Update on invasive shot hole borers: Online training now available

By Sabrina Drill, Natural Resources Advisor, UCCE Ventura

Invasive shot hole borers (ISHB) are a pest and disease complex potentially affecting over 200 tree species, but posing a strong risk to box elders, sycamores, and other riparian and urban trees, as well as being a nuisance pest for avocado. The beetles have also been shown to attack a wide variety of common and less common ornamental species. For a complete host list, visit https://ucanr.edu/sites/pshb/overview/SHB_Reproductive_Hosts/. The tiny beetles burrow into the trunks and branches of trees and create galleries where they cultivate a fungus that utilizes the trees own circulatory system, harming and in some cases killing the tree. We know the beetle can reproduce in over 60 species of trees, and they have devasted natural riparian areas, though we are beginning to see recovery of some infestations. Currently, the most effective management method is to remove infested wood, sometimes entire trees, and chip what is removed to minus 1”.

ISHB are now firmly established in Ventura County, with finds throughout the Santa Clara River Valley in traps from South Mountain to Toland Park in Santa Paula, and several infested box elder and sycamores in Meiner's Oaks and Ojai. It appears not to have crossed into the county in the south, but there are still active infestations in western Los Angeles County.

Personnel from UC ANR, CDFA, the Ventura County Agricultural Commissioner, and the US Forest Service have taken lead roles in developing a statewide ISHB action plan for the California Invasive Species Advisory Committee, with an initial investment of $5 million to work on controlling the pest. Plan elements include research in the ecology and control of the pest, including work to develop biocontrol; an early detection and surveillance program; addressing green waste and other pathways of spread, and outreach and education.

In terms of education and outreach, an area where Cooperative Extension in Ventura, LA, Orange, and San Diego counties have led, we're excited to announce the release of a new on-line training course. Available through our website, https://ucanr.edu/sites/pshb/, the course is actually served by the eXtension on-line learning platform (and users will need to create a free account). The course consists of 4 chapters including history and impacts, biology, symptoms and look-alike pests, and monitoring and management. While it can't fully replace a field training, it can be a good way to familiarize new staff to the issues.

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. The leaking sap is the tree's attempt to staunch the wound (More about the sugar can be found at: https://www.sciencedirect.com/science/article/pii/S0254629911001372 ). Any wound that might be caused by woodpeckers, pickers or little kids climbing the trees will damage the bark, and where the damage has occurred, the sugar will form.  So fire damage can cause wounding and so can insect infestation like shot hole borer. Any wound will cause the sugar to leak out in a response to heal the damage. This sugar exudate is a sing of health in the tree, showing that it can respond to attack/infestation/disease. No response is a bad sign.

Physical damage from kids clambering around in a tree

Fire/heat damage exudate

Shot Hole Borer Damage

There are also diseases that can cause a wound that will exude the sugar sap.  Three of these are due to water stress of some form that allows infection to occur. These cankers can be quite a problem in avocado, as well as some other tree species, during drought years. With rainfall, the sugar stain is washed away and if there is adequate rainfall, the cankers might even heal. But they can easily reappear once the right stress conditions reappear. The other tree species don't exude the white sap, which is unique to the laurel family. The cankers have also appeared regularly in orchards that have irrigation and salinity management problems. All of these diseases can lead to unthrifty looking trees, which can lead one to conclude that they have Avocado Root Rot. Often, though the trees can have the cankers and the whole canopy can look quite healthy. 

One of these trunk cankers is bacterial – Bacterial Canker –caused by Xanthomonas campestris. The name” campestris” means “field” in Latin, and it is a bacterium commonly found in nature. So the bacterium is widespread, and it is not unusual to see a large part of an orchard infected, but it is not commonly found in most orchards. The infection causes a pocket of infection that will ooze sap. The oozing pockets will often appear in a series along a branch or the trunk. It is associated with poor water distribution, and irrigation timing and water/salinity stress. It can be quite a sight, but it rarely kills trees and when the water problems are identified and corrected the cankers will dry up on their own. More on Avocado Bacterial Canker: http://ipm.ucanr.edu/PMG/r8101111.html.

