HPIPM:Bacterial Spot

Author: Howard F. Schwartz and David H. Gent

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Xanthomonadales
Family	Xanthomonadaceae
Genus	Xanthomonas

Scientific Name

Xanthomonas vesicatoria

Scientific Name Synonyms

Xanthomonas campestris pv. vesicatoria

Xanthomonas vesicatoria pv. vesicatoria

Xanthomonas axonopodis pv. vesicatoria

Phytomonas vesicatoria

Common Name

bacterial spot

Identification and Life Cycle

Bacterial spot is caused by two species of bacteria, Xanthomonas axonopodis pv. vesicatoria and X. vesicatoria. Bacterial spot can be damaging to both pepper and tomato in the High Plains during warm (75 to 86ºF), humid, rainy weather. Bacterial spot does not affect eggplant. Bacteria are introduced onto plants by planting contaminated seed and transplants, splashing rain or irrigation water, aerosols, or on contaminated equipment. The bacteria multiply on leaves to form large populations before penetrating through natural openings or wounds created by wind-blown sand, insect feeding, or mechanical injury. The bacterial spot pathogens survive between susceptible crops in and on weed, volunteer plants, infested crop debris, culls, and contaminated seed and transplants.

Plant Response and Damage

The bacterial spot pathogens can infect all aboveground plant parts. Disease symptoms begin as small, brown, water-soaked lesions, which turn brown with necrotic centers. Leaf lesions are generally sunken on the upper surface, but are raised on lower surfaces. Lesions are rarely larger than 0.12 inch, but when conditions are favorable for disease, lesions coalesce and from large blighted areas. Infected leaves turn yellow and drop prematurely. Fruit lesions begin as small (0.04 inch) circular green spots, but turn brown and become cracked and roughened with age. Bacterial spot reduces both yield and fruit quality. Infected fruit is generally unmarketable.

Biological Control

Bacteriophage, viruses that attacked bacteria, control bacterial spot but must be applied at dusk at least twice weekly to be effective. Nonpathogenic Xanthomonas spp. provide some control of bacterial spot.

Cultural Control

Plant only high quality seed and transplants free from the bacterial spot pathogens. Hot water treatments can reduce seed contamination, but may reduce germination. Practice a three-year or longer crop rotation between susceptible crops; do not plant tomato and pepper consecutively in a field. Eliminate weeds, volunteers, crop debris, and cull piles that can serve as inoculum sources. Avoid reuse of irrigation tail water and overhead irrigation if possible. Resistant varieties are available, but should be chosen carefully to match the most prevalent pathogenic races of the pathogen present. Eleven pathogenic races are known to occur; no single resistance gene will provide resistance to all pathogenic races.

Chemical Control

Resistance to copper bactericides and streptomycin are widespread in the bacterial spot pathogens. Tank-mixing copper bactericides with EBDC fungicides such as maneb can provide some suppression of copper-tolerant strains of X. axonopodis pv. vesicatoria. The plant activator Actigard can provide effective control of both copper sensitive and tolerant strains of the pathogen, but can reduce yields in the absence of disease. Chemical controls are most effective when combined with as many cultural and biological controls as possible.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.