Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: avocado

White Stuff on Avocado Trees

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: ). 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:

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 :

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:


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:,


Posted on Thursday, May 30, 2019 at 9:08 PM
Tags: avocado (278), cankers (1), exudate (1), wjote stuff (1)

Drought or Root Rot? - 12 Steps

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.  

Posted on Monday, May 27, 2019 at 11:22 AM
Tags: avocado (278), cinnamomi (4), Phytophthora (23), root rot (21)

SWEEPing Citrus

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.


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. 


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.

Posted on Friday, May 24, 2019 at 10:44 AM
Tags: avocado (278), cdfa (4), citrus (318), grants (5), irrigation (78)

Five Snail Speakers for the Price of One

Snails and Slugs (May 22, 2019 from 3-4pm)

Presenters: (!) Dr. Cheryl Wilen (UC IPM), (2) Dr. Rory Mc Donnell and (3) Dr. Dee Denver (Oregon State University), (4) Dr. Adler Dillman and (5) Dr. Irma De Ley (UC Riverside). The webinar will cover an overview of snail and slug biology, damage and management with emphasis on brown snail and Italian white snail, and current research on slug biocontrol using nematodes. One DPR CE unit (other) and one CCA CE unit (IPM) are approved.


And What Else Are the

                   UC Ag Experts

                             Talking About? 

Event NameDate
UC Ag Experts Talk: Snails and slugs 5/22/2019
Uc Ag Experts Talk: Management of Weeds in Citrus Orchards 6/19/2019
UC Ag Experts Talk: Citrus Dry Root Rot 7/24/2019

What is involved in the webinars?

A series of 1 hour webinars, designed for growers and Pest Control Advisors, will highlight various pest management and horticultural topics for citrus and avocados.  During each session, a UC Expert on the subject will make a presentation and entertain write-in questions via chat during and/or after the presentation. As we develop this program, we may expand to other crops.  

Topics: pests and diseases of citrus, avocado and other crops

And Next up is:

Management of Weeds in Citrus Orchards (June 19, 2019 from 3-4pm)

Dr. Travis Bean, assistant weed science specialist in UCCE, will discuss the importance of weed management in citrus, tree age and variety considerations, scouting and weed identification, cultural and mechanical practices, and pre- and post-emergence herbicides. One DPR CE unit (other) and one CCA CE unit (IPM) are pending.

Register in advance for the webinars by clicking on the event links above.

Are there Continuing Education units?

When the subject discusses pest or disease management, continuing education units will be requested from DPR (1 unit per session).  Participants will pre-register, participate in the webinar and be awarded the unit.  The sessions will be recorded and hosted on this web site for future study.  However, continuing education units will be awarded only to the participants who attend the live version of the webinar.  

Who is involved?

This webinar series is brought to you by Ben Faber (UC ANR Ventura Advisor) and Dr. Beth Grafton-Cardwell (Depart of Entomology UC Riverside Extension Specialist) with the technical support of Petr Kosina (UC IPM Contect Development Supervisor) and Cheryl Reynolds (UC IPM Interactive Learning Developer).  

giant land snail
giant land snail

Posted on Wednesday, May 15, 2019 at 1:21 PM
Tags: avocado (278), citrus (318), disease (57), pest (9), subtropicals (8), webinars (2)

USDA Forecasts for Avocado And Citrus and .....

Here's an example of the kind of information that can be both exciting and disappointing - forecasts of the future of the citrus and avocado industries and many other fruit and nut crops.  The latest forecasts are available form the USDA - Economic Research Service:


Avocado imports could rise further in 2018/19 as a weak crop outlook in California reduces domestic supply

Wednesday, April 24, 2019

Imports play a significant role in meeting the U.S. demand for avocados. Since the mid-1990s, imports of avocados have grown sharply as per capita consumption has grown, representing 87 percent of domestic use in the 2017/18 marketing year. USDA forecasts that imports will make up an even larger share of supply in 2018/19, mainly because California's crop is expected to be smaller than in recent years. Contributing factors to this reduced crop include record-breaking heatwaves in July 2018 followed by record-breaking wildfires, as well as recent rains and cold weather, and the general alternate-year-bearing nature of avocado trees (whereby a large crop one year is followed by a smaller crop the next year). Because over 80 percent of all U.S.-produced avocados each year are from California, California's low harvest in 2018/19 should boost U.S. demand for imported avocados (especially from Mexico) even higher than it has been in recent years. If USDA's forecast is realized, imports in 2018/19 will represent 93 percent of the domestic avocado supply. This chart appears in the ERS Fruit and Tree Nuts Outlook newsletter, released in March 2019.

Fruit & Tree Nuts
Provides current intelligence and forecasts the effects of changing conditions in the U.S. fruit and tree nuts sector. Topics include production, consumption, shipments, trade, prices received, and more.

Can the past foretell the future?

avocado grower 1910
avocado grower 1910

Posted on Wednesday, May 1, 2019 at 5:56 AM
Tags: avocado (278), citrus (318), economics (4), forecasting (3), fruits (3), nuts (2)

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