Lower limb dieback (LLDB) is an emerging problem within many almond orchards throughout California. Observations of orchards with LLDB occur independent of soil types, irrigation systems, varieties planted, and planting spacings, while typically affecting orchards that are in their eighth leaf or older.
The problem tends to be associated with smaller diameter branches in the lower canopy of orchards. Often enough, however, the problem extends to larger diameter branches, causing branch loss that can extend to 10 feet or more from the ground. Symptoms include wilted, yellow leaves that eventually fall from the tree. Bark removal will reveal a brown canker with little or no gumming that usually does not completely girdle the branch. The fungal canker can be observed on the top side of the affected branches, which is often found proximal to the yellowing leaves. Often, the canker is associated with a dead spur or small branch. The fungus appears to move up the branch to the point of attachment with the main scaffold, but does not appear to enter into the main scaffold.
The first appearance of symptoms has been reported in April, with shoots continuing to collapse throughout the summer. Branch collapse is often noticeable about a week after a hot spell in which the evapotransipiration rates are very high. Isolations made by Themis Michailides (UC Extension Plant Pathology Specialist) from almond trees in orchards affected by LLDB have identified two commonly isolated genera of fungi, Botryosphaeria and Phomopsis.
Sampling of diseased limbs from 10 orchards in 2005 (Glenn, Madera, and Stanislaus Co.) and 18 orchards in 2006 (Butte, Colusa, Fresno, Glenn, and Kern Co.) identified Botryosphaeria spp. and Phomopsis spp. from 52 percent and 56 percent of the isolations made, respectively. Isolation of these pathogens was higher in late summer/fall than in spring/early summer. Interestingly enough, in the Sacramento Valley, fall isolations frequently occurred from both limbs with and without symptoms.
Corresponding pathogenicity tests of isolated fungi on thrifty and unthrifty trees indicated that the isolates of Botryosphaeria spp. and Phomopsis spp. can cause disease on almond trees. It was also found that the isolates of Botryosphaeria spp. tend to be more virulent than Phomopsis spp. There is some uncertainty of these fungi being the true cause of the disease, however, as they have been considered to be fungi that colonize weak tissues that are predisposed from some other stress.
Several fungicide trials by Roger Duncan, UCCE Stanislaus, and Bruce Lampinen, UC Extension Pomology Specialist, have consisted of spring and fall applications of several fungicides. Fall (October – December) applications of copper hydroxide, liquid lime sulfur, Pristine fungicide, Nutriphyte P (0.5 gallons per acre), and Plant Shield, a commercial formulation of Trichoderma harzianum (a biological control agent), did not lower the incidence of LLDB the following year. May applications of Captan 80 WDG, Pristine, and Agri-fos, all applied with a bark penetrant, also failed to reduce LLDB symptoms. These trials indicate that chemical control for LLDB is either not possible or has not been identified.
Orchard water management may play a role in the incidence of LLDB. Through the use of soil moisture monitoring systems and a pressure chamber, research by Bruce Lampinen has demonstrated that orchards with LLDB frequently are over-watered in the early season (April-June). Research is ongoing, but evidence suggests that excessive water in the early season can prevent proper root growth and development, which would lead to increase tree stress during periods of high evapo-transpiration. This stress would lead to the reduction of tree resources being sent to branches of the lower canopy, weakening these branches, and allowing invasion by the above mentioned fungi. Growers with LLDB affected orchards should evaluate their water management practices to prevent over-watering during the early season.
Other orchard problems may also contribute to LLDB. Hull rot, scale infestations, and herbicide drift can damage the lower branches and kill spurs, providing an entrance for fungi. These problems occur frequently on younger trees (less than eighth leaf), before the onset of LLDB, which suggest that they may predispose the tree to LLDB. Therefore, it may be important for growers to implement orchard practices that will reduce damage to the lower canopy. Then again, any practice that reduces tree damage should be adopted to increase orchard longevity.
Until the true cause of LLDB is determined, it is advised that growers prune out infected limbs. Work done by Roger Duncan showed a reduction of LLDB in orchards in which affected limbs were removed, and suggests that this is currently the best way to reduce LLDB. Affected limbs should be removed as soon as possible by pruning 4-6 inches beyond the canker margin to ensure complete removal of the fungal pathogens. It is not advised to make major cuts on scaffolds or large branches to ensure a full 4-6 inches. Aggressive rouging out of infected branches during the summer months may reduce inoculums levels and prevent infections in the fall or dormant period.
For more information on LLDB, please check with your local UC Extension office or the Web site of the California Almond Board.
• Crops affected: Almond, predominantly found on Padre and Butte, but also found on many other varieties including Nonpareil, Fritz, Carmel, Wood Colony, and Mission.
• Causal organisms: Unknown, but high isolation frequency of Botryosphaeria spp. and Phomopsis spp.