Bacterial Canker

A group of fungi, which we once labelled as Dothiorella, causes another canker but we now know a much larger group of fungi that includes Botryosphaeria and Phomopsis causes the canker. On leaves, the symptoms are called blight; on stems, called dieback and on larger branches and trunks, called simply cankers. UCR plant pathologists have actually identified at least seven different species of fungi 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 these fungal infections, so they are pathogenic. They also are saprophytic on dead tissue and can survive in mulch. The cankers will appear in blotches or patches on the trunk and branches.

Again, these cankers most commonly occur in orchards with irrigation management problems, although there are exceptions where it is unclear what the underlying cause might be. When drought issues are addressed, these cankers will often heal on their own. Read on about Dieback and Canker : http://ipm.ucanr.edu/PMG/r8100611.html.

Trunk Canker

The third cause of sugary cankers is Black Streak, the cause of which has been unclear. It has been tested as a virus, viroid, fungus and bacteria, but it does not seem to fall into any of those groupings. It acts like Trunk Canker, but so far, it has defied a fungal classification. Unlike Trunk Canker, it will usually show up as a widely scattered area of small cankers, often on the undersides of branches and along the trunk.

The correction is similar to Trunk Canker and they mostly appear after a low rainfall year, where irrigation pressures are insufficient, where emitters have clogged and where general water or salinity stress has occurred. More on Black Streak: http://ipm.ucanr.edu/PMG/r8100311.html.

Black Streak

The bacteria and fungi that cause these cankers are everywhere 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 mengei (previously P. citricola); a relative of Avocado Root Rot called Crown Rot, but this fungus attacks the crown roots and lower trunk.  The environment that encourages this canker is 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/fog/rain.  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 you do have cankers appear, though, it responds to the same materials used for root rot control, but the materials should actually be sprayed right on the canker.

Crown Rot

So here we have four different trunk diseases all caused by water management. The first three usually from amount and timing and how salts are managed. Crown Rot really is simply irrigation splash on trunks. All four of these can easily be managed with improved irrigation management. You can read more about drought-induced problems in orchards at:

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=20381,

We recently had a series of workshops on Avocado Root Rot and ways to manage it.  A common question was how to figure out whether the tree was diseased with Phytophthora cinnamomi  or just stressed from lack of water.  Drought is also compounded and confused by salt accumulation, which is a reflection of how water is being managed. It might be the right amount, but not timed correctly.  Too much at one time means the water goes beyond the shallow root system, too little at an irrigation and the salts contained in the water start being taken up by the roots.  These “extra” salts need to be leached; otherwise, they actually compete with the tree for soil water.  By “extra”, these are the salts like sodium and chloride that can be harmful to the tree, rather than the nutrient salts that are necessary for tree growth, but will also be leached when trying to achieve a balance by removing the harmful salts.

So there are several steps to follow to figure out a droughted tree from a root rotted tree.  If the tree is stressed from drought, eventually though, it quite likely can lead to root rot.   Looking at wilted leaves is an indication of a stressed root system which is common with a lack of water, but can happen when the roots are soaked for too long from rain, a leaky irrigation system or sediment accumulation that can occur with flooding.  Wilting is also one of the first symptoms of root rot, because there are not sufficient roots to keep up with the tree's water demand.

Step I. Wilting

Wilting is going to be the first step in alerting you to a soil/root/water problem, but it is just the first alert and there are more steps to a field diagnosis.  The steps take on three different parts of the tree:

First, look at the canopy overall and then more closely in the canopy

Then, look AT the ground

Then, look IN the ground

If you look at the tree from a distance and the canopy is thinning with dieback (staghorning)

Step 2: Thinning canopy.

This means that it is something that has been going on for a longer time that just to cause the leaves to flag (wilt)

And when you look more closely, the leaves are small, yellow, have tip burn and there are lots of flowers

Steps 3, 4, 5: Small, yellow leaves; tip burn; profuse flowering

This again means that it's something that just didn't happen with a missed irrigation or two, or a stopped up emitter.  Something has been going on for maybe more than a season.

And if there is fruit, if it is sunburned which means it probably isn't saleable, it means there isn't enough canopy to protect income

Step 6: Small, sunburned fruit

Now you definitely know there is a problem with the roots. The roots mirror the canopy.  When they go wrong, they canopy goes wrong.  All these thinning symptoms in the canopy, also means the root system is thinning.  Also, when the canopy goes wrong, the roots have problems.  When the canopy can't feed the root system it is less able to fend off disease, if that is the cause of the thinning canopy problem.  At this point, it's not definitive that it is root rot causing the problem, but a sad canopy can lead eventually to a root rot problem because of lack of energy generated in the canopy.

