Some call it tip burn which is often what you see on an avocado as it goes into flowering. The areas where avocado are grown typically have a lot of salts in the water, but also specific salts like sodium and chloride. Over the irrigation season (which is all year long there is little or no rain), the salts in the water/soil are taken up by the tree. In adequate rainfall years, there is enough water to leach those accumulated salts from the root system. When we go for several years with low rainfall and we keep irrigating with the poor quality irrigation water, the trees develop die back at the tips and is conditions worsen more and more of the leaf is called. This can get to the point where you can not call it die back any longer. It's called leaf drop. I've recently seen a number of orchards that are completely defoliated. No leaves. We have had a number of homeowner calls asking what the problem is and what they can do about it. The damage is done and those leaves are not coming back. It's possible to reduce the damage if one acts early on by applying more water than is usually applied to aid the leaching process, but if it is poor quality water, there will still be damage, but possibly not defoliation. With high priced water or where water is being rationed, many growers and homeowners do not have themake the option of putting on the excess water. There is no chemical or equipment that is going to make the situation better. When you trees defoliating, you want to cut out those that are diseased or you know have been poor producers and put what water you have on the remaining trees in better condition.
This advice is good for other evergreen tree crops like citrus, although they are not as sensitive as avocado. Avocado is an indication of how bad it really is.