Introduction to Bacterial Spot in Tomatoes and Peppers
Bacterial spot is a serious bacterial disease that causes significant economic losses globally.
It is more commonly found in tropical and subtropical regions characterized by warm temperatures and high humidity.
The disease was first reported in the 1920s.
It primarily affects the production of tomatoes (Solanum lycopersicum) and peppers (Capsicum species).
The pathogen thrives in warm, humid climates.
It leads to the development of water-soaked spots on leaves, stems, and roots.
The disease results in defoliation and yield losses.
Causal organism of Bacterial spot of tomato and peppers
Bacterial spot of tomato and peppers is mainly caused by four species of Xanthomonas: Xanthomonas euvesicatoria, Xanthomonas perforans, Xanthomonas vesicatoria, and Xanthomonas gardneri.
These are gram-negative, rod-shaped bacteria that belong to the family Xanthomonadaceae.
They are characterized by yellow pigmentation, which results from the production of the carotenoid pigment Xanthomonadin.
Xanthomonadin helps protect the bacteria against UV light and oxidative stress.
Symptoms of Bacterial spot of tomato and peppers
Symptoms of bacterial spot appear on leaves, stems, and fruits of tomato and pepper plants.
The severity and appearance of symptoms are less consistent compared to other bacterial diseases.
Symptom expression depends on factors such as environmental conditions, host susceptibility, and the virulence of the infection.
The disease can be distinguished from other bacterial or fungal infections by the presence of Xanthomonas ooze from infected tissues.
This bacterial ooze is observable under a microscope, aiding in diagnosis.
Symptoms on leaves and stems
In tomatoes, early symptoms include water-soaked spots on leaves surrounded by a yellow halo.
As the infection progresses, these spots turn necrotic, becoming brown or black.
Severe infections lead to extensive leaf spotting and defoliation.
In peppers, irregular spots appear on leaves that enlarge and turn reddish brown with dark margins as the disease advances.
The leaf lesions in peppers are often angular, following the vein patterns.
On stems and petioles, dark, elongated lesions are observed.
These lesions may girdle the plant, temporarily disrupting nutrient supply.
Symptoms on fruits
On both tomatoes and peppers, initial symptoms on fruits include water-soaked lesions.
As the lesions mature, they enlarge and become raised.
These lesions may darken and crack, reducing the visual appeal and market quality of the fruit.
Damaged fruit tissue can become a site for secondary infections.
Infection of leaves lowers the rate of photosynthesis.
The host plant becomes more susceptible to environmental stress due to the infection.
Disease cycle of Bacterial spot of tomato and peppers
Overwintering
The bacteria overwinter in plant residues from infected plant debris left in the field after harvest, acting as a primary source of infection.
Seedborne transmission is another significant pathway for the spread of the disease.
Alternative host plants, especially weeds from the Solanaceae family, serve as reservoirs for the pathogen.
In some cases, the bacteria persist in the soil or on contaminated tools, equipment, and greenhouse surfaces.
Xanthomonas has the ability to form biofilms on plant surfaces or other substrates.
These biofilms enable the bacteria to survive adverse environmental conditions until the next growing season.
Primary Infection
During primary infection, the disease-causing bacteria establish themselves on new crops.
This occurs through viable inoculum that has overwintered from the previous infection cycle.
Bacteria enter the plant through natural openings such as stomata and hydathodes, or through wounds caused by mechanical damage or insect feeding.
Successful colonization requires optimal environmental conditions, particularly warm temperatures (25–30°C) and prolonged leaf wetness from rain, dew, or irrigation.
Once inside the host, the bacteria multiply rapidly, leading to symptoms such as water-soaked spots on leaves and fruits.
Colonization and Development of Symptoms
After infection, the bacteria multiply in the intercellular spaces within the host tissue.
They secrete enzymes and toxins that break down plant cell organelles.
This breakdown provides nutrients for the bacteria and suppresses the plant’s defense mechanisms, allowing the pathogen to establish itself.
As the infection progresses, disease symptoms become more severe, especially on leaves and fruits.
Necrotic spots turn brownish, and premature leaf fall may occur.
Infected tissues may ooze a bacterial exudate, which serves as an additional source of inoculum for further disease spread.
Secondary Infection and Spread
During secondary infection, the pathogen spreads from infected plants to healthy ones during the growing season.
The primary mode of spread is rain splash, where raindrops or irrigation water carry bacterial cells from lesions to nearby plants.
Wind-driven rain can disperse the bacteria over longer distances.
Human activities such as pruning, harvesting, or other fieldwork can spread the pathogen via contaminated hands, tools, or clothing.
In some cases, insects may act as vectors for localized spread of the bacteria.
Once introduced to a