Is it a disease? Is it a beetle boring into the tree? It looks scary when your avocado branches have volcanoes or streaks of white powder on them. What is that white stuff?
It is the dried sap. When a branch is injured in some way, the sap runs just like your blood runs when you scrape your knee, and then this sap dries into a white powdery form on the outside.
More specifically, this avocado sap is a seven-carbon sugar called mannoheptulose, or in a slightly different form it’s called perseitol. That name might seem familiar if you remember that the botanical name of the avocado is Persea americana.
I can’t taste the seven carbons, but I can taste the sugar. You can wipe a little powder off with your finger and lick it and taste the sweetness.
Why is it happening?
Why is a tree oozing sap? Is it anything to worry about? Avocado trees exude sap for various reasons, but they are all related to stress or injury.
The injury can be straightforward, such as a kid climbing the tree and scraping its bark, or your pruning of a branch. In such cases, seeing the white exudate is actually a good thing. It means that the tree is healthy and it is merely trying to protect its wound.
Also, avocado trees periodically shed branches, especially small branches on the interior of a large tree’s canopy where there is not a lot of sunlight. Often, I have noticed, some white powder will form at the base of such a branch as it dies and dries. (See an example of this in the photo at the very top.)
Then there are many other less obvious reasons for the white powder to appear on avocado branches or trunks, and they can be harder to diagnose, and they can even be a challenge to correct.
What to do about it?
Recently, Ben Faber gave a webinar presentation on these harder-to-diagnose issues, which he collectively referred to as avocado cankers. Faber is a University of California farm advisor for Ventura and Santa Barbara Counties. He gave an overview of six stresses, or pests or diseases, that will cause some form of white powder to appear on avocado trees.
If you’ve got white powder appearing on your avocado tree’s branches or trunk, and you don’t think it’s related to a pruning wound or some other minor physical damage, or a small branch dying naturally, then watch Faber’s presentation, or see this blog post by Ben Faber, or look at the University of California Integrated Pest Management page for causes of cankers on avocado trees.
Do note, however, that most of these diseases appear in avocado trees that are under water stress, says Faber. And often they can also be corrected by good irrigation. These are diseases with names like Black Streak, Bacterial Canker, and Botryosphaeria Canker.
My experience certainly corroborates this. After the record heat wave in early July 2018 (117 degrees in my area), there were a high number of branches with white exudate on my Hass avocado tree. Many of these branches also died back.
In a sense, it was due to water stress but also it was due to the extreme level of heat. At a point, avocado trees can’t handle heat so high no matter how well you’re watering. But if you’re watering well in general, the symptoms will be short lived in a mature tree, says Faber.
So just after that record heat, my Hass tree had symptoms consistent with Botryosphaeria Canker (sometimes called Limb Blight), but I watered as best I knew how, we got great rains last winter, and I believe I watered very well this past summer too. The tree has recovered well and is carrying a decent crop.
It has long been observed that, for avocados in California, getting the watering right is getting almost everything right.
So what is that white powder? It’s dried avocado sap. What does it indicate? Usually nothing serious, but possibly a stress or disease that is related to watering. Any other possibilities? There is the remote chance that you’re dealing with a pest/disease complex like Fusarium Dieback (caused by shot hole borers) or a very serious one that has yet to appear in California but has devasted trees in Florida, called Laurel Wilt. If you suspect that beetles are boring into your avocado tree, do check out these links.
Lastly, should you leave the white powder alone or wipe it off? I don’t know if it matters. I’ve always left it alone because I think of it as similar to a scab. Except that sometimes out of curiosity I have to check if it’s still sweet.
(Read more about the workings of mannoheptulose and perseitol in avocado trees in the South African Journal of Botany.)
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