of the above, or none of the above it can help.

I'm not sure where you are posting from so make sure you know the outdoor temperatures (especially the lows) before putting a plant outdoors. Don't outdoor your plant if you are going to have freezing temps.

FataMorgana