Avocado Trunk Cankers
This has been a low rainfall year, and often with the low rainfall, cankers will seem to suddenly appear on the woody parts of the tree. There are a number of causes for the white exudate from cankers on the trunk and limbs of avocado. Any wound will cause the tree sap to run and crystalize on the surface. It is a seven carbon sugar of mannoheptulose, or its alcohol form perseitol. It's sweet. So any wound that might be caused by woodpeckers or little kids climbing the trees will damage the bark, and where the damage has occurred, the sugar will form. There are also diseases that can cause a wound that will exude the sugar. Three of these are due to water stress of some form that allows infection to occur. One of these is a bacterial – Bacterial Canker. Another is caused by a fungus which in the past was called Dothiorella Canker. We now know it as a different name and UCR plant pathologist has actually identified seven different species of fungus that invade the wood and can eventually weaken the tree so limbs can break and the tree becomes unthrifty. In the case of very young trees, they can be killed by the fungus. A third cause of sugary cankers is Black Streak, the cause of which was unclear until recently when Eskalen has possibly identified it coming from a similar set of fungi that cause Dothiorella Canker. It makes sense, because in all three of these cases, they most appear after a low rainfall year, where irrigation pressures are insufficient, where emitters have clogged and where general water or salinity stress have occurred. The bacteria and fungi that cause these cankers are everywhere in most orchards and are just waiting for the stressed tree to appear. The grower just needs to identify where this stress is occurring, correct the problem (clogging, low pressure, poor irrigation design, infrequent scheduling, inadequate leaching, etc.) and if the damage is not too extensive, often these symptoms will disappear with time.
The fourth cause of canker is caused by Phytophthora citricola, a relative of avocado root rot. This is caused by a moist trunk, either from irrigation water hitting the trunk, or on the north side of the tree that doesn't dry out from morning dew. This is a much slower acting disease than root rot, although it can rapidly kill young trees. The cankers occur at about 18 inches from the ground and gradually girdle the tree. The first thing to do before ever seeing this disease is to make sure irrigation water isn't hitting the trunks. If you do have cankers appear, though, it responds to the same materials used for root rot, but should actually be sprayed right on the canker.
Oh, and there's a 5th cause you occasionally see. woodpeckers having their way with the tree. The holes and dripping sugar are always in a nice line around the trunk.
