Rotten Time, Phytophora
In almond orchards from Red Bluff to Bakersfield, trees of every age and variety, on Class One soils and hillside clays, flood irrigated, and on drip, are collapsing and dying in greater numbers than has been seen in decades. In some cases, entire orchards succumb.
The killer: Phytophthora fungi. Generally associated with heavy soils and excessive moisture, such as trees doused by sprinklers, Phytophthora cause crown and root rot, but some species (there are many) attack trunks and scaffolds as well.
Phytophthora fungi have long been known to be the leading killers of almond trees, but the extent of damage in recent years and the unusual occurrences have growers and tree experts alike scratching their heads over causes and prevention there is no cure once the dreaded organisms set to work.
Consider what Joe Connell, Butte County farm advisor has seen in recent years. "It's showing up more now, and in young nonbearing trees as well as in bearing orchards," he observed. "Last year we had aerial Phytophthora with more pruning wound Phytophthora and above-ground cankers. This year it's mostly root and crown rot."
A "surprise" to Connell, the disease in his area is as common in traditional orchard soils around Chico and Durham as it is on newly-planted, marginal soils. Which is not the case in Glenn and Colusa counties, says John Edstrom, farm advisor for the central Sacramento Valley. "It's more of a problem in the marginal soils of the west side of the valley, and with young trees," he says. "We have a significant amount in one- to four-year-old orchards."
Edstrom's seen entire orchards of 30 to 40 acres die. Typically,though, it's 10 to 15 percent of a young orchard that has to be removed. "We used to think 1 to 3 percent was typical, but it has doubled and tripled in the last four years," he notes.
Down south, Mario Viveros, Kern County farm advisor, first began noticing Phytophthora invasions in orchards around Shafter, a dozen miles northwest of Bakersfield. "That was two years ago," he recalls, "and every year it has gotten worse." What puzzles him, and others who have observed the phenomenon, is the disease's widespread occurrence in well-managed, older, otherwise healthy orchards on sandy soil that are flood, sprinkler and low volume irrigated, and that have no history of Phytophthora outbreaks.
"What I saw looked like Phytophthora, but is was very unlikely because of the well-drained soil and flood irrigation," he said, "and we just don't get Phytophthora in Kern County."
They do now, observes Gerry Guthrie, Blue Diamond® field representative for the area. "It's showing up all over, in trees from 3 years to 20 years old and, oddly, usually in the crotch where there are no wounds," he said.
Once the symptoms appear _one-inch, light-orange gum balls on the trunk or scaffolds _ a tree lasts about a season and a half before it collapses, usually in mid-summer when it can't draw enough water past the damaged tissue to support itself, says Viveros.
While the total count of trees lost in the area isn't great yet, individual growers have suffered heavy losses. Now, the "fear factor" has set in, said Guthrie, because the disease is showing up everywhere and in the least likely blocks. The concern is, he said, "How many trees will we lose?"
Search for Why
No one knows for certain why Phytophthora is on the increase in the southern end of the valley. Viveros says work is underway to determine that, but he has some ideas.
Kern County has a lot of mature orchards, Viveros notes. "When a tree gets to be 12 to 14 years old, it forms deep pockets at the crotch where the main scaffolds come together. These pockets collect leaves, nuts and, in a wet year, a significant amount of moisture, creating ideal conditions for Phytophthora. The recent outbreaks of the disease were, perhaps, related to the heavy rains of 1994, '95 and '96 and those ideal conditions."
He observes that the heaviest-hit orchards are 14 years old or older, which tends to confirm his theory.
After digging through old texts on the disease, Viveros found a 1947 circular, number 103, Almond Culture in California, that seemed to confirm his suspicions. The circular advised growers to prune their young trees in a way that would avoid the creation at maturity of deep pockets where the main scaffolds meet the trunk of the tree. "There was a big emphasis on staggering limbs up and down the tree to avoid forming a water pocket," he relates. "I thought it interesting that it was a concern back in the Forties."
Careful tree training is as good an idea today as it was a couple of generations ago, says Viveros. "One of the best things a grower can do when training his trees is to stagger the limbs. It creates a stronger tree and it helps prevent disease."
"Proper tree training is extremely important," Viveros emphasizes. "Too often growers assign the job to unskilled people. It's scary to think that they are putting that big of an investment in their future, really _ in the hands of minimum wage workers. In the old days, tree pruners were special people. Growers were extremely particular about how their trees were pruned. Maybe we need to go back to that. To do otherwise is to put the productive lives of our trees in jeopardy."
Wind A Factor
In Butte County, Connell blames the wind as well as the rain for the harder hit that growers in his area are taking from Phytophthora this year. "Heavy winds in December whipped and flexed our trees, creating depressions in the soil around the base of the trunks," he explains. "Intense rains in February and March filled those depressions with water that the Phytophthora could breed in and launch their attack on the trunk and main roots." That plus soil that remained saturated for long periods contributed to this year's outbreak of Phytophthora, he conjectured.
Marginal Ground Chancy
In Edstrom's area, Phytophthora is primarily a problem in young orchards, and largely of the root and crown rot varieties. "We have a significant amount of it in one- to four-year-old orchards," he observes. "It began showing up in significant amounts in the last two years."
Edstrom attributes the increase to two leading causes: standing water and poor underground drainage. "First, excessive rains in recent years saturated soils and created surface puddles, raising the likelihood of spores germinating and infecting trees," he explained. "Second, new plantings have expanded into nontraditional orchard areas, usually on heavier soils that are less well drained and that have unknown subsurface drainage characteristics. Some of these areas had saturated soils for long periods."
