Management of almond leaf scorch disease: long term data on yield, tree vitality, and disease progress

Authors: Sisterson, Mark; Ledbetter, Craig; HIGBEE, BRADLEY - Paramount Farming Company, Inc; GROVES, RUSSELL - University Of Wisconsin; Chen, Jianchi; DAANE, KENT - University Of California

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2012
Publication Date: 6/11/2012
Citation: Sisterson, M.S., Ledbetter, C.A., Higbee, B., Groves, R., Chen, J., Daane, K. 2012. Management of almond leaf scorch disease: long term data on yield, tree vitality, and disease progress. Plant Disease. 96:1037-1044.

Interpretive Summary: Almond leaf scorch (ALS) disease has been present in California for more than 60 years. This disease is caused by the bacterium Xylella fastidiosa, which causes several other important plant diseases including Pierce’s disease of grapes. If ALS-affected trees are detected in orchards, growers must decide to retain or remove affected trees. The decision to retain or remove ALS-affected trees is a function of two components: yield loss due to infection and risk of ALS-affected trees serving as a source for in-field, secondary pathogen spread. Yields of ALS-affected and unaffected trees were compared over a five year period. Yield loss due to ALS was consistent over the study period with yields of ALS-affected trees reduced by 20 and 40% relative to unaffected trees for the cultivars Nonpareil and Sonora, respectively. Orchards surveys conducted over a 6 or 7 year period determined that ALS incidence increased linearly and that 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. Collectively, results from orchard surveys suggested that ALS-affected trees retained in orchards did not serve as a source for secondary spread. Finally, to establish typical levels of ALS incidence, 61 orchards containing the cultivar Sonora were surveyed. ALS 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 ALS-affected trees may be economically beneficial in older orchards.

Technical Abstract: Almond leaf scorch (ALS) disease 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, ALS-affected trees may be more economically beneficial as ALS-affected trees produced a reasonable yield and did not die over a 3 year period. As almond orchards are kept in production for ~25 years, longer term data are needed to fully evaluate the merits of retaining ALS-affected trees. Extension of yield evaluations from 3 to 5 years demonstrated that yield loss due to ALS was consistent over 5 years with yields of ALS-affected trees reduced by 20 and 40% compared to unaffected trees for the cultivars Nonpareil and Sonora, respectively. To assess risk of ALS-affected trees serving as a source of inocula for secondary (tree-to-tree) spread and to evaluate vitality of ALS-affected trees, previous surveys of two orchards were extended from 3 to 6 or 7 years. The relationship between disease incidence and survey year was linear for all cultivars examined at both orchards. Further, 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 ALS-affected trees retained in orchards did not serve as a source for secondary spread. Over the 6 to 7 year study period, death of ALS-affected trees was rare with only 7% of ALS affected trees dying. Orchards used in this study had relatively high disease incidence. To establish typical levels of disease incidence, 61 orchards containing the cultivar Sonora were surveyed. ALS 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 ALS-affected trees may be economically beneficial in some cases.