- Author: Ben Faber
Avocado Trunk Cankers
2016/17 was a normal rainfall year in the avocado growing region of California. But this was on top of lots of winters with such low rain amounts that there was not adequate salt leaching. No rain, no leaching. There's irrigation water to leach, but it's got salt in it and it just slowly accumulates over the irrigation season, and then from year to year without winter rain, those salts add up. Well, it looks like the little winter leaching we had is starting to lose its effect and one of the symptoms of low rainfall and salt buildup are the cankers that can show up on trunks and branches.
These seem to spontaneously appear, often in the weaker areas of an orchard, where water pressures are lower or inadequate water is being applied. I almost always saw it at the tops of windy hills, where emitters were clogged or there was high soil textural variability and the sandier areas were getting inadequate application frequency. Recently I saw trunk cankers on a heavier soil that was wet, even though it was getting water only once every two weeks or so. It was heavy soil, but the leaves were wilted. So, the trees were water stressed.
Some, but not a lot of roots in heavy soil.
Cupping, drooping leaves on heavy soil, but wet.
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. When the canker is pierced, it will usually bleed liquid sap that has collected in a pocket within the canker. The cankers usually follow in a line up the trunk and will sometimes ooze on their own, like in the photo.
Another is caused by a fungus which in the past we simply called the canker, Dothiorella Canker. We now know that it is caused by a whole suite of fungal species 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. These fungi can go after not just the trunk, but also stems and leaves. Then cankers are usually more diffuse than those of bacterial canker. They also attack many other tree species besides avocado, including citrus and many landscape trees. http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=13095
A third cause of sugary cankers is Black Streak, the cause of which is still unclear. It's thought to be a viroid and has been studied for over 75 years with no definitive causal agent and could actually be a strictly abiotic source of tree response.
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.
Citricola crown rot with the cankers girdling the trunk. It appears near the ground.
The point being made about all these cankers is that they are related to irrigation management and during low rainfall periods, they are more likely to appear. Keep your eyes open and make sure irrigations are being done right.