Nemaguard’ xylem sap is not an effective mouth wash for Xylella fastidiosa-infected vectors

Authors: Krugner, Rodrigo; Ledbetter, Craig; Rogers, Elizabeth

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/5/2019
Publication Date: 10/29/2019
Citation: Krugner, R., Ledbetter, C.A., Rogers, E.E. 2019. Nemaguard’ xylem sap is not an effective mouth wash for Xylella fastidiosa-infected vectors. In: Proceedings of the Second European conference on Xylella fastidiosa 2019, October 29-30, 2019, Ajaccio, France. p. 86.

Interpretive Summary: Almond leaf scorch disease is caused by the bacterium Xylella fastidiosa, which is transmitted by several species of sharpshooters and spittlebugs or the use of infected bud wood. ‘Nemaguard’ was selected at USDA stations in Fort Valley, GA, and Fresno, CA and released in 1959 as a rootstock for almond and stone fruits due to resistance to nematodes and enhanced scion vigor. Today, ‘Nemaguard’ is the most common rootstock for almond and stone fruit in California. ‘Nemaguard’ happens to be resistant to X. fastidiosa, but nothing is known about potential chemical and physical mechanisms of resistance. The goal of this study was to determine whether feeding on ‘Nemaguard’ can reduce bacterial loads in infected insect vectors, and therefore reduce spread of the disease. After acquiring X. fastidiosa from infected grapevines, vector feeding periods of up to 14 days on ‘Nemaguard’ did not reduce bacterial populations in vectors or transmission of X. fastidiosa to susceptible plants. Results suggest that natural chemical properties of ‘Nemaguard’ xylem sap are not involved in resistance to X. fastidiosa and that future research should focus on identification of potential physical traits that prevent bacterial multiplication or movement within the plant.

Technical Abstract: ‘Nemaguard’ was selected by the United States Department of Agriculture and released in 1959 as a rootstock for almond and stone fruits due to resistance to nematodes and enhanced scion vigor. 'Nemaguard’ happens to be resistant to Xylella fastidiosa, the causal agent of almond leaf scorch disease. Previous research showed that prior to June-budding this rootstock can prevent infection of almond nursery stock by X. fastidiosa. Further, the rootstock also promotes an apparent complete pathogen elimination and remission of leaf scorching symptoms in infected susceptible scions. However, nothing is known about potential chemical and physical mechanisms of resistance to X. fastidiosa infection. The goal of the current study was to determine whether insect vector feeding periods on ‘Nemaguard’ can reduce bacterial populations in vectors or transmission efficiency to susceptible plants. After acquiring X. fastidiosa from infected grapevines, vector access periods of up to 14 days on ‘Nemaguard’ did not reduce pathogen population densities in vectors or transmission efficiency of X. fastidiosa to susceptible plants when compared to control. Results suggest that natural chemical properties of ‘Nemaguard’ xylem sap are not involved in resistance to X. fastidiosa and that future research should focus on identification of potential physical traits that prevent bacterial multiplication or movement within the plant.