Management of Almond Leaf Scorch Disease: Long-Term Data on Yield, Tree Vitality, and Disease Progress

Sisterson, M. S., Ledbetter, C. A., Chen, J., Higbee, B. S., Groves, R. L., and Daane, K. M. 2012. Management of almond leaf scorch disease: Long-term data on yield, tree vitality, and disease progress. Plant Dis. 96:1037-1044. Almond leaf scorch disease (ALSD) has been a chronic problem for California almond growers. This disease is caused by the bacterial pathogen Xylella fastidiosa and is transmitted by xylem-feeding insects. Previous research suggested that retaining, rather than roguing, ALSD-affected trees may be more economically beneficial because ALSD-affected trees produced a reasonable yield and did not die over a 3-year period. Because almond orchards are kept in production for approximately 25 years, longer-term data are needed to fully evaluate the merits of retaining ALSD-affected trees. Extension of yield evaluations from 3 to 5 years demonstrated that yield loss due to ALSD was consistent over 5 years, with yields of ALSD-affected trees reduced by 20 and 40% compared with unaffected trees for ‘Nonpareil’ and ‘Sonora’, respectively. To assess risk of ALSD-affected trees serving as a source of inocula for secondary (tree-to-tree) spread and to evaluate vitality of ALSD-affected trees, previous surveys of two orchards were extended from 3 to 6 or 7 years. The relationship between disease incidence (percentage of trees infected) and survey year was linear for all cultivars examined at both orchards. Furthermore, at each orchard, the spatial location of infections detected after the first survey was random with respect to the spatial location of infections identified during the first survey, suggesting that ALSD-affected trees retained in orchards did not serve as a source for secondary spread. Over the 6to 7-year study period, death of ALSD-affected trees was rare, with only 9% of ALSD-affected trees dying. Because orchards used in this study had relatively high disease incidence, 61 orchards containing Sonora were surveyed to determine typical levels of ALSD incidence. ALSD was widespread, with at least one infected tree in 56% of orchards surveyed, but incidence was typically low (mean incidence = 0.47%). Collectively, the results suggest that retaining ALSD-affected trees may be economically beneficial in older orchards. The majority of the world’s almond crop is produced in the Central Valley of California. For California almond growers, almond leaf scorch disease (ALSD) has been a chronic problem for more than 60 years (1). This disease is caused by the xylem-limited bacterium, Xylella fastidiosa (10). Various strains of X. fastidiosa cause disease in a wide variety of cultivated and ornamental plants, including grape, alfalfa, and oleander (11). In addition, X. fastidiosa can be found in weeds grown in and near agricultural sites (23). The pathogen is transmitted by xylem-sap-feeding insects (20), and the green sharpshooter (Draeculacephala minerva) appears to be the principal vector of X. fastidiosa in California’s Central Valley (6). The green sharpshooter is abundant in permanent pastures and weedy alfalfa fields (6,19,25). Movement of green sharpshooters into almond orchards is incidental because almond is not a preferred host (19), although grassy weeds in and around almond orchards can sustain green sharpshooter populations (6). Because almond trees are not a primary host for green sharpshooters, risk of ALSD may be reduced by distancing orchards from green sharpshooter habitats and by removing weeds that support vectors in and around orchards (6,25). Risk of ALSD can also be reduced by cultivar selection because cultivars vary in their susceptibility, with ‘Sonora’ being one of the more susceptible cultivars (4,24). If ALSD-affected trees are found in orchards, growers must decide to retain or remove infected trees. Historically, ALSD-affected trees were reported to decline and die over a 3to 8-year period (1,18,21). If ALSD-affected trees die, the decision to remove them is simple because they have no value. However, Sisterson et al. (24) monitored yields of ALSD-affected trees over a 3-year period and found that ALSD-affected trees produced reasonable yields and did not die during the study period. If ALSDaffected trees produce reasonable yields over a long period, there is incentive to retain rather than remove ALSD-affected trees. The economic benefit of retaining ALSD-affected trees must be balanced against the risk of ALSD-affected trees serving as a source for secondary (tree-to-tree) spread of X. fastidiosa. Studies conducted over a 1to 3-year period suggest that risk of secondary spread is low (4,9,24). Because orchards are kept in production for approximately 25 years (7), longer-term data are needed to better assess the risk of infected trees serving as a source for secondary spread and to determine whether ALSD-affected trees continue to produce a reasonable yield over the long term. Sisterson et al. (24) compared yields of ALSD-affected and unaffected trees over a 3-year period, short of the 3to 8-year period over which ALSD-affected trees were anecdotally reported to decline (1,18,21). In this study, yield evaluations of Sisterson et al. (24) were extended to 5 years. In addition, some growers reported that yields of unaffected trees located next to ALSD-affected trees compensated for yield loss due to ALSD. In this scenario, unaffected trees located next to ALSD-affected trees developed larger canopies due to reduced growth of ALSD-affected trees. To test this hypothesis, yields of unaffected trees located next to ALSDaffected trees were compared with yields of unaffected trees that were located next to unaffected trees. Corresponding author: M. Sisterson, E-mail: [email protected] Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. The United States Department of Agriculture is an equal opportunity provider and employer. Accepted for publication 10 February 2012. http://dx.doi.org/10.1094 / PDIS-08-11-0693-RE This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.