Bonsai-gecko

You could fertilize at any time, but you shouldn't. In most cases you should only fertilize a tree when it is in active growth. This time a year in your area fukien will grow very slowly(if at all) during te winter and should only get a very very mild dose of fertilizer even if it is actively growing.Its wise never to fertilize a stressed tree as the fertilizer may overwhelm the already weak roots.

Black Spot is a type of fungus(mold) attack.Its common on roses, etc. Fukien is very suceptible to it. The yellowing leaves aren't a cause of it, but allow it to grow and spread even more rapidly. Outdoors fukien almost never falls prey to black spot because the wind blows away the spores. Indoors the spores aren't dispersed and land right on other healthy leaves and the disease can spread shockingly fast. For fulien t is not even a good idae to let dead leaves sit in or near the pot as they can develope spores and infect the healthy tree.Its also not advised to mist the leaves of a fukien at night or in low light levels, as the moisture will only increase the likelyhood of a black spot infection.

Fukien is VERY sensitive to pesticides/fungicides/chemicals, if you develope a problem you need to treat make sure you ask around and reseach whether the chemical is safe for fukien.I know any amount of diazon(among others) is fatal to these trees.

Glad to hear your trees doing better already.Some people have problems with fukiens sensitivity, but once you learn their quirks they are almost impossible to kill.