A call from a small grower, surprised at the sudden decline of the avocado trees. It must be a disease was the grower's thought. Well driving up to the site, there were numerous trees with canopies indicating drought stress. In fact most of the trees looked like they had had the water turned off. When I got to the orchard, all the trees had a similar look (see photo below). The fringe of the canopy had turned brown/red where the leaves had collapsed rapidly, while the interior leaves were often still green. All the trees had a similar cast. It turns out the water district had required a cutback just when temperatures were going into the 100's. NO water, no cooling effect of transpiration and the outer fringe of leaves collapsed. This is called the “clothesline” effect. It's like a sheet on a clothesline where the margins of the sheet dry first and gradually the body of the sheet dries. The same thing happens in a canopy. The outside leaves are the first to dry out and then the rest of the canopy goes. When you see a whole orchard go down suddenly, that does not fit into a disease pattern. There's usually an epicenter where it starts – where it's colder, wetter, dryer, hotter, more overgrown, etc. and spreads out from there if it is going to spread. It turns out that the automatic irrigation system had gone down and the grower hadn't noticed until too late. When you see reddish tinged leaves, it means the leaves went down fast. When they are brown, it means they slowly went down over weeks or months.
With all the dead points in the tree, it is now open to disease – twig/leaf blight caused by one of the Botryosphaerias. These decay fungi are everywhere in an orchard decaying organic material on the orchard floor. With the dead material in the tree, now the tree becomes a potential feast for the fungi. The dead stuff has to come out, or the fungus will start eating into the tree. I suggested that instead of pruning out all those little points of death, that they cut back the whole canopy to major scaffold branches. In doing so, it would rapidly and cheaply remove the dead material and reduce the water demand.
At a recent meeting the question came up about the fate of nitrogen fertilizer applied through the irrigation system. If it is applied as urea, how long does it take to convert it to nitrate? If applied as ammonium, how long does it take to convert to nitrate? Urea and nitrate pretty much move wherever water moves and is very susceptible to leaching. Because of the positive charge on ammonium, it is not as mobile as nitrate, but once bacteria transform it to nitrate, it moves with water.
This is an important question, since if more water is applied than is needed by the plant, the nitrate is going to move out of the root system and no longer be available to the plant and ends up heading to ground water. Reading the literature, growers get the sense that all this transformation takes time, maybe a long time.
It turns out that soils in coastal California have a pretty rapid conversion of nitrogen. Francis Broadbent at UC Davis did a bunch of studies back in the 1950's and 60's and found enzyme hydrolysis of urea to ammonium occurring within hours. Other researchers have looked at nitrification, the conversion of ammonium to nitrate by soil bacteria, occurring within days and much of the conversion occurring within a week depending on soil temperature (see chart below).
So there is all this nitrate present and the key is what happens to it. It turns out that most plants when actively growing absorb nitrate at about 5 pounds of nitrogen per day. So with a 100% efficiency, applying 20 pounds of nitrogen, all of it would be taken up in four days. Of course, nothing in nature is that efficient. But the point is a big slug of nitrogen applied is not going to be taken up immediately and if more water is applied after that than is needed by the crop, it likely is pushed out of the avocado root zone.
Of course all the nitrogen a plant uses does not come from applied fertilizer. The bulk is coming from soil organic matter that is slowly decomposing. This nitrogen is being released at a rate that is probably in balance with the growth of the tree.
The applied fertilizer, however, is much more unstable and needs to be handled accordingly. The rule of thumb is to break the irrigation application into thirds. In the first third, run the irrigation to fill the lines and wet the soil. In the second third, run the fertilizer. This spreads it through the system and onto the ground. The last third is clear the irrigation system of the material and to move the fertilizer into the root zone. Then given time, the tree will take up the applied nitrogen. At the next irrigation then the bulk of that nitrogen will have been taken up and little will be pushed through the root system.
Low and High Nitrogen Avocado Leaves
Chart showing rapid conversion to nitrate with soil temperature
Avocado is a tree that has a good ability to respond to fire damage, if it is not too extensive. However, often a tree will recover only to collapse later on in the year or years because of the damage. So a tree may appear to do well and then suddenly collapse. In an orchard setting, fire damage can kill one tree completely, whereas the one just beside it recovers completely. This poses a major problem with irrigation management. How to irrigate the slowly regenerating tree that gradually needs more water, less frequently, next to trees that are recovering at a different rate or not at all. This becomes a management nightmare. Often the result of the difficulty of water management, the remaining trees develop root rot and they eventually die from that and not the original fire damage.
There is a general rule of thumb I have learned and used – when more than 50% of the trees have succumbed, it is best to replace the whole orchard. This is due to the issues of irrigation management and the loss of return from the unused portion of the grove.
So, from a pure economic management aspect, where there is any fire damage, that area should be considered a loss. If you look at your aerial survey and just measure the areas that show fire damage and take that as a proportion of the total planted area, you should be able to assess the extent of the damage incurred in the fire. So measuring the brown areas relative to green should give you a good assessment of the damage incurred in the fire.
It may be possible to nurse back individual trees with a lot of attention and if it's a small enough area, go ahead. But on commercial scale of acres, it often doesn't pay from a management point of view to nurse the orchard to an economic production level.
The destruction after a fire can be pretty gruesome and sad. Many times, though the fire moves through the orchard so fast that, even though the canopy has turned brown, there is a good chance the trees can come back. It all depends on how much damage has been done to the trunk. If the fire has substantially damaged the base of the trunk, it is unlikely to come back, even if the canopy is still green. That is the saddest thing, because you think you've dodged the bullet, but if the trunk is too damage, the canopy collapses gradually over a few weeks. However, the canopy may look a goner, but if the trunk is still intact, the tree will come back and may still be as productive as before.
The thing to look for at the base of the trunk is the discoloration. If it's black, it's probably not coming back. However, it can come back if after a few weeks you don't see the pencil-sized cankers that indicate that the sap has bubbled to the surface (see photo 1 below). If after a few weeks, the trunk is still clear of those bubbles (photo 2), even though the canopy looks gone, it is quite likely to come back.
It's also possible that the trunk may be damage in just one part of the trunk and not on the rest. In this case, it can also come back. The problem with these ones, is that they may come back and given enough time will be good trees again. But if they haven't recovered enough and there's a sudden heat spell, they may go down suddenly as if they had been burned again. That's the way they will look, like they have been burned again. Given enough time, though the avocado will grow from good areas to cover the poor areas and the tree may be productive again.
Avocados are amazing in their ability to recover. Eucalyptus can do it. But you singe the trunk of a lemon, and good bye lemon.
Photo 1 and 2.
In a recent meeting the topic of where to go for irrigation information came up. Well there's no substitute for attending a class in irrigation, such as offered at Cal Poly SLO (http://www.itrc.org/classes/iseclass.htm ,
but here's some written sources to get you started thinking.