Most plants are susceptible to one form of root rot or another. Avocado trees are no exception. Avocado can develop phytophthora root rot which, in its early stages, causes leaves to grow small, yellow, wilt and eventually drop. As the disease progresses, new growth becomes rarer. Small branches at the top of the tree will begin to die back, causing the underlying branches to become sun burnt. Eventually, root rot will cause the infected tree to die but not before spreading to other trees in the area. Prevention is the best treatment for root rot.
Spread gypsum at the base of your avocado tree from the trunk to the end of the drip line. The gypsum will slowly leach calcium into the soil to suppress the growth of phytophthora root rot. A medium-sized avocado tree will need roughly 25 lbs. of gypsum. When all of the gypsum dissolves into the soil, reapply it.
Keep a 4- to 6-inch layer of organic mulch underneath the avocado tree. Mulch increases the fertility and microorganism level in the soil, which inhibits the development of phytophthora root rot. Coarse mulch like yard and tree trimmings and hardwood chips work best. The mulch should be spread all the way to the drip line of the tree but kept at least 1 foot away from the trunk of the tree.
Monitor your avocado tree's water. Avoid over-watering your avocado tree. Never water if the top inch or two of the soil are wet. This will water-log the soil and provide the ideal breeding ground for root rot. Consider having your water tested by your local county extension office. Water that is high in salt, boron, chloride or sodium makes avocado roots more susceptible to root rot. Surface and runoff water sources may even be contaminated with the disease.
Fertilize your avocado trees accurately. Healthy trees are much more resistant to root rot. Each year in autumn, take a leaf and soil sample into your local county extension office for chemical analysis. The analysis will indicate what nutrients your avocado tree needs to thrive and will prevent under- or over-fertilization. Transpiration reports can help you determine exactly how much water your tree needs.
Keep your avocado tree's soil pH around 5.5. Your annual soil test will tell you the pH of your soil and suggest amendments to raise or lower it.