- Author: Ben Faber
LAUREL WILT AND PHYTOPHTHORA ROOT ROT SURVEY
BACKGROUND
Despite the importance of laurel wilt (LW) and Phytophthora root rot (PRR) in avocado producing states, and the research conducted to date, there are still major gaps in our knowledge. Specifically, little is known about grower decision-making about management and the economic costs and benefits of management options. Understanding grower decision-making, economic costs, and the benefits of management options to growers is crucial to the establishment of rational and sustainable disease management strategies. That's why in this project we are organizing a multi-state survey to collect data about management, to understand the situation and highlight effective management strategies for each state.
Note: Laurel wilt (LW) disease is not Fusarium dieback. (Fusarium dieback is the disease spread by the invasive shot hole borers.) Laurel wilt is currently reported only in the eastern and central US.
And here's the survey that we are asking growers to complete:
https://ufl.qualtrics.com/jfe/form/SV_6fEn9M00UGmnwXA
Thank you.
University of CA, Florida, Hawaii and TAMU (Texas) personnel working on avocado disease
- Author: Ben Faber
Anthracnose, Sunblotch, and Armillaria Root Rot
Avocado Trunk Canker and Collar Rot
Avocado Branch Canker (Botryosphaeria)
- Author: Ben Faber
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.
- Author: Ben Faber
Upcoming CAS/UC/CAC Seminar Addresses
Mulch,
Phytophthora and
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:
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28917
- Author: Ben Faber
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.
In the case of this grove, it was at the top of the hill where it got all the wind and a full day's sun above the typical coastal fog.
So the grower asked about the damage. There was the heat damage from the July heat wave.
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.