some varieties are susceptilbe to yellow tips. As long as the bulbing has already occured it isn't an issue that will affect yield.

Low fertility, particularly nitrogen can cause either yellowing tips or yellowing of the entire leaf, it seems to vary.

You don't have much longer to wait until harvest so I would suggest if you think low N is the problem, add a *small* amount of fast acting N like blood meal, fish emulsion or, if you are a synthetic user urea. If you add too much N at this point you will put production into leaves instead of the bulb/cloves.

Otherwise, just wait it out. N isn't terribly important for bulbing and by now they are forming nice heads. If it were me I think I would just let them be at this point in our zone. If I ferted them at all I would use fish emulsion as it seems a bit 'gentler' and doesn't "force" green growth.