Phenology of Xylella fastidiosa and its vector around California almond nurseries: An assessment of plant vulnerability to almond leaf scorch disease

Authors: Krugner, Rodrigo; Ledbetter, Craig; Chen, Jianchi; SHRESTHA, ANIL - Fresno State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2012
Publication Date: 10/1/2012
Citation: Krugner, R., Ledbetter, C.A., Chen, J., Shrestha, A. 2012. Phenology of Xylella fastidiosa and its vector around California almond nurseries: An assessment of plant vulnerability to almond leaf scorch disease. Plant Disease. 96(10):1488-1494.

Interpretive Summary: Almond leaf scorch disease is caused by the bacterium Xylella fastidiosa, which is transmitted by xylem-fluid feeding insects including sharpshooters and spittlebugs. Within almond orchards, tree-to-tree spread (secondary spread) of the pathogen appears to be rare or inexistent, thus most infections are due to the movement of inoculative vectors into the orchard (primary spread). To our knowledge, primary spread of X. fastidiosa through infected almond nursery stock has not been considered. The objectives of the study were: 1) to elucidate the fate of bacteria in planta after inoculations in almond nursery plants; 2) to monitor the movement and seasonal abundance of insect vectors in vegetation near commercial almond nurseries; and 3) to monitor plant infection by X. fastidiosa in habitats surrounding almond nurseries to determine their potential to serve as hosts for the pathogen and/or vector. Draeculacephala minerva Ball was the only vector of X. fastidiosa found. Population densities of D. minerva were higher in irrigated pastures than in alfalfa or mixed weedy environments. A total of 87 plant samples tested positive for X. fastidiosa (6.3%), with a higher number of X. fastidiosa-infected plants found in weedy alfalfa fields than in other habitat types. Among plant species infected by X. fastidiosa, 33% were winter annuals, 45% were biennials or perennials, and 22% were summer annuals. Collectively, these findings identified a potential pathway for X. fastidiosa infection of almonds.

Technical Abstract: Management of almond leaf scorch disease requires knowledge of all possible infection pathways. The disease is caused by the xylem-limited bacterium Xylella fastidiosa, which is transmitted by several species of leafhoppers. The objectives of this research were to elucidate the fate of bacteria in planta after inoculations in almond nursery plants and to determine patterns of insect vector population dynamics and temporal distribution of X. fastidiosa-infected plants relative to host plant assemblages in habitats surrounding commercial nurseries. In an experimental nursery, disease incidence was markedly affected by rootstock type. Prior to bud grafting, ‘Nemaguard’ rootstock seedlings were immune to bacterial infection. After bud grafting with a susceptible scion, scions were susceptible to infection regardless of rootstock genotype. Surveys in commercial nurseries revealed that only habitats with permanent grass cover sustained vector populations throughout the season. A total of 87 plant samples tested positive for X. fastidiosa (6.3%) using ELISA, with a higher number of X. fastidiosa-infected plants found in weedy alfalfa fields than in other habitat types. Among plant species infected by X. fastidiosa, 33% were winter annuals, 45% were biennials or perennials, and 22% were summer annuals. Collectively, these findings identified a potential pathway for X. fastidiosa infection of almonds.