XYLEM-INHABITING FASTIDIOUS BACTERIA

 XYLEM-INHABITING FASTIDIOUS BACTERIA

• Fastidious xylem-inhabiting bacteria are rod-shaped cells 0.2 to 0.5 micrometers in diameter by 1 to 4 micrometers in length, surrounded by a cell membrane and wall. 

• They lack flagella and are usually undulating (having a smoothly rising and falling form or outline) or rippled. Most fastidious xylem bacteria are gram negative, with Xylella fastidiosa being the first plant pathogenic bacterium with a completely sequenced genome. 

• Only xylem-inhabiting bacteria causing sugarcane ratoon and Bermuda grass stunting are gram positive and belong to the genus Clavibacter. 

• They can grow on complex nutrient media and produce tiny colonies(1–2 millimeter), but cannot grow on conventional bacteriological media.

• Gram-negative xylem-inhabiting fastidious bacteria are transmitted by xylem-feeding insects like sharpshooter leafhoppers and spittlebugs, 

• which can acquire and transmit the bacteria in less than two hours. 

• Carrier adult insects can transmit the bacteria for life but do not pass them on to progeny. 

• No insect vector is known for gram-positive xylem-inhabiting fastidious bacteria, 

• but one can be transmitted mechanically by cutting implements during harvest. 

• Symptoms of fastidious xylem-inhabiting bacteria often consist of marginal leaf necrosis, stunting, and general decline and reduced yields. However, some diseases, like phony peach, do not cause marginal leaf necrosis, and the only diagnostic symptom is stunting and an internal discoloration of the stalk.

• Fastidious vascular bacteria are sensitive to several antibiotics but have proven impractical in field treatment. 

• High temperatures can also be used to treat fastidious vascular bacteria, such as heat treatment of entire plants or propagative organs. 

• Important plant diseases caused by fastidious xylem-limited, gram-negative bacteria include 

1. Pierce's disease of grape(Xylella Fastidiosa), 

2. citrus variegated chlorosis, 

3. phony peach disease, 

4. almond leaf scorch, and 

5. plum leaf scale.

• The also very important ratoon stunting disease of sugarcane is caused by the xylem-limited, gram-positive bacterium Leifsonia xyli (formerly Clavibacter xyli subsp. xyli).

• Heat treatment of plants or propagative organs, either in water at 45-50°C for 2-3 hours or in hot air at 50-58°C for 4-6 hours, has successfully treated grapevines from Pierce's disease and sugarcane from ratoon stunting disease.

References-Agrios, G. N. (2005). Plant pathology. Academic Press.

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