Major symptoms: In early summer (late June), leaves appear with marginal leaf scorch (brown, necrotic (dead) leaf tissue. Usually, a narrow band of yellow (chlorotic) tissue is inward from the dead tissue, but the sudden appearance of leaf scorch symptoms prompted by hot weather may not allow the several days for the narrow chlorotic band to develop. As the disease progresses, affected twigs on limbs die back from the tip (see S.M. Mircetich et al. 1976 for a description). The most characteristic symptom of almond leaf scorch (ALS) is leaf scorching followed by decreased productivity and general decline. Even very susceptible varieties take many years to die completely, but nut production is severely reduced within a few years in most varieties. The first summer after infection by insect vectors, there will only be a few scorched leaves. During the winter many of these infections fail to survive, depending on winter severity. By the second summer after infection, growers may be able to see new infections and prune them off about 3 feet below the lowest symptoms, however even by the second year symptoms may not be prominent enough to see in most cases.
ALS symptoms spread m
ore rapidly and affect a larger number of trees in some varieties than in others in The varieties Peerless and Long IXL are extremely sensitive. Non-pareil is also sensitive. Carmel and Mission are fairly resistant or tolerant.
Distribution: California (see Purcell 1980 and Almeida and Purcell 2003, on vector transmission) – Trees demonstrating signs of ALS were first observed in the mid-1930s near Riverside. ALS was also noted in a few randomly scattered almond trees in Los Angeles and Contra Costa counties in the early 1950s. ALS has since spread to commercial orchards throughout California. The disease is still (2003) rare in the San Joaquin Valley, even in orchards next to vineyards with Pierce’s disease – and the reasons for this are not known. Strains of the causal bacterium, Xylella fastidiosa, that cause Pierce’s disease cause ALS, but not all strains from almond with ALS can cause Pierce’s disease (Hendson et al. 2001; Almeida and Purcell 2003, Applied and Environmental Microbiology), which can explain how ALS can occur at high levels in almond without Pierce’s disease in adjacent vineyards, as occurs in the Delta region of Contra Costa County. Reports of ALS in India (Jindal and Sharma 1987 have not been confirmed by isolation of the causal bacterium.
Effects of glassy-winged sharpshooter: Some increase in ALS is expected in California almond orchards that are near breeding habitats (such as citrus groves) of the glassy-winged sharpshooter (GWSS), but as yet it is hard to predict how severe the problem with GWSS will be for ALS. Remember that most new infections are not visible until the second or third year after infection, and this may take even longer with tolerant varieties of almond. GWSS have been observed feeding but not reproducing on almonds from just after leaves appear in February until March and early April. It is recommended that growers work contact regional control agencies (County Agricultural Commissioners can provide the latest information) to discuss ways prevent large numbers of GWSS entering almond from citrus or other breeding habitats so as to prevent ALS problems from developing. This will require coordinated efforts with growers that have crops (citrus) that produce populations of GWSS. Insecticides in orchards to control vectors that have entered almond orchards will probably not be very effective because the vectors may continue to enter the orchards from outside sources. So far there are not protocols for controlling GWSS in almond. Growers should contact their local University of California Cooperative Extension office for the latest guidelines.
Selected references:Davis et al. 1980 (etiology); S.M. Mircetich et al. 1976 (description); Purcell 1980 (vector transmission).