- Author: Ben Faber
At a recent conference in Florida, University of Florida entomologist Daniel Carrillo reported some very disturbing news. There is a fungus/pest complex in Florida avocado and related native laurel species that is similar to a complex found in the California - Shot Hole Borer/Fusarium fungus complex. There it is called Laurel Wilt Disease and is a complex of the ambrosia beetle Xyleborus and the fungus Rafaelea lauricola. It is a fungus/insect complex that causes death in avocado and the relatives of avocado trees. The California complex can cause the death of many tree species, such as sycamore, coast live oak and willow, as well as the decline of avocado. The complex and disease are called Fusarium Dieback here, caused by Euwallacea ambrosia beetle and Fusarium fungus.
What Carrillo and other colleagues have found is that there are similar species to their introduced Rafaelea species of ambrosia beetle that are now attacking live avocado trees. These so-called cryptic species are members of a group of beetles that normally do not attack live trees. These beetles are typically some of the first group of decomposers that go after dead trees. These newly identified insects are morphologically very similar to the original beetle, but are native members of the Florida environment. They too are now attacking live trees. There are now ten potential species of ambrosia beetle that can introduce pathogenic fungi.
To exacerbate the situation, there are other fungi now that have been associated with these beetles that may be similarly as pathogenic as the original fungus. These fungi are genetically distinct from the species causing damage in California. However, this ability of different fungi to adapt to a new invasive beetle species and the ability of other beetle species to pick up the pathogenic fungal species is a scenario that might appear in California.
As the world becomes smaller and more living materials are moved around and they mix, this may be the new reality we are facing.
To read more about the Florida findings check out the article:
http://www.mdpi.com/2075-4450/7/4/55
http://ucanr.edu/sites/socaloakpests/Polyphagous_Shot_Hole_Borer/
![shot hole borer shot hole borer](https://ucanr.edu/blogs/Topics/blogfiles/39998.jpg)
- Author: Richard Stouthamer
The recent find of the Kuroshio shot-hole borer in Santa Barbara shows that the beetle is expanding up the coast and it comes on top of the finding earlier this year of a single Kuroshio shot-hole borer in San Luis Obispo. Earlier yet in 2014 a single beetle identified as Euwallacea fornicatus was found by the CDFA monitoring in Santa Cruz county, unfortunately this specimen was only identified using morphological characters and therefore we do not know which of the three cryptic species of the Euwallacea fornicatus species complex we are dealing with for that particular find. After the single find (2016) in San Luis Obispo a several additional traps were placed in the vicinity of the first find but no additional beetles have been caught. In a single location in Irvine KSHB has also been detected last year (2015). Recently, the Kuroshio Shot-hole borer has also been reported in Tijuana Mexico, which is not surprising since the heavily infested Tijuana river valley park in San Diego county is less than 0.6 miles from the border with Tijuana. It is clear from these detections that the KSHB is on the move, just like the PSHB. These long distance moves by the beetles are most likely caused by human transport, and the most likely culprit is wood transported after trees have been cut down or trimmed. Both in San Luis Obispo and the location in Santa Cruz no additional finds have been reported, often the density of insects following an invasion of a new area remains low while the population is expanding and followed by it reaching such levels that they are “suddenly” detected in many locations.
![pshb insect pshb insect](https://ucanr.edu/blogs/Topics/blogfiles/39587.jpg)
- Author: Ben Faber
This bulletin applies to avocado and citrus too.
DuPont Pioneer researchers have discovered a protein from a non-Bacillus thuringiensis (Bt) bacterium source that exhibits promise as an alternative means for controlling corn rootworm in North America and Europe. Science Magazine published the finding this week. “This research represents a breakthrough for addressing a major challenge in agriculture,” said Neal Gutterson, vice president, Research & Development, DuPont Pioneer. “We have discovered a non-Bt protein that demonstrates insecticidal control of western corn rootworm with a new and different mode of action than Bt proteins currently used in transgenic products. This protein could be a critical component for managing corn rootworm in future corn seed product offerings. The work also suggests that bacteria other than Bt are alternative sources of insecticidal proteins for insect control trait development.” An extremely destructive corn pest, corn rootworm larvae and adults can cause significant economic loss for growers. The current biotech approach for insect control sources proteins from Bt soil bacteria. Field-evolved insect resistance to certain Bt proteins has been observed in some geographies. Another Pioneer study related to non-Bt insect control, recently published in Scientific Reports, shows how RNA interference (RNAi) can be applied to control corn rootworm feeding damage. RNAi is a biologically occurring process that happens in the cells of plants, animals and people. By employing the RNAi process, a plant can protect itself by carrying instructions that precisely target specific proteins in pests. - See more at: https://www.morningagclips.com/discovery-of-potential-insect-control-traits/?utm_content=articles&utm_campaign=NLCampaign&utm_source=Newsletter&utm_term=newsletteredition&utm_medium=email#sthash.nF99oEWC.dpuf
![amorbia 1 amorbia 1](https://ucanr.edu/blogs/Topics/blogfiles/39111.jpg)
- Author: Ben Faber
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.
![clothesline effect clothesline effect](https://ucanr.edu/blogs/Topics/blogfiles/38123.jpg)
- Author: Ben Faber
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
![nitrogen avocado nitrogen avocado](https://ucanr.edu/blogs/Topics/blogfiles/38167.png)
![nitrification nitrification](https://ucanr.edu/blogs/Topics/blogfiles/38238.png)