Hi Tui, last season our 4 x 20 year old avo trees looked really sad - drooping and losing leaves. They did still set a lot of very small fruit in spite of looking like they were dying. We have some new growth on some of the trees but they still look bad and parts of them died off. Should we putting citrus fertilizer on them?
Avocados respond to hard pruning and that might be what you need to do to reinvigorate the trees. Losing leaves and drooping leaves does sound like a root rot disorder called phytophthora, once that gets into the soil it is very hard to fix and plants do not usually recover. It is caused by poor drainage or the trees drying out due to drought and then receiving a lot of water. This causes the root cells to burst and the soil fungus enters the plant through the roots. It can also spread to other healthy plants.
Use Tui Organic Seaweed Plant Tonic on the trees at a rate of 70-100ml per 9L watering can. Being such well established trees a watering can may not be sufficient. If you want to feed your avocados use Tui Novatec Premium, it is used by commercial tree growers and orchardists.
Thanks, Tui Team.