Many of the new plantings in Edstrom's area are in foothills on the west side of the Sacramento
Valley. "While surface drainage is relatively good, as might be expected in hilly terrain, pockets and low spots pose a problem and subsurface drainage can be unpredictable," Edstrom said. "It appears that a hidden phenomenon is at work in some areas in which an underground sheet of water slowly seeps out of the hills, taking months to finish draining." This leaves tree roots under stress for an extended period, he said, setting orchards up for disease.
What Can A Grower Do?
According to IPM for Almonds, once you have the disease in an orchard, chances are that it will remain. Eradication is impossible and most species of Phytophthora will eventually kill a tree. Some species will cause a tree to slowly decline, others will cause it to suddenly collapse in spring or early summer with the arrival of warm weather. The severity and rate of disease development depends in part upon the rootstock and the species of Phytophthora involved.
Slow decline results from roots infected by species that attack feeder roots, IPM for Almonds explains. Other species attack roots en masse, causing the entire root system to decay and rapid death of the tree. Trees with crown rot infections usually decline rapidly. If invaded in the fall, they can be girdled by spring. Their stored energy will produce blossoms and leaves, but when the weather turns warm and water is needed from the roots, the tree quickly dies. Trunk and branch cankers caused by Phytophthora grow rapidly. However, trunk cankers stop growing during high summer temperatures, resuming their growth when conditions are more favorable. When conditions become less favorable for root rot fungi, infected trees are sometimes able to grow new roots and recover.
All Phytophthora species (there are at least 11) that attack almond trees are soil-borne fungi, according to IPM for Almonds. They are present throughout the almond-producing regions of the state, although they may not infest every orchard, nor do all species occur in all soils. They require free water to reproduce and infect a host.
IPM for Almonds notes that Phytophthora do not require wounds to infect a tree. The fungi enter a tree where infested, waterlogged soil or flood water touches the root, crown or trunk. If the pathogen is present in water used to sprinkle-irrigate an orchard, it may enter the trunk or branches where water collects.
No chemicals are registered for use on this disease in producing orchards, which focuses attention on orchard management and disease prevention. IPM for Almonds offers the following management guidelines:
Proper water management is the most important aspect in controlling root and crown rot. Water should not be allowed to accumulate or stand around crowns of trees. Low spots in the orchard, areas that flood frequently, and places where water penetration is poor should be provided with adequate drainage or left unplanted.
To protect the trunk and scaffolds from infection, use low-angle sprinklers and remove weeds from around the base of the trees. Weeds tend to maintain high humidity and provide a favorable environment for disease development.
Edstrom adds that the most dangerous times for infection are March/April and October/November, so one should avoid irrigating then unless the soil is dry. Young trees are the most vulnerable to infection, he notes, so make certain water doesn't puddle at the bases of these trees. He advises refilling soil pockets at the bases of trees caused by trunks bending in the wind. Consider double-staking such trees to prevent recurrence.
Avoid Spreading Infection:
Be careful not to introduce the pathogen into uncontaminated soil by infected plant material, infested irrigation water, or contaminated soil on farm equipment.
If you are replanting an area where Phytophthora is present, plant trees on small mounds, as shallowly as possible, or on broad ridges with the upper roots near the soil level. Establish berms before planting. The ridges should be 8 to 10 inches high. Planting depth after settling should be no deeper than in the nursery, and the graft union should be well above the soil line.
Use a rootstock that has some tolerance for Phytophthora. Of the rootstocks used in almond orchards, almond is the most susceptible to crown and root rot, followed by almond X peach hybrids, Nemaguard, and Lovell. Marianna 2624 is the most tolerant.
Edstrom notes that high demand for new trees has reduced availability of the more popular choices of varieties and rootstocks. Lovell peach has been the preferred rootstock in the Sacramento Valley, given its reputation for having slightly higher tolerance of wet soil conditions than Nemaguard peach. But Edstrom thinks this reputation may not have any practical value.
Tests on layered-clay soil at Nickels Lab in the Sacramento Valley showed Nemaguard to out-yield Lovell. "If Nemaguard competes with Lovell under these difficult conditions, maybe it should be considered for more widespread use when Lovell supplies are tight," he says. All commercial almond varieties are compatible on both Lovell and Nemaguard, and cultural requirements are basically the same, he adds.
He recommends against switching from Lovell to peach/almond hybrid rootstock, unless the soil is deep and light-textured. Marianna might substitute for Lovell on fine-textured soils or where wet conditions are common, he says, but some almond varieties are either incompatible (Nonpareil) or risky (Butte and Monterey) on M2624. Almonds on M2624 produce smaller, shorter-lived trees and are subject to graft union disorders, including brown line and mild etch, he notes.
The choice of rootstock becomes more of a gamble as growers move out into marginal soils and areas not heretofore planted to almonds, says Edstrom. "Lovell and Nemaguard have been the mainstays for many years, but that was in the best soils."
Joe Connell has seen good results with Marianna rootstock in new orchards in Butte County. "We haven't had much problem with Phytophthora in these orchards," he says. "They are planted on Marianna plum root, on mounds, in marginal soils, and they are doing okay."
Edstrom suggests removing, narrowing or avoiding altogether intercrops where Phyto- phthora infections occur.
While no chemicals are registered for treatment of Phytophthora, two _ Alliete and Ridomil, are approved as preventive treatments in non-bearing orchards. It isn't clear how effective they are, but it may be worth trying them.
While Phytophthora is causing heartache in many orchards this year, future infections may be prevented or at least minimized by following the above suggestions.