Leaf scorch symptoms are not correlated with bacterial populations during Pierce’s disease
G. A. Gambetta1, J. Fei2, T. L. Rost2 and M. A. Matthews1,*
1Department of Viticulture and Enology, University of California, Davis, CA 95616, USA
2Section of Plant Biology, University of California, Davis, CA 95616, USA
* To whom correspondence should be addressed. E-mail: firstname.lastname@example.org
Received 6 August 2007; Revised 25 September 2007 Accepted 27 September 2007
Xylella fastidiosa (Xf) is a xylem-limited bacterium that lives as a harmless endophyte in most plant species but is pathogenic in several agriculturally important crops such as coffee, citrus, and grapevine (Vitis vinifera L.). In susceptible cultivars of grapevine, Xf infection results in leaf scorch, premature leaf senescence, and eventually vine death; a suite of symptoms collectively referred to as Pierce’s disease. A qPCR assay was developed to determine bacterial concentrations in planta and these concentrations were related to the development of leaf-scorch symptoms. The concentration of Xf in leaves of experimental grapevines grown in the greenhouse was similar to the concentration of Xf in leaves of naturally infected plants in the field. The distribution of Xf was patchy within and among leaves. Some whole leaves exhibited severe leaf-scorch symptoms in the absence of high concentrations of Xf. Despite a highly sensitive assay and a range of Xf concentrations from 102 to 109 cells g–1 fresh weight, no clear relationship between bacterial population and symptom development during Pierce’s disease was revealed. Thus, high and localized concentrations of Xf are not necessary for the formation of leaf-scorch symptoms. The results are interpreted as being consistent with an atiology that involves a systemic plant response.
Key words: Bacterial wilt, disease resistance, pathogenesis, water deficits
Journal of Experimental Botany, doi:10.1093/jxb/erm260 . This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/).