Answer: Well, let's make sure the spider mites are gone. They are fairly tenacious about hanging around. Use a magnifying glass and check the undersides of the affected leaves for a "dirty" or "rusty" appearance. Maybe that's what the brown speckles you see are. Look for very, very fine webbing. You didn't say how frequently you sprayed with the plant food, but spider mites are best controlled by washing the plant off daily with a strong stream of water. If the problem is bad, you may need to treat them with insecticidal soap.
If you determine that the brown speckles aren't mites, it could well be a rust, which is a fungus that can be transferred via water splashing on the soil, etc. Cut off any affected leaves. Mints can typically take a fair amount of pruning, and although they may not look vigorous over the winter, they should come back next spring. Also, you didn't say if the radiator is on when the plants sit on it, but warm heat wouldn't be a good location for them. I hope this info helps!
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