Xylella fastidiosa: an examination of a re-emerging
In memory of Professor Bruce Kirkpatrick, respected scholar, mentor and friend. You will always be missed.
†, BARBARA BLANCO-ULATE
Department of Plant Pathology and Microbiology, University of California, Riverside, CA 92521, USA
Department of Plant Sciences, University of California, Davis, CA 95616, USA
Department of Viticulture and Enology, University of California, Davis, CA 95616, USA
Xylella fastidiosa is a Gram-negative bacterial
plant pathogen with an extremely wide host range. This species
has recently been resolved into subspecies that correlate with
host specificity. This review focuses on the status of X. fastidiosa
pathogenic associations in plant hosts in which the bacterium is
either endemic or has been recently introduced. Plant diseases
associated with X. fastidiosa have been documented for over a
century, and much about what is known in the context of host–
pathogen interactions is based on these hosts, such as grape and
citrus, in which this pathogen has been well described. Recent
attention has focused on newly emerging X. fastidiosa diseases,
such as in olives.
Bacteria; Gammaproteobacteria; family Xan-
thomonadaceae; genus Xylella; species fastidiosa.
(0.25–0.35 3 0.9–3.5 lm), non-flagellate, motile via Type IV
pili-mediated twitching, fastidious.
Xylella fastidiosa has a broad host range that
includes ornamental, ecological and agricultural plants belonging
to over 300 different species in 63 different families. To date,
X. fastidiosa has been found to be pathogenic in over 100 plant
species. In addition, it can establish non-symptomatic associa-
tions with many plants as a commensal endophyte. Here, we list
the four distinct subspecies of X. fastidiosa and some of the agri-
culturally relevant diseases caused by them: X. fastidiosa ssp.
fastidiosa causes Pierce’s disease (PD) of grapevine (Vitis vinif-
era); X. fastidiosa ssp. multiplex causes almond leaf scorch (ALS)
and diseases on other nut and shade tree crops; X. fastidiosa
ssp. pauca causes citrus variegated chlorosis (CVC) (Citrus spp.),
coffee leaf scorch and olive quick decline syndrome (OQDS) (Olea
europaea); X. fastidiosa ssp. sandyi causes oleander leaf scorch
(OLS) (Nerium oleander). Significant host specificity seemingly
exists for some of the subspecies, although this could be a result
of technical biases based on the limited number of plants tested,
whereas some subspecies are not as stringent in their host range
and can infect several plant hosts.
Most X. fastidiosa-related diseases
appear as marginal leaf necrosis and scorching of the leaves. In
thecaseofPD,X. fastidiosa can also cause desiccation of berries
(termed ‘raisining’), irregular periderm development and abnormal
abscission of petioles. In olive trees affected with OQDS, leaves
exhibit marginal necrosis and defoliation, and overall tree decline
occurs. Plants with ALS and OLS also exhibit the characteristic leaf
scorch symptoms. Not all X. fastidiosa-related diseases exhibit the
typical leaf scorch symptoms. These include CVC and Phony Peach
disease, amongst others. In the case of CVC, symptoms include
foliar wilt and interveinal chlorosis on the upper surfaces of the
leaves (similar to zinc deficiency), which correspond to necrotic,
gum-like regions on the undersides of the leaves. Additional symp-
toms of CVC include defoliation, dieback and hardening of fruits.
Plants infected with Phony Peach disease exhibit a denser, more
compact canopy (as a result of shortened internodes, darker green
leaves and delayed leaf senescence), premature bloom and
reduced fruit size. Some occlusions occur in the xylem vessels, but
there are no foliar wilting, chlorosis or necrosis symptoms .
Keywords: citrus, grapevine, olive, Pierce’s disease, tyloses,
In the late 1800s, Newton B. Pierce described a severe grapevine
disease occurring in Anaheim, California. This disease was initially
called Anaheim disease (Pierce, 1892) and was first attributed to
a viral infection because: (i) no causal microorganism could be
*Correspondence: Email: email@example.com
†These authors contributed equally to this work.
2017 BSPP AND JOHN WILEY & SONS LTD
MOLECULAR PLANT PATHOLOGY (2018) 19(4), 786–800 DOI: 10.1111/mpp.12585