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Xylella fastidiosa Wells et al. 1987 - (citrus variegated chlorosis)
The species is a Gram-negative bacterium which infects the xylem of certain plants, causing the diseases citrus variegated chlorosis, Pierce's disease of grapes and leaf scorch of almond, coffee, pear, pecan and other trees. It is wide-spread in tropical and subtropical parts of the Americas and has been also reported from some other regions. E.g. X. fastidiosa subsp. pauca has been reported infecting olive trees in Italy since 2010 (see Martelli et al., 2016). The different forms infecting specific host plants often have a more restricted distribution, like Pierce's disease is restricted to the south-western and south-eastern parts of North America. Symptoms include scorching of the leaves, yellowing, stunting, dwarfing, die-back and the development of small fruits, depending on the host plant. The symptoms often resemble drought effects. Leaf scorch, the main symptom, affects the marginal parts of the leaves which dry off and are separated from the green leaf areas by a yellow margin.
|• English:||citrus variegated chlorosis
pecan bacterial leaf scorch
plum leaf scald
phony peach disease
Pierce's disease of grapes
almond leaf scorch
leaf scorch of pecan
leaf scald of plum
leaf scorch of almond
|• Français:||maladie de Pierce du raisin
échaudure bactérienne des feuilles
|• Português:||clorose variegada dos citros|
The disease is transmitted by a variety of xylem feeding leafhoppers (Cicadellidae) and also by some species of froghoppers (Cercopidae). For a review on the insect transmission of this disease agent see Overall and Rebek (2017). The bacteria attach themselves to the foregut wall of the leafhopper vector and are passed back into the xylem with the saliva of the vector. Management of the disease has proven difficult. Providing plants with enough moisture, e.g. through mulching, can delay the disease development. Also using resistant cultivars has shown some promise. Since many regions are free from the disease, quarantine procedures are of particular importance.
The bactrium can be cultured on artificial media but grows very slowly. It has a rod-shaped form, about 1-3 µm long, and has pili (hair-like extensions) on its surface. While the bacterium can easily move upwards with the xylem fluid of the plant, the pili also enable the bacterium to move slowly downwards.
The following subspecies have been described:
• Xylella fastidiosa subsp. fastidiosa - infects grapevine
• Xylella fastidiosa subsp. multiplex - infects peach, elm, plum, sycamore, almond, pecan and olive
• Xylella fastidiosa subsp. morus - infects mulberry
• Xylella fastidiosa subsp. pauca - infects citrus, coffee and olive
• Xylella fastidiosa subsp. piercei - infects grape, alfalfa, almond and maple
• Xylella fastidiosa subsp. sandyi - infects oleander
• Xylella fastidiosa subsp. tashke - causes leaf scorch on Chitalpa tashkentensis trees
For details see the respective page in BugwoodWiki.