Symptoms and Signs
Beginning in late April or May, 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 substantially affected. Less affected are Nonpareil, Carmel, and Aldrich varieties.
The occurrence of lower-limb dieback on almonds is still being researched. Thus the etiology is unknown, but it may be a physiological disorder related to water potential and light levels. Affected trees may be first weakened by pre-existing root problems, such as overly wet soils in the spring, low light, or possibly other causes including herbicides, fertilizers, or anything that may damage tree roots. Hull rot may also contribute to lower limb dieback because infected branches may continue to die in the following growing season after initial infections. Toxins such as fumaric acid may accumulate in large branches when multiple spurs are infected. Predisposed trees and dieback from hull rot 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.
Good management strategies have not yet been determined. Keep trees strong by proper irrigation management and maintain good control of scale insects. Dormant copper sprays and in-season fungicide sprays have not been shown to be effective in University trials.