The next step is to look AT the ground surface and see if there's natural leaf mulch.  If the tree lacks energy to produce leaves, there won't be any leaf drop and now leaf accumulation. These should be leaves in various stages of brown, indicating they have been there for a while.  This mulch protects the roots from drying out and also produces an environment hostile to the root rot organism. No leaves to feed the fungi and bacteria that compete and destroy Phytophthora, eventually Phytophthora will come to dominate the system. No energy to produce leaves; no canopy to protect leaf mulch from wind?  And, then the wind blows the leaves away.  On hillsides, gravity can act against mulch creation and also exposes trees to more wind, but a healthy tree can create its own mulch in harsh hillside environments.

Step 7: No natural leaf mulch

With a sick canopy and no natural leaf mulch, this is the time to think there is something seriously wrong.  There is something wrong with the water uptake in this tree.  Either a lack of water or a lack of roots.  Is it the timing, amount or distribution of the water? These are all issues that can be corrected if there is sufficient water to do so.  Maybe the soil is too wet?  It could be asphyxiation.  Lack of air.  That can be corrected by identifying the cause of the lack of air or too much water.

Step 8: Asphyxiation

But if the soil is not too wet, when you apply water, does the tree perk up?  Give it a couple of days.  This could always have been the problem.  Does the water come on?  Is a valve shut down?  Is the system not working?  Is there poor water distribution.  This infrastructure problem is common in hillsides irrigation with cheap parts that are easily damaged by coyotes, rabbits, and pickers.

Step 9: Turn on the water

But if the tree does not or has not responded to applied water, then start digging. It's time to look IN the ground. This is something that should be done on a regular basis just to see how those roots are doing, anyway. 

And when you start digging, there's no roots

Step 10: NO roots

Or only big roots

Step 11: Only big roots

And, if you do find any little roots, they are blackened and brittle

Step 12: dead root tips

And you have applied water and the tree doesn't perk up, then the tree probably has Avocado Root Rot disease caused by Phytophthora cinnamomi.

There can be other reasons, for a tree collapse like this, like a gas pipe leak, gopher activity in young trees, a chemical/fertilizer spill.  Probably other things that kill roots, but a field diagnosis like this process can pretty much identify the problem as root rot.  It can then be verified by a lab test to make sure.  However, there are times of the year and disease conditions when a test will come back negative and it might be necessary to retest with another sample at another time of year.  

Most groves that have been in the ground for many years and have been harvested by outside commercial crews quite likely have the root rot organism present in the orchard.  The lack of disease is because the stress that brings on disease is lacking – water management, frost/heat damage, flooding, too much rain, too much fruit, pruning, etc. – anything that predisposes the tree to infection.  It is when several stresses are present that the trees start declining and if identified soon enough can be corrected and the decline stopped and reversed.  

Spotlight on SWEEP in Citrus

Shulamit Shroder, UCCE climate smart agriculture specialist - Kern County

In 2014, Bruce Kelsey in Kern County received a grant through the California Department of Food and Agriculture's State Water Efficiency and Enhancement Program (SWEEP). He used the funds to set up 8-foot-wide plastic weed mats underneath his mature organic citrus trees. He also decreased his electrical consumption by about 30% and installed soil moisture sensors, a water flow meter, and a pressure-sustaining device.

Benefits

Labor: The installation of the weed mat was a labor-intensive process, but it ended up paying off in the long term. It diminished weed populations so that he no longer has to weed under his citrus trees. Now he only mows with a small mower in the lanes between his trees.

Water usage: His overall water usage decreased by about 10%. The weed mat decreased evaporation and weed pressure while the other devices allowed him to better manage and schedule his irrigation. 

Drawbacks

Pests: Bruce experienced an increase in earwigs in the weed mat orchard. The plastic covering provided the perfect humid environment for the insects.

Organic certification: The weed mats will eventually start to disintegrate, which could contaminate his soil. To maintain his organic certification, he will have to rip them up once they start to break down. Smaller, younger trees do not protect the plastic from the sun, which quickly destroys the plastic. For this reason, he recommended against using weed mat in immature orchards.

Figure 1. Weed mat in place.