Instructor Resources: Chapter Three Textbook Summaries

Common Plant-Pathogenic Bacterial Genera: Characteristics and Examples

Gram-negative

Agrobacterium
aerobes, peritrichous flagella, abundant EPS
Example: Agrobacterium tumefaciens

Erwinia and closely related newer genera: Brenneria, Pantoea, Pectobacterium
facultative anaerobes, peritrichous flagella; pectolytic enzymes (some species)
Examples: Erwinia amylovora (no pectolytic enzymes), Pectobacterium carotovorum (pectolytic enzymes)

Pseudomonas and closely related newer genera: Acidovorax, Burkholderia, Herbaspirillum, Ralstonia
aerobes, polar flagella; some fluoresce under ultraviolet light and chelate (remove) iron from their environment
Example: Pseudomonas syringae (many pathovars)

Xanthomonas
aerobes, one polar flagellum, yellow colonies; source of "xanthan gums"
Examples: Xanthomonas axonopodis pv. citri, X. axonopodis pv. vesicatoria

Gram-positive

Clavibacter
aerobes, irregularly shaped rods, nonmotile
Examples: Clavibacter michiganensis subsp. michiganensis, C. michiganensis subsp. sepedonicus

Streptomyces
aerobes, branched filaments, spore-forming (actinomycete)
Example: Streptomyces scabies

Mollicutes

phytoplasmas
aerobes, pleomorphic
Example: aster yellows phytoplasma

spiroplasmas
aerobes, helical shape
Example: Spiroplasma citri

Legume nodule bacteria

Nitrogen-fixing bacteria (e.g., Rhizobium and Bradyrhizobium) associated with legumes, such as soybean and alfalfa, are related to the crown gall bacterium and cause the formation of small galls or "nodules" on host roots. Although dependent on the plant for food, these bacteria are desirable because of their ability to "fix" atmospheric nitrogen into a form that the plant can absorb. Strains vary in their host plant preferences and in their efficiency as nitrogen-fixers. Legume seeds are inoculated with selected strains to reduce the need for nitrogen fertilizers.

Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria

Xylem-limited bacteria
• Symptoms: water-stress symptoms including wilt, stunting, scorch on leaves

• Pathogen: Xylella fastidiosa
• Shape: bacilliform (rod-shaped)
• Vectors: insects that feed in xylem, e.g., sharpshooters and spittlebugs
• Example diseases: bacterial leaf scorch of shade trees, citrus variegated chlorosis, Pierce's disease of grapevine

Phloem-limited bacteria
• Symptoms: yellowing, stunting, witches' brooms

Phytoplasmas
• Pathogens: no official names yet; Phytoplasma genus proposed
• Shape: mollicute (no cell wall); pleomorphic (variable shape)
• Vectors: leafhoppers
• Example diseases: aster yellows, lethal yellowing of coconut, X disease of cherry

Spiroplasmas
• Pathogens: Spiroplasma citri, S. kunkelii
• Shape: mollicute (no cell wall); helical shape
• Vectors: leafhoppers
• Example diseases: brittle root of horseradish, citrus stubborn, corn stunt, periwinkle spiroplasmosis

Phloem-colonizing walled bacteria
• Pathogens: Serratia marcescens, Liberobacter spp.
• Shape: bacilliform (rod-shaped)
• Vectors: psyllids, leafhoppers, one true bug
• Example diseases: citrus greening, cucurbit yellow vine disease

Xylem-Limited Fastidious Bacterium

Although cultured and named as recently as 1978, Xylella fastidiosa is the most common xylem-limited pathogen. Its entire genome was recently sequenced in Brazil, where it causes citrus variegated chlorosis. Brazil produces one-third of the world's oranges and half of the orange juice, so the disease is of great concern there. X. fastidiosa also causes a severe scorch disease of many landscape hardwood trees in the mid-Atlantic states. The pathogen was first identified and cultured in the study of Pierce's disease of grapevine. Although this disease has been a problem in California for decades, the 1989 introduction of a new insect vector, the glassy-winged sharpshooter, has threatened the wine industry of the entire state. This vector feeds on more than 133 host plants. Grape plants with Pierce's disease must be eradicated to prevent spread. Studies of the gene sequence of X. fastidiosa have already helped to explain how the bacterium obtains food from xylem fluid and xylem cells and how it adheres to xylem cells and to the stylets of vectors. It is hoped that new management strategies also will be identified.

Management Strategies for Bacterial Diseases

Exclusion
Impose quarantines (local and international).
Grow plants in dry environments.
Plant pathogen-free seed or stock.

Eradication
Rotate to nonhosts.
Rogue weed hosts.
Destroy infested plant debris.
Use heat treatment of seeds or propagative material.
Apply antibiotics.
Apply insecticides (to kill infective vectors).

Protection
Apply copper chemicals.
Apply antibiotics.
Apply bacterial antagonists.
Minimize leaf wetness.
Plant resistant hosts.