1 Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, China,

2 Floral and Nursery Plants Research Unit, Agricultural Research Service, United States Department of Agriculture,
Beltsville, Maryland, United States of America,

3 Molecular Plant Pathology Laboratory, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland, United States of America,
4 Istituto Agronomico Mediterraneo, Via Ceglie 9, 70010, Valenzano (BA), Italy

Abstract

Xylella fastidiosa causes bacterial leaf scorch in many landscape trees including elm, oak,
sycamore and mulberry, but methods for specific identification of a particular tree host species-
limited strain or differentiation of tree-specific strains are lacking. It is also unknown
whether a particular landscape tree-infecting X. fastidiosa strain is capable of infecting multiple
landscape tree species in an urban environment. We developed two PCR primers specific
for mulberry-infecting strains of X. fastidiosa based on the nucleotide sequence of a
unique open reading frame identified only in mulberry-infecting strains among all the North
and South American strains of X. fastidiosa sequenced to date. PCR using the primers allowed
for detection and identification of mulberry-infecting X. fastidiosa strains in cultures
and in samples collected from naturally infected mulberry trees. In addition, no mixed infections
with or non-specific detections of the mulberry-infecting strains of X. fastidiosa were
found in naturally X. fastidiosa-infected oak, elm and sycamore trees growing in the same
region where naturally infected mulberry trees were grown. This genotype-specific PCR
assay will be valuable for disease diagnosis, studies of strain-specific infections in insects
and plant hosts, and management of diseases caused by X. fastidiosa. Unexpectedly but interestingly,
the unique open reading frame conserved in the mulberry-infecting strains in the
U. S. was also identified in the recently sequenced olive-associated strain CoDiRO isolated
in Italy. When the primer set was tested against naturally infected olive plant samples collected
in Italy, it allowed for detection of olive-associated strains of X. fastidiosa in Italy. This
PCR assay, therefore, will also be useful for detection and identification of the Italian group
of X. fastidiosa strains to aid understanding of the occurrence, evolution and biology of this
new group of X. fastidiosa strains.