Or perhaps the avocado mite?

Well, since you have several, I suggest you begin fertilizing one of them at least. Avocados are hungry plants, very vigorous, and so they can make use of nutrients. That first one may already be too far gone. Josh

R...you have a couple of things going on. 1. Usually tip burn is caused by salts in your water that has built up in your soil. This is common...and very seldom a major problem. Flush out your soil real well....then mildly fertilize after that. Also, it appears to me that you may have some insect munching on the leaves. Spray for that.

Willows - Salix species - are prone to a host of fungal and insect issues but few that bother foliage enough to be a concern. Too often folks freak out about leaf spotting or minor insect issues and rush to spray something........usually unnecessarily. Plants get problems and blemishes - pretty much a fact of life and the natural course of events. At this time of year, as ken notes, there is nothing you need or should do other than clean up fallen leaves and discard.