Lower-limb dieback first shows up as yellowing leaves.
Symptoms shown here in the orchard are characteristic
of the disorder, which is also identified by dead tissue
found under the bark. Photos: Themis J. Michailides,
Kearney Agricultural Center.
Research funded by the Almond Board dating back to 2004 indicates lower-limb dieback (LLD) is a syndrome or physiological disorder primarily induced by excessively wet soils early in the season.
Work done by almond specialist Bruce Lampinen indicates water management resulting in excessively wet soils in early season is the primary factor causing LLD. Lampinen looked at soil moisture conditions in orchards with symptoms, and found they were very wet in April through June. These early-season wet conditions led to shallow rooting and sensitivity to water stress as the season progressed, with the result that trees became stressed at soil moisture levels that normally would be considered adequate.
UC plant pathologist Themis J. Michailides, Kearney Agricultural Center, has found that fungi are not the likely cause of LLD symptoms, but may be present as secondary invaders attacking weakened trees. While he found Botryosphaeria spp. and Phomopsis spp. in trees with LLD symptoms, he also noted these fungi were equally present in both affected and healthy trees. Furthermore, inoculation experiments with these fungi did not induce symptoms.
Growers with LLD problems should make sure irrigation is managed to avoid overwatering. Furthermore, in the affected orchards studied, optimal irrigation management could only be done using both plant- (e.g., pressure chambers) and soil-based monitoring. “Managing Irrigation for Optimum Efficiency” can be found on the Almond Board website.