Recent papers on Xylella fastidiosa
Several papers on Xylella fastidiosa (EPPO A1 list) and its associated diseases were recently published.
A review of the current knowledge about this complex bacterium has been published by Hopkins & Purcell (2002). In particular, it gives details on Pierce’s disease in California and the epidemics which has followed the introduction of an efficient vector Homalodisca coagulata. A table also summarizes crop and forest tree diseases confirmed or suspected* (Koch’s postulates not confirmed) to be caused by X. fastidiosa with their distribution.
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Plant diseases
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Reported occurrence
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Grape strains
Pierce’s disease
Alfalfa dwarf
Almond leaf scorch
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North and Central Americas, Peru, Kosovo1
California (US)
California (US), Argentina
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Peach-plum strains
Phony peach
Plum leaf scald (US)
Citrus variegated chlorosis
Coffee leaf scorch
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Southeastern USA
Southeastern USA
Brazil, Argentina (known there as pecosita)
Brazil
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Unknown cross-infectivity strains
Oak leaf scorch (related to peach strains)
Maple leaf scald
Elm leaf scorch
Sycamore leaf scorch
Mulberry leaf scorch
Plum leaf scald (South America)
Periwinkle wilt
Pear leaf scorch*
Pecan leaf scorch
Oleander leaf scorch
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Eastern USA
Eastern USA
Eastern USA
Eastern USA
Eastern USA
Paraguay, Brazil
Florida (US)
Taiwan
Louisiana (US)
California, Florida (US)
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1) see EPPO RS 98/006 and 98/157: this record is considered as dubious. Material came from Cermjan, Kosovo (near Albanian border) but isolations and further studies were done in USA. Lack of detailed information on the origin of the material and of further study in the area concerned leaves considerable doubt about the nature of the original material.
Host range of different strains
Strains of X. fastidiosa isolated from diseased citrus and coffee in Brazil can incite symptoms of Pierce’s disease after artificial inoculation into 7 grapevine cultivars grown in glasshouse conditions. These cultivars are commercially grown in Brazil and California. Although this was observed under artificial conditions, this result appeared slightly surprising because earlier work had showed that strains from grapevine and citrus were the most distantly related within X. fastidiosa (Li et al., 2002).
Vectors
Studies done in Florida (US) showed that Oncometiopa nigricans (Homoptera: Cicadellidae) is a vector of citrus variegated chlorosis disease. This insect is commonly found feeding on citrus in Florida. Although X. fastidiosa is present in Florida, citrus variegated chlorosis does not occur there. But the presence of a suitable vector adds to the threat this disease represents for the citrus industry (Brlansky et al, 2002).
Diagnostics
A real-time PCR technique has been developed by Schaad et al. (2002) to detect X. fastidiosa within 1-2 hours in the field (with a portable thermocycler). This technique can be used to detect the bacterium in asymptomatic grapevines, either on naturally exuding sap (as soon as vines are coming out of dormancy in early spring) or on wood chips (during dormancy). This technique could be useful in areas where the disease occurs to map field infections and destroy infected plants early in the season to prevent any further spread.
Taxonomy
As X. fastidiosa is a heterogeneous species, Schaad et al. (2003) proposed new taxa on the basis of DNA homology and ITS sequencing. 26 strains from 10 host plants were compared and 3 new taxa and names are now proposed:
- taxon A (grape, alfalfa, maple, and 2 almond strains) X. fastidiosa subsp. piercei
- taxon B (peach, elm, plum, wild grape, 1 almond, and sycamore strains) X. fastidiosa subsp. agglomeri
- taxon C (citrus strains) X. fastidiosa subsp. idiotraposa
Sources
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Brlansky, R.H.; Damsteegt, V.D.; Hartung, J.S. (2002) Transmission of the citrus variegated chlorosis bacterium Xylella fastidiosa with the sharpshooter Oncometopia nigricans.
Plant Disease, 86(11), 1237-1239.
Hopkins, D.L.; Purcell, A.H. (2002) Xylella fastidiosa: cause of Pierce’s disease of grapevine and other emergent diseases. Plant Disease, 86(10), 1056-1066.
Li, W.B.; Zhou, C.H.; Pria, W.D. Jr; Teixeira, D.C., Miranda, V.S.; Pereira, E.O.; Ayres, A.J.; He, C.X.; Costa, P.I.; Hartung, J.S. (2002) Citrus and coffee strains of Xylella fastidiosa induce Pierce’s disease in grapevine. Plant Disease, 86(11), 1206-1210.
Schaad, N.W.; Opgenorth, D.; Gaush, P. (2002) Real-time polymerase chain reaction for one-hour on-site diagnosis of Pierce’s disease of grape in early season asymptomatic vines.
Phytopathology, 92(7), 721-728
Schaad, N.W.; Postnikova, E.; Fatmi, M.; Lacy, G.H.; Chang, C.J. (2003) Xylella fastidiosa taxonomy. Abstract of a paper presented at the APS Annual Meeting (Charlotte, US, 2003-08-09/13). Phytopathology 93(6), supplement, S76.
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