Insights regarding resistance of ‘Nemaguard’ rootstock to the bacterium Xylella fastidiosa

Authors: Krugner, Rodrigo; Rogers, Elizabeth; Burbank, Lindsey; Wallis, Christopher; Ledbetter, Craig

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2022
Publication Date: 3/6/2022
Citation: Krugner, R., Rogers, E.E., Burbank, L.P., Wallis, C.M., Ledbetter, C.A. 2022. Insights regarding resistance of ‘Nemaguard’ rootstock to the bacterium Xylella fastidiosa. Plant Disease. 106(8):2074-2081. https://doi.org/10.1094/PDIS-01-22-0136-RE.
DOI: https://doi.org/10.1094/PDIS-01-22-0136-RE

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 propagation material. Understanding the mechanism of host plant resistance to X. fastidiosa can be valuable for development of new crop varieties and strategies for disease management. Nemaguard, a X. fastidiosa-resistant rootstock commonly used in California almond production, was tested to determine whether X. fastidiosa could move within the plant. X. fastidiosa did not move far away from the point of inoculation in Nemaguard; this is in contrast to susceptible almond cultivars where X. fastidiosa moves long distances throughout the plant. All plants produce a variety of naturally-occurring chemical compounds in their xylem sap. To test whether compounds in Nemaguard sap could reduce the number of X. fastidiosa inside sharpshooter vectors, the sharpshooters were allowed to feed on grapevines infected X. fastidiosa, and then allowed to feed on either Nemaguard or on healthy grapevines. There was no effect of Nemaguard feeding on the ability of sharpshooters to transmit X. fastidiosa, therefore Nemaguard xylem sap was not an effective mouthwash for X. fastidiosa infected vectors. There are likely to be other factors involved in Nemaguard resistance to X. fastidiosa.

Technical Abstract: Nemaguard is a commonly used rootstock for almond and stone fruits due to resistance to nematodes and enhanced scion vigor. Nemaguard also 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 chemical and physical mechanisms of Nemaguard resistance to X. fastidiosa infection. The first objective of this study was to compare distribution of X. fastidiosa in almond and Nemaguard and evaluate the chemical profile of infected versus non-infected plants of each species. Results showed limited or no movement of X. fastidiosa beyond the point of mechanical inoculation in Nemaguard, whereas in almonds X. fastidiosa was detected and isolated from all plant parts above the point of inoculation. Large differences in the concentration of phenolic compounds between Nemaguard and almond suggested a potential effect of xylem stem chemistry in X. fastidiosa population growth in plants and insects. Therefore, the second objective of this study was to determine whether insect vector feeding periods on Nemaguard could reduce bacterial populations in insect mouthparts or its transmission efficiency to susceptible plants. After acquiring X. fastidiosa from infected plants, 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 suggested that natural chemical properties of Nemaguard xylem are not involved in resistance to X. fastidiosa and that future research should focus on identification of potential physical traits that prevent bacterial attachment, multiplication, or movement within the plant.