Seasonal Behavior of Xylella fastidiosa Causing Almond Leaf Scorch Under Field Conditions and Detection of the Bacteria by Means of Array-PCR

Authors: Chen, Jianchi; LIVINGSTON, S. - UC DAVIS; Civerolo, Edwin; KIRKPATRICK,, B.C. - UC DAVIS

Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Proceedings
Publication Acceptance Date: 12/7/2007
Publication Date: 12/12/2007
Citation: Chen, J., Livingston, S., Civerolo, E.L., Kirkpatrick,, B. 2007. Seasonal Behavior of Xylella fastidiosa Causing Almond Leaf Scorch Under Field Conditions and Detection of the Bacteria by Means of Array-PCR. CDFA Pierce's Disease Control Program Research Symposium. p.127-129.

Interpretive Summary: Almond leaf scorch disease (ALSD) is caused by Xylella fastidiosa and has re-emerged as a serious threat to almond industry in the San Joaquin Valley of California. In addition to laboratory characterizations, this project focused on studying how X. fastidosa behaved in plant hosts under field conditions. Two almond orchards in Fresno County, California, were selected for the experiment. One ALSD tree and one non-ALSD tree were identified from each orchard. By analyzing samples collected from these trees, we found that in 2007, the earliest occurrence of leaf scorching symptoms was in July, one month later than that in 2006. In both 2006 and 2007, PCR detected the presence of X. fastidiosa in plant tissue one month ahead of symptom expression. X. fastidiosa was not evenly distributed within an almond tree such that limited sampling could result in a false negative assay. Using a highly sensitive molecular marker, we detected variation among X. fastidiosa strains within a single tree. We also developed an array-PCR protocol with improved accuracy of X. fastidiosa detection.

Technical Abstract: Diseases caused by Xylella fastidiosa have re-emerged as a serious threat to several economically important crops, such as grape and almond, in the San Joaquin Valley of California. Knowledge of bacterial behavior in plant hosts under field condition is important for disease control. This research characterized populations of X. fastidiosa almond leaf scorch (ALSD) strains in almond orchards. In 2006, two almond orchards were selected based on known history of ALSD. One ALSD tree and one non-ALSD tree were identified from each orchard. The branch pattern of each tree was mapped. Samples were taken every month from each scaffold at both distal and proximal positions. We previously reported population dynamics of ALSD bacteria and the pattern of leaf scorching symptoms in 2006. We continued to study disease development in these same trees in 2007. Presence of leaf scorch symptoms were monitored, bacteria were isolated from different regions of the trees, and genotypes determined by PCR analysis of both conserved house keeping genes and the hypervariable pspB locus. In 2007, the earliest occurrence of leaf scorching symptoms was in July, almost one month later than in 2006. In both 2006 and 2007, PCR detected X. fastidiosa in plant tissue one month ahead of symptom development. PCR was slightly more sensitive than cultivation method for early bacterial detection. However, uneven bacterial distribution and random sampling errors may contribute to the differences among the assays. Correlation between cultivation and PCR detection was over 90%. Analyses of tandem repeat numbers (TRNs) at the pspB locus showed that two TRN genotypes coexisted in the same almond tree, although one genotype predominated. To reduce PCR error that result from large volume sample processing, we developed an array-PCR protocol using primers from eight housekeeping genes. This array-PCR significantly improved detection accuracy.