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.
Upcoming CAS/UC/CAC Seminar Addresses
Gibberellic Acid Use
The California Avocado Society will host the first of its 2019 California Avocado Growers Seminar Series with workshops focused on mulch and Phytophthora. Dr. Ben Faber, Dr. Tim Spann and Dr. Carol Lovatt will deliver presentations at the seminars.
Dr. Ben Faber, UC Cooperative Extension Soils/Water/Subtropical Crops Farm Advisor, will speak about the benefits of using mulch in avocado groves. Ben will discuss the various types of mulch that can be used, how and when to apply them and the benefits of using mulch in avocado groves.
Dr. Tim Spann, California Avocado Commission Research Program Director, will cover Phytophthora 101. Tim will discuss what phytophthora species affect avocados, how to recognize symptoms of phytophthora infection in avocados and best management practices for dealing with phytophthora.
Dr. Carol Lovatt, UC Riverside Emeritus Professor of Plant Physiology, will discuss the use of gibberellic acid (GA) plant growth regulator on avocados. A special local needs registration was obtained in early 2018 for use of GA on avocado in California. Carol will discuss the benefits of using GA, and when and how to apply it for those growers interested in trying this new tool.
The seminars will be held as follows:
Tuesday, February 5, 1:00 p.m. – 3:00 p.m.
SLO Farm Bureau, 4875 Morabito Place, San Luis Obispo, CA 93401
Wednesday, February 6, 9:00 a.m. – 11:00 a.m.
UC Cooperative Extension Office Auditorium, 669 County Square Drive, Ventura, CA 93003
Thursday, February 7, 12: 30 p.m. – 2:30 p.m.
Fallbrook Public Utility District Board Room, 990 East Mission Road, Fallbrook, CA 92028
And read more about Mulch Myths:
So I got called out to see an orchard that had dieback. Over the phone it sounded like “Dothiorella Leaf Blight” (http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=10354). This symptom shows up when there has been water stress of some sort, over a period of a few weeks to months.
It can be a branch or a whole tree. It usually is due to a watering schedule that doesn't meet the needs of the plant. It is exacerbated by competition from more aggressive plants nearby, like eucalyptus windbreaks. With increased competition over time, root rot can also start taking its toll because of the stress, as well. Where you see areas cleared of trees, and where there are struggling trees in various stages of survival, it's a good indication of a rough environment.
Trees with root rot symptoms may have been stumped previously, and with their weakened root systems are more prone to leaf blight.
Then there was the damage that had showed up earlier, the leaf blight that was associated with the ongoing water stress. In a way, both of these symptoms are of water stress, one very rapid, hence the reddish tint to the leaves. The other a slower collapse, the leaf blight from prolonged water stress.
Over time, this type of stress had allowed root rot to take hold and trees were stumped in order to help them survive. The trees were also started on a regular phosphonate program. In general, the rest of the orchard looked good, except for this area near the competitive windbreak trees. The heat damage was mostly in this area, as well. The grower had been forewarned of the impending heat and irrigated the trees prior to the hard weather.
In the case of heat damage or blight, and leaf/stem blight, it's not fatal for mature trees. Younger trees under two years can be killed. They usually need whitewashing to prevent sun burn damage.
But even they can be revived if the watering is improved.
PULLMAN, Wash. Soil pathogen testing - critical to farming, but painstakingly slow and expensive - will soon be done accurately, quickly, inexpensively and onsite, thanks to research that Washington State University scientists plant pathologists are sharing.
As the name implies, these tests detect disease-causing pathogens in the soil that can severely devastate crops.
Until now, the tests have required large, expensive equipment or lab tests that take weeks.
The soil pathogen analysis process is based on polymerase chain reaction (PCR) tests that are very specific and sensitive and only possible in a laboratory.
The new methods, designed by WSU plant pathologists, are not only portable and fast, but utilize testing materials easily available to the public. A paper by the researchers lists all the equipment and materials required to construct the device, plus instructions on how to put it all together and conduct soil tests.
Responding to growers needs
"We've heard from many growers that the time it takes to obtain results from soil samples sent to a lab is too long," said Kiwamu Tanaka, assistant professor in WSU's Department of Plant Pathology. "The results come back too late to be helpful. But if they can get results on site, they could make informed decisions about treatments or management changes before they even plant their crop."
Some diseases from soil pathogens may not be visible until weeks after the crop has sprouted, Tanaka said. That could be too late to treat the disease or could force farmers to use more treatments.
WSU graduate student Joseph DeShields, a first author on the paper, said it took about six months of work to get their device to work in the field. It relies on magnets to capture pathogens' DNA from the soil.
"It turns out, it's really hard to separate and purify genetic material from soil because soil contains so much material for PCR tests," said DeShields "So we were thrilled when we made that breakthrough."
Rachel Bomberger is a WSU plant diagnostician who helped with the concepts of the machine testing. She said she's impressed by what Tanaka and the team accomplished.
"We removed a huge stumbling block when it comes to soil testing," said Bomberger, one of the co-authors on the paper. "We found the missing piece that makes the testing systems work in the field without expensive lab equipment or testing materials."
The system was tested on potato fields around eastern Washington, Tanaka said, but it will work on soil anywhere in the world.
"It's a really versatile method," he said. "You could use it for nationwide pathogen mapping or look at the distribution of pathogens around the country. We started small, but this could have huge implications for testing soil health and disease."
Tanaka said it was important for this discovery to be available in an open-access video journal.
"We're always concerned about helping every grower and the industry as a whole," Tanaka said. "We want everybody to look at this and use it, if they think they'll benefit from it."
The results were published in the Journal of Visualized Experiments, an open-access journal that includes a video showing how to assemble and used the system and a full list of materials needed to use their method.
This research is supported by the Northwest Potato Research Consortium and the Washington State Department of Agriculture - Specialty Crop Block Grant Program.
See the video here:
And the article here: