The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can't find roots within 6 inches of the soil surface or if you do find them they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. And one of the things a gardener will often do, is start watering the diseased tree more, thinking it is lack of water, which if it is diseased only makes the condition worse. Adding more water to a tree that can not easily take it up only creates an asphyxiation that makes matters worse. Irrigation and mulch are the two most important factors for avoiding the disease.
So what do you do if you have disease? There are fungicides that are available from the nursery, but there are a number of things that you can do before applying something like that. First of all get a handle on the irrigation. Make sure you are irrigating to the tree's needs. Check soil moisture before irrigating. Make sure the tree is not getting supplemental water from another area such as a lawn sprinkler. Make sure there is a good thick, woody mulch under the canopy. Adding gypsum (15-20 pounds per tree) evenly spread under the canopy can also help, but reviewing and modifying the irrigation practice is the most important thing that you can do.
