- Author: Michael Hsu
Wet winter, El Niño create favorable conditions for aerial Phytophthora pathogen
With heavy rains in the forecast amid strengthening El Niño conditions, almond growers should be on the lookout for a rare disease that can cause severe damage to their orchards, according to Florent Trouillas, a University of California Cooperative Extension specialist in fruit and nut pathology.
Phytophthora, soilborne microorganisms dubbed “water molds” because of their dependence on water, typically cause root and crown rot at the base of trees. But a few aerial Phytophthora can travel upwards and infect the higher parts of the tree. One species – Phytophthora syringae – is drawing special attention due to an unprecedented outbreak last winter, fueled by the atmospheric rivers that lashed California.
“It was found statewide – meaning in every almond-producing county – and disease incidence in orchards ranged from 10% of the trees infected to 75%,” said Trouillas, a UC Davis plant pathologist whose lab is based at UC Agriculture and Natural Resources' Kearney Agricultural Research and Extension Center in Parlier.
Trouillas and his colleagues, UC Davis graduate student Alejandro Hernandez and UC Riverside plant pathology professor Jim Adaskaveg, recently published a detailed online article describing the pathogen, which can infect a range of crops but mainly impacts almonds in California.
Although it doesn't kill the tree, the disease causes branch dieback that requires significant additional work and expense for almond growers. In 2022, almonds were the state's fourth-highest valued commodity, at $3.52 billion.
During last year's aerial Phytophthora outbreak, researchers also observed a new and troubling phenomenon: P. syringae, historically known to attack the cuts caused by pruning, was directly infecting the young shoots on almond trees – without any wounds.
“This was really the first time we had seen widespread evidence of infection on the twigs,” Trouillas said.
Although generally rare, outbreaks of P. syringae have been traditionally associated with wet El Niño years, according to Trouillas – and recent and persistent rain across the state should have growers on high alert.
Prune in dry weather, monitor, mitigate if necessary
While almond growers tend to prune during the downtime of winter, they should keep an eye on the forecast and aim for a 10- to 14-day window of dry weather to perform those tasks, whether training young trees or maintaining the established ones.
“If growers were to prune around a rain event – before, during or shortly after – this increases the likelihood of infection because this pathogen moves around with water,” Trouillas explained.
Researchers speculate that P. syringae, normally found in the soil, gets carried into the upper parts of a tree through strong winds and heavy rain. Alternatively, harvest processes like shaking and sweeping also produce air movements that may blow the microorganism into the canopy, where it waits for a favorable wet environment. The pathogen then attacks the wounds or young shoots, producing characteristic cankers and gumming.
The patterns and colors of the gum balls are keys to diagnosing an infection of this particular aerial Phytophthora. Starting around bloom time (mid-February), growers should monitor pruning wounds and young shoots on their trees, especially in the canopy, for signs of the disease.
The unique coloration of the gum balls – ranging from gold and amber to dark burgundy to bright red (see photos) – generally indicates P. syringae infection. But growers are urged to contact their local Cooperative Extension advisor for confirmation.
“It is super critical for growers that, whenever they see gumming, not to assume that it is this aerial phytophthora, because there are many other diseases that can cause gumming on the tree,” Trouillas said.
If the diagnosis is confirmed, growers may apply a compound that can mitigate the infection. The plant pathologists' recent writeup describes several curative treatment options, as well as a preventive measure that reduces the amount of pathogen in the soil and thus the likelihood of infection.
For more information on the pathogen's history and biology, as well as various options for disease management, visit the article on Sacramento Valley Orchard Source: https://www.sacvalleyorchards.com/almonds/trunk-soil-diseases/aerial-phytophthora-outbreaks-in-wet-years.
/h3>/h3>- Author: Steven Swain
Sudden oak death (SOD) is a disease syndrome that has killed millions of native oak trees (Figure 1) along the west coast of the United States, from Big Sur in California up to Southern Oregon. The disease may involve several organisms, but its main driver is the fungus-like organism (known as water mold), Phytophthora ramorum. This plant pathogen is spread in the springtime by windy rainstorms. It infects the bark of oak trees, frequently creating bleeding trunk cankers that interfere with water uptake and sugar transport.
Death of SOD-infected trees can be accelerated by attacks from bark and ambrosia beetles. In the absence of beetle attacks, infected oaks may take years to die.
Many common disorders (other than P. ramorum infections) can cause damage that resembles SOD, so laboratory testing is needed to confirm the diagnosis.
Sudden Oak Death Host Range
Sudden oak death isn't always sudden, nor does it infect just oaks. The potential host list of P. ramorum includes hundreds of plant species, many of which are natives of California's woodlands and forests. In most of these host species, the pathogen simply causes small necrotic spots on the leaves (Figure 2). In a few instances, notably in certain susceptible Rhododendron cultivars, the disease may progress from the leaves down into the stem and kill the plant.
