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 bacterial – Bacterial Canker. Another is caused by a fungus which in the past was called Dothiorella Canker. We now know it by a different name and a 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 Dr. Akif Eskalen possibly identified it. It was coming from a similar set of fungi that cause Dothiorella Canker. It makes sense, because in all three of these cases, they most often appear after a low rainfall year, where pressures in irrigation systems are insufficient, where emitters have clogged and where water or salinity stress have occurred. The bacteria and fungi that cause these cankers are widely distributed 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 cankers appear, they respond to the same materials used for root rot, but should actually be sprayed right on the canker.
Images
Top: Black Streak can occur on trunks or branches as small, scattered cankers. The name comes from the blackening that occurs around the infected area.
Next: Bacterial Canker causing discrete silver dollar-sized cankers. The exudate running down from the bottom edge results when a knife is poked into it. The cankers tend to run in a line up the trunk or branch.
Next: Citricola crown rot with the cankers girdling the trunk. It appears near the ground.
Bottom: Dothiorella Canker (soon to have a new name) can occur anywhere on the trunk or branches
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