How to Manage Pests
UC Pest Management Guidelines
Lower Limb Dieback
(Reviewed 3/09, updated 3/09)
In this Guideline:
SYMPTOMS AND SIGNS
Beginning in late April, leaves on lower limbs of affected trees first turn yellow, then brown. The limb eventually dies, often right up to the point of attachment, but the large wood of the scaffolds remains apparently unaffected. If the bark on dying limbs is scraped away with a knife, brown spots are evident in the wood. The symptoms can be confused with normal shade-out of low limbs. However, as lower limb dieback progresses, limbs receiving adequate sunlight several feet high in the tree can eventually become affected.
COMMENTS ON THE DISEASE
Padre appears to be the most seriously affected variety, although Butte can also be affected. Less affected are Nonpareil, Carmel, and Aldrich. The occurrence of lower-limb dieback on almonds has been recognized only recently in California and is still being researched. Thus the etiology is unknown, but it may be a physiological disorder related to water potential and light levels. It is believed that affected trees are first weakened by pre-existing root problems, such as overly wet soils in the spring, low light, or possibly other causes including herbicides and fertilizers that may damage tree roots. These predisposed trees are susceptible to infection by secondary pathogens such as Botryosphaeria dothidea and Phomopsis amygdali, or other species that may colonize and eventually girdle the limbs, resulting in limb death. Another possible injury that may be colonized by secondary pathogens includes hull rot infections where dieback occurs.
Good management strategies have not yet been determined. Currently, keeping trees strong by proper irrigation management and maintaining good control of scale populations are the primary recommendations for controlling this disease. Fungicide sprays are not effective.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside