Since lower limb dieback (LLDB) was first identified in almonds more than 10 years ago, the disease has confounded growers and researchers alike.
“Lower limb dieback is one of the most mysterious diseases of almonds,” said Florent Trouillas, University of California Cooperative Extension (UCCE) plant pathologist. “There are a lot of hypothesis around and different ideas and different suggestions, but nothing really straightforward.”
David Doll, UCCE farm adviser in Merced County, agrees.
“It's a complex problem. There are likely three to four issues involved,” he said.
Trouillas is a new plant pathologist at the Kearney Agricultural Research and Education Center near Parlier. He is approaching the malady with an open mind.
His LLDB work is part of a larger project looking at trunk and scaffold cankers, funded in part by the Almond Board of California.
Before determining which direction the LLDB research will take, Trouillas plans to survey growers.
“First we have to define what we’re talking about with LLDB,” the plant pathologist said.
“Some issues are related to shading and others are tied to over watering and over-fertilization. There are also cases where the problem can be caused by high infections of hull rot or a high scale population.”
Trouillas recently visited an almond orchard near Five Points where a heavy infection of hull rot caused by the Rhizopus or bread mold fungus was likely responsible for LLDB. In some orchards north of there, he’s seen high scale populations associated with LLDB.
In addition, Trouillas is talking to colleagues in Australia who share that the country’s almond industry is equally concerned about LLDB.
As the name implies, lower limb dieback affects limbs in the lower half of the canopy. It is typically found in 7-8 year-old trees and then progresses as the orchard ages.
LLDB is more pronounced in Butte and particularly Padres, but also has been seen to a lesser extent in the Aldrich, Fritz, NePlus Ultra, Nonpareil, Sonora, Wood Colony, Mission, and Carmel varieties.
Limbs can look fine as the growing season begins. From late May though late July, leaves begin to turn yellow and the limbs become girdled from enlarging cankers. Eventually the entire branch collapses and dies. Symptoms may progress through much of the summer.
One theory suggests over saturation of soils whether from unexpected rains or over irrigation during the spring. The soil may remain saturated for longer periods which reduces the amount of oxygen in the soil which can cause deeper roots to die.
This makes the tree dependent on shallower roots unable to pull needed soil moisture during the hot summer. The tree can respond by shutting down the lower limbs.
Doll says the problem seems more pronounced in years with a cool spring, followed by a hot spell. This year, May was unseasonably cool. LLDB showed up in July - about a month later than usual.
Other suspect contributors to LLDB could include soil compaction, shade out, soil with low rates of infiltration, severe hull rot infections, or severe scale infestations.
Even without a causal agent, Doll says growers should irrigate based on soil- and plant-based moisture readings to reduce tree stress. They should also conduct dormant spur sampling for scale, follow proper orchard sanitation, and remove dead limbs.
Based on several fungicide field trials, Roger Duncan, UCC farm adviser in Stanislaus County, says LLDB does not appear to be caused by a foliar or canopy disease.
Duncan says spring or fall treatments with several fungicides had no significant effect in reducing the symptoms. Among the products tested were copper hydroxide, liquid lime sulfur, Pristine, NytriPhyte P, plus PlantShield, a commercial formulation of the biological fungicide Trichoderma harzianum.
Applications of Captain 80 WDG, Pristine, and Agri-fos in May, all applied with a bark penetrant, produced no significant differences compared to the untreated control.
Duncan is currently conducting a trial with dormant sprays in two 6-7 year-old Butte and Padre orchards with minor LLDB symptoms. Parts of the orchards received dormant sprays of 12 pounds of Kocide 2000 plus oil mixed in 100 gallons of water while other areas were not sprayed.
After two years, there were no visible differences between the treated and untreated plots, and LLDB symptoms were substantial throughout.
Duncan plans to continue the trial through at least this season to determine if there is a long-term cumulative effect of dormant copper sprays on LLDB.