A relatively new problem in almonds, researchers have been investigating the causes and potential cures of lower limb dieback (LLDB) since 2004. It was first identified in Stanislaus and Butte counties, and has also been reported in Glenn, Madera, Fresno and Kern counties. Padre and Butte varieites appear to be the most affected, but symptoms have also been found in Carmel, Nonpareil, Sonora, and some others.
Symptoms have been observed to develop on Butte/Padres when the trees are about seven to eight years old, and worsen as the trees age. Dead tissue found under the bark of affected limbs suggests to researchers that either a toxic agent (such as an herbicide) or a pathogen (such as a fungus) may be associated with LLDB.
Outwardly, symptoms begin with the yellowing of leaves in the lower canopy around late April. These leaves turn brown and some fall off while others stick on the affected branches. The condition spreads, and by late summer, a significant number of limbs in the lower canopy may die.
UC plant pathologist Themis J. Michailides, Kearney Agricultural Center, Parlier, and almond specialist Bruce Lampinen, UC Davis, are the lead researchers in an ongoing project funded by the Almond Board of California to discover the cause of LLDB, and once that is known, to develop orchard management practices or treatments to prevent the problem.
Of note is Almond Board-funded work by Bruce Lampinen on canopy development, spur dynamics and irrigation management related to plant-based mid-day stem water potential, which has helped almond growers maintain orchard productivity while conserving inputs such as nitrogen and water. His work on water management may have implications for LLDB.
Role of fungal diseases
Michailides and his associates have been sampling tissue from affected trees every year since 2004, finding a high incidence of two fungi, Botryosphaeria spp. and Phomopsis spp. in trees with LLDB symptoms. These fungi are known to cause canker diseases in almonds in California and other growing regions of the world.
The researchers found, however, that in many cases Botryosphaeria and Phomopsis were equally present in both symptomatic limbs in the lower canopy and in healthy tree limbs in the same area of the canopy. This suggests that these fungi are not likely causing the symptoms, but may be present as secondary invaders attacking trees weakened by LLDB symptoms, or, in fact, may have no role in the condition at all.
There is further evidence that the two fungi are not implicated in causing LLDB:
A fungicide trial conducted by Farm Advisor Roger Duncan, Stanislaus County, had no effect against LLDB.
The Stanislaus County orchard with the worst symptoms had low levels of the two suspect pathogens in 2008.
While Botryosphaeria and Phomopsis are frequently found at high levels in symptomatic trees, innoculating healthy trees with Botryosphaeria and Phomopsis did not result in LLDB symptoms.
High soil moisture
Excessively wet soils in early season appear to be a more likely condition leading to lower limb dieback in almond trees. In 2007, Lampinen monitored three orchards in Stanislaus County that had a history of LLDB. Augering holes in the three orchards in mid-June revealed that two of the orchards, which were flood-irrigated, were uniformly wet down to 5 feet. LLDB symptoms increased in these orchards until mid-June, and leveled off as both soil moisture and mid-day stem water potential of tree leaves fell into the normal range.
Combining these 2007 results with soil-moisture monitoring in affected orchards in 2008, six out of seven orchards were very wet during the April to June timeframe. The seventh orchard had very wet soil even later into mid-summer, yet the trees were stressed, suggesting that excessive moisture may have led to root damage.
The Almond Board of California is funding further studies investigating the causes and possible prevention of lower limb dieback in the coming year. These include:
A survey of affected orchards early in the season to identify the first symptoms of LLDB, and what pathogens, if any, and other characteristics are commonly associated with the condition.
An experiment with herbicide applications to determine if herbicide drift could be a “toxic agent” that enters under tree bark, causing tissue death.
Trials with varying levels of soil moisture and tree water content to establish whether excessive moisture plays a role in LLDB.
Although these studies will help us unravel the mystery of what causes lower limb dieback in almonds, research to date implicates early season overwatering. The research also suggests that fungal diseases associated with LLDB are simply opportunistic secondary invaders attacking weakened trees. Growers with LLDB-affected orchards should first look at their water management practices to prevent overwatering, which leads to excessive soil moisture and root damage.