Phytophthora ramorum has evolved as a foliar pathogen, primarily spreading from the leaves of infected hosts such as bay laurel (Umbellularia californica), tanoak (Notholithocarpus densiflorus), and rhododendrons. Therefore, it does not spread easily to other plants from oak trunk cankers. Thus, when oaks become infected, they are referred to as terminal hosts. Tanoak acts as both a foliar host, spreading spores from its leaves in wet and windy weather conditions, and a terminal host, developing cankers that are almost always lethal to the infected plant (Figure 3). Perhaps because infected tanoak leaves rain spores onto their own trunks, their SOD survival rates are among the lowest of all trees that may become afflicted by the disease in North America.
Despite its fearsome reputation, SOD doesn't always kill infected oaks. Valley oak (Quercus lobata), blue oak (Q. douglasii), and Oregon white oak (Q. garryana) are not known to develop cankers in nature, while coast live oak (Q. agrifolia), black oak (Q. kelloggii), Shreve oak (Q. parvula var. shrevei), and interior live oak (Q. wislizeni) are considered susceptible. Recent studies have demonstrated that there may be considerable variance in some coast live oaks, with measured resistance varying from about 16-40%. Many of these resistant or tolerant trees do become infected but are able to defeat the pathogen before trunk cankers enlarge to life-threatening sizes. Casual observations suggest that resistance levels seen in coast live oaks are likely similar to those in Shreve oak and black oak populations. Because interior live oak trees grow in comparatively hotter, dryer, environments than the other susceptible oaks, they almost never become infected, so resistance levels have not been studied. Tree age seems to play a role too. Oak trees under four inches in trunk diameter at chest height are not typically susceptible to infection by P. ramorum.
Treatment Approaches
Many different treatment approaches have been trialed, a few of which have shown promise.
Prevention
Potassium phosphite compounds (AgriFos, Reliant, Garden Phos, etc.) work best as preventive treatments. Most of the efficacy achieved by these compounds appears to be the result of stimulating the trees' natural defensive systems, although individual oaks vary widely in their immune responses. Thus, the application of potassium phosphite compounds is a bit of a gamble as to whether it will actually help an individual tree. Trees that are already showing symptoms of infection when treated have a significantly lower survival probability than trees that appear healthy during treatment.
There are two different recommended application methods for potassium phosphites—surface sprays made directly to the bark (using a surfactant such as Pentrabark) and trunk injections. Some applicators have claimed increased efficacy from the trunk injections, albeit at the cost of potential damage to the tree from wounding. Others have claimed equivalent efficacy with repeated bark sprays, when carefully timed. Application of calcium to the root zones of oaks treated with potassium phosphite has been shown to further improve resistance rates.
Potassium phosphite treatments should be made in the spring and fall, regardless of the application method used. This is because the uptake of potassium phosphite by the tree is dependent on high transpiration rates. High transpiration rates in turn depend on both adequate available soil moisture, and warm and sunny weather, preferably with a light breeze.
Another preventive treatment option that has proven to be effective is removal of foliar hosts that are near highly valued oak trees (Figure 4). Removing bay laurels that have foliage within about 30 feet of an oak trunk decreases the chances of that oak becoming infected. Removal is especially effective for small, understory foliar hosts, such as young bay laurels, poison oak, and rhododendron. This is not a recommendation for the wholesale removal of bay laurel trees, which are important parts of the California forest ecosystem and should be retained where appropriate. This management approach should only be adopted after careful consideration of the ecological function of the forest or woodland as a whole.
Management of Active Infections
Bark scribing, or cutting away the outer portion of infected bark to let the infection site dry out, was previously touted as an effective treatment for infected oaks. However, rigorous testing has since shown that it does not significantly increase the odds of an oak surviving. Similarly, the application of whitewash to tree trunks has not been shown to make measurable improvements to survival of treated oaks as compared to no treatment at all.
Mefanoxam (Subdue Maxx, Stergo MX, etc.) is a fungicide with a proven record of suppressing Phytophthora activity. It has been used to successfully treat infected plants, keeping them alive as long as treatment continues, typically over the span of a year or two. However, in most cases, once treatment stops, Phytophthora begins growing again within several months, and the infected plants will eventually succumb to the disease. While mefanoxam fungicides may be effectively used in certain limited situations, they should not be considered effective curative treatments. Resistance to this active ingredient has developed repeatedly, rendering it unacceptable for long-term use.
In summary, no silver bullet exists for preventing SOD, and there are no effective tools that will reliably save a tree that is already showing symptoms of infection. This does not mean that any oak that exhibits SOD trunk cankers or bleeding symptoms will die. Bleeding from the bark is a normal response to substantial damage, whether from a pest, pathogen, or, sometimes, even mechanical damage. Even if bleeding is the result of infection by P. ramorum, symptomatic trees have recovered with no intervention, usually in association with a few dry years. The most effective tools for managing SOD are preventive, in nature. For more information and best management practices, please see the California Oak Mortality Task Force website at: https://www.suddenoakdeath.org.
/h3>/h3>/h2>/h2>
- Author: Emily C. Dooley, UC Davis
Outbreaks similar to El Niño-influenced issues of the 1990s
The wave of atmospheric rivers that swept across the state this winter has created the right conditions for plant pathogens that haven't been seen for decades in California. University of California, Davis, plant pathologist Florent “Flo” Trouillas is getting more calls from growers and farm advisors concerned about potential crop damage.
“Generally, whenever you have rain events, you're going to have problems,” said Trouillas, a Cooperative Extension specialist who is based at the Kearney Agricultural Research and Extension Center in Parlier. “In wet years we get really busy because most pathogens need and like water.”
Trouillas is like a disease detective. He splits his time between the field and the lab, working to diagnose pathogens, diseases and other ailments that strike fruit and nut crops such as almonds, cherries, olives and pistachios.
On a recent visit to an almond orchard near Fresno, Trouillas joined Mae Culumber, a nut crops farm advisor for UC Cooperative Extension Fresno County. A few weeks earlier, the two had walked the orchard, taking note of the base of some trees that had gumming — a thick, jelly-looking substance indicating a pathogen had taken hold.
“A lot of what Florent is doing is trying to assess patterns on a landscape,” Culumber said. “Sometimes things may look like they are one thing, but it could be another problem.”
When the two returned weeks later, the amber-colored gumming had moved into the canopy, looking like gumballs stuck to branches, some of which were already dead. “It's getting out of control from before,” Trouillas says. “This branch was killed. This is widespread.”
From the field to the lab
Lab testing confirmed what Trouillas believed was the culprit: Phytophthora syringae, a pathogen that can affect almond crops but is rarely seen in California. If it is found, generally the site of infection are wounds caused by pruning, but that is not the case here, where the infection began in the canopy at twigs, or small branches.
It is a threat to a key crop, which according to the California Department of Food and Agriculture, generates $5 billion annually. The last time Phytophthora syringae hit California was in the 1990s after a series of El Niño-influenced storms. Trouillas, who has a photographic memory, remembered reading about it in an old manual.
“It's rare for California and one that we see mostly following atmospheric rivers,” he says.
“The disease will only happen following these extremely wet winters.”
Phytophthora is soilborne, mostly found in tree roots, and doesn't generally spread up into branches. But the intense storms created the right conditions for the pathogen to “swim” up trunks as winds blew spores into the air and rain dropped them back down into the canopy, Trouillas said.
Some of the trees in this orchard will die; others can be saved by pruning infected branches and applying a recommended fungicide, he said.
Identification, diagnosis, education
Trouillas is one of more than 50 Cooperative Extension specialists at UC Davis and each is charged with identifying problems and developing solutions for those issues in support of agriculture, the ecosystem and communities throughout the state.
In his role, Trouillas focuses not only on pathology and research but also on educating growers, nursery staff, pest control advisers and others in agriculture about ways to manage potential threats and how to prevent crop damage.
“His role is very crucial,” said Mohammad Yaghmour, an orchard systems advisor for UC Cooperative Extension Kern County. “He's not only on this mission to educate growers but he's also a source of education for us.”
Trouillas typically conducts one or two site visits a week, usually after a farm advisor reaches out about a problem they can't solve on their own.
“This allows us to be at the forefront of disease detections in California,” he said.
He likens these visits to house calls a doctor would make, only to fields instead. And one of those calls recently took him to a cherry orchard in Lodi.
“These guys help me quite a bit,” said Andrew Vignolo, a pest control adviser with Wilbur-Ellis who asked for a consult. “I bug them a lot.”
The visit starts like any consult in a doctor's office, only the questions come fast as they walk around the Lodi orchard where branches are dying, there is gumming and the trees appear stressed. Some look to be sunburned from exposure. Old pruning wounds show cankers, indicating that past disease treatments didn't get rid of whatever was affecting the trees.
Trouillas asks about the cultivar of the trees because some varieties are more susceptible to pests or diseases. He focuses on stress because that opens the door to disease.
Do they prune in the dormant winter months or in summer when pathogens are more prevalent? Does the soil get tested? How old are the trees? What about nutrition?
“I'm trying to figure out how they got infected so bad,” Trouillas said, walking the orchard. “Bacterial canker is a very mysterious disease.”
He thinks it might be a bacterial canker disease and shaves some bark to take to the lab for testing. He wants to come back next winter to take some samples to see where the pathogen is overwintering.
“We'll know in a few weeks if we have a fighting chance,” Vignolo said.
Be it Lodi, Fresno or elsewhere in the state, Trouillas focuses on local conditions. But what is learned in one field can be passed on to others, providing early warnings or advice for those in similar situations. “All these efforts at collaboration, from the field, to the lab, going through research projects, there's only one goal here — to help the farmers of California.”
/h3>/h3>/h3>In California, P. ramorum is primarily a leaf pathogen and thrives in coastal tanoak/redwood forests and oak woodlands. There are two categories of hosts for P. ramorum. In trunk hosts, the pathogen infects the bark and can cause bark cankers that may ooze a thick, sticky sap. Trunk hosts of the pathogen include many species including tanoak, coast live oak, California black oak, and Shreve's oak.
On leaves, foliar symptoms range from leaf spots to twig dieback. Sudden oak death can be lethal to trees in both forest settings and residential landscapes. Foliar hosts include California bay laurel, rhododendron and camellia.
To learn more about this disease, read Pest Notes: Sudden Oak Death and visit the UCCE Sonoma County Sudden Oak Death website.
- 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.