Agriculture Reference
In-Depth Information
XYLELLA FASTIDIOSA
- BIOSECURITY THREAT
The xylem-limited bacterium,
Xylella fastidiosa
, causes
disease in more than 100 plant species, including
perennial fruit crops and forest trees.
The bacterium occurs in sub-tropical and tropical regions
of North, Central and South America, with limited
distribution outside these areas. The main factors
limiting the pathogen's distribution are cold winter
temperatures and suitable, overwintering insect vectors.
The organism exists as various strains and subspecies,
which differ in several characteristics, including host
range. Three subspecies are currently recognised:
X. fastidiosa
subspp.
piercei
Fig 2.31 Vineyard infected with Pierce's disease showing areas of
lost vines. Inset: symptoms on a vine cane.
X. fastidiosa
subspp.
multiplex
X. fastidiosa
subspp.
pauca
The pathogen
X. fastidiosa
is not present in Australia and
is subject to quarantine.
Symptoms
Symptom development depends on the rate and extent
of colonisation of the xylem vessels of the host. The
symptoms produced are usually those associated with
water stress.
Symptoms typically include leaf scorch, veinal chlorosis,
wilt and dwarfing. In grape, the disease is also
commonly associated with leaf scorch, defoliation,
uneven hardening of shoots, shoot dwarfing and drying
of fruit clusters.
Affected citrus trees have mottled leaves on one or more
branches and may be stunted with twig dieback. The
undersides of leaves often develop brown, gummy lesions.
Fruit is small and hard and ripens early.
Fig 2.30 A severe infection of Pierce's disease on grape leaves.
Left: chardonnay; right: merlot.
Importance and hosts
Diseases caused by
X. fastidiosa
include phoney peach
disease, citrus variegated chlorosis (CVC), Pierce's
disease of grapevines, coffee leaf scorch, plum leaf scald,
and leaf scorch of almond, coffee, oleander, mulberry
and pear. CVC disease has been responsible for severe
losses of citrus in Brazil and Argentina. Pierce's disease
has caused extensive losses of grapevines in Florida and
California in the past decade. Pierce's disease is caused
by the subspecies
piercei
of the pathogen, which also
infects lucerne, maple and almond.
CVC is caused by the subspecies
pauca,
which also causes
coffee leaf scorch disease. The subspecies
multiplex
has a
wide range of hosts, including peach, plum, almond, tree
species and herbaceous weeds.
There is evidence that cross-infection between the
subspecies can occur; for example, the CVC and coffee
subspecies has infected grape in transmission experiments.
Fig 2.32 Leaf symptoms of citrus variegated chlorosis (CVC).
