Download Xylella fastidiosa Wells et al. 1987 Dutch name: n/a English name

Xylella fastidiosa Wells et al. 1987
Dutch name: n/a
English name:
Leaf scorch diseases on oleander, olive, almond, coffee, pecan, pear,
grapevine, amenity tree species, etc.; citrus variegated chlorosis;
grapevine Pierce’s disease; plum leaf scald; peach phony disease;
dwarf lucerne.
Status
Xylella fastidiosa (hereafter abbreviated as: “Xf ”)has a quarantine status within the EU, Category
IAI. This means that:
- It is a harmful organism that can occur in multiple plant species or plant products
- Absent in the EU (as far as is known) and poses risks to the entire community, hence it is controlled for
in imports.
Systematics
It concerns a Gram-negative, non-spore forming, rod-shaped bacterium.
Host plants
Economically (and/or socially) important host plants are:
Citrus latifolia (tahiti lime), Citrus reticulata (tangerine), Citrus reticulata x paradisi (tangelo), Citrus
sinensis (navel orange);
Vitis rupestris (sand grape), Vitis vinifera (grapevine) ;
Prunus angustifolia (mountain cherry tree), Prunus dulcis (almond), Prunus persica (peach), Prunus
salicina (Japanese plum) ;
Pyrus sp. (pear); Ulmus sp. (elm) Nerium oleander (oleander);
Coffea sp. (coffee); Medicago sativa (lucerne), Morus alba (white mulberry),
Acer saccharum, Carya ilinoinensis, Liquidambar styraciflua, Platanus occidentalis.
Additionally, there are numerous wild and/or economically less valuable host plants, such as: Brachiaria
sp. (signal grass), Cynodon sp., Cyperus sp., Digitaria sp., Echinochloa frumentacea (Japanese millet),
Fragaria vesca (wild strawberry), Lolium sp. (tufted grass), Paspalum dilatatum, Passiflora foetida,
Quercus rubra, Rubus sp., Trifolium sp. (clovers), Vinca minor, and others.
There are also host plants that show no symptoms. Sometimes they may show very slight stunted
growth.
Geographical distribution
Xf has been reported to occur in:
Asia: [China (Taiwan), Turkey]; Africa: [Morocco (not confirmed)]; North America: [Canada (Ontario),
Mexico, USA (27 states)]; Central America: [Costa Rica]; South America: [Argentina, Brazil (9 states),
Paraguay, Venezuela]. Europe: found at a single location in Italy in 2013, measures were taken.
Biology
Within the Xf genus, there are various strains that associate with just one host plant or that have a
preference for a certain host plant. For example, it emerged that Brazilian Xf strains from citrus and
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coffee were pathogenic on grapevine. However, no natural infection on grapevine has been found in
Brazil. To date, Xf strains from grape are not known to be pathogenic for peach and Xf strains from
peach not for grape. Studies into the association of strains with host plants are few and incomplete.
According to current insights, 4 groups can be distinguished that can possibly be defined
taxonomically to a sub-species level. However, there is no full consensus on this matter yet.
Xf principally spreads across large distances by means of infected plant material and (vegetative)
propagation materials. Across shorter distances (on site), Xf mainly spreads naturally by various vector
insects. The familiar vector insects generally cannot travel far on their own, but borne by the wind they
can transmit Xf infections over large distances.
Virtually all species of sucking insects that feed on sap of the Xylem (order of Hemiptera, families:
Cicadellidae and Cercopidae) are potential vectors for Xf. Among them, the “Glassy-winged sharpshooter”
(Homalodisca coagulata, a leafhopper) is capable of transmitting the bacterium. Uptake by insects is very
fast and effective, within 2 hours. The bacterium sticks to the mouth parts and can be transmitted to a next
plant directly upon subsequent feeding. Xf propagates within the insect but does not circulate in the
hemolymph. No latency period is required for the transmission of the bacterium. A study conducted in
the USA showed that 75 out of 100 plants examined can function as (intermediate) host plant for Xfbearing vector insects. The establishment of the bacterium in natural vegetation (potential reservoir
for Xf) therefore constitutes a major risk. Following infection of a plant, Xf colonises the Xylem (wood)
tissue of roots, stems and leaves.
During severe and lengthy winters, infections may disappear as a result of the bacterium dying in the
dormant plant tissue. In moderate areas with regular frost periods, the bacterium can survive in plant
material that was infected early in the season. No indications have been found of disease transmission
through contact between grape branches in moderate areas with regular frost; probably because the
infection sets in too late.
Symptoms / damage
Citrus, “Variegated chlorosis”. Young affected trees develop stunted and distorted growth. Twigs
and branches can wilt and the crown thins out. Leaf symptoms: interveinal chlorosis occurs, which
strongly resembles a zinc deficiency. As the leaf ages, small, slightly raised, brown and necrotic spots
emerge on the underside of the leaves, directly beneath the chlorotic spots.
Grapevine, “Pierce’s disease”. Sudden partial brown dying off of the leaves, followed by yellow or
red discolouration of adjacent leaf tissue. The desiccation then spreads across the leaf surface. The leaf
then drops off, while the live stalk remains intact. Brown and green spots emerge on the stalk as a result
of irregular ageing.
Peach, “Phony disease”. Sick young shoots show stunted and retarded growth with greener and
denser leaf growth. Side branches grow horizontally or downward, giving the tree a compact and round
appearance.
“Leaf scorch” diseases. Brown discolouration of the leaf edges and leaf ends. In an early stage of the
infection, the leaf can show a slight yellowing.
An assessment of symptoms does not yield definitive information about plants’ health situation during
transportation or trade. Inconspicuous or young infections can go unnoticed. Zinc deficiency can cause chlorosis
symptoms in grapevine and citrus that resemble Xf, but the patterns are distinguishable by a trained observer.
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In viticulture, a plant pathogenic fungus can cause symptoms that resemble grapevine’s “Pierce’s
disease”. This “Esca” disease occurs in France early on in the season (June/July), after a number of hot
days, while the symptoms of “Pierce’s disease” emerge after grapes have begun to turn colour.
Indications for the presence of vector insects are a whitish powder on the plant surfaces. This is the
result of watery excrement. Due to the large quantities of Xylem sap consumed daily by the insects
(hundreds of times their own body volume), they also eject large quantities of moist excrement. This
dries up to form whitish powder.
Observation / sampling period
Container or pot plants kept under warm conditions can, in principle, show symptoms at any time of the year, but particularly after a
few weeks of rapid growth.
In general, one should take into account that a period of plant growth and a warm period for the development of Xf
should have preceded the moment of symptom assessment. Particularly for grapevines, the best observation time is late
in the season, after the grapes have begun to turn colour.
Illustrations
Citrus leaf with interveinal chlorosis and slightly raised brown necrotic spots on the underside.
Dropped leaves (grapevine)
Partial dying (leaf scorch)
Phony disease of peach
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Upper left: Coffea Arabica with yellowing and leaf scorch
Right: Olive with leaf scorch
Below, all three: leaf scorch in oleander
Olive trees with “quick decline” and leaf scorch
© 2014 Wageningen. Het auteursrecht van deze publicatie berust bij de Nederlandse Voedsel en Warenautoriteit
Niets uit deze uitgave mag zonder voorafgaande toestemming van genoemde dienst op enigerlei wijze worden vermeerderd, openbaar gemaakt of
voor commerciële doeleinden worden gebruikt.
Versie 3
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Pagina 4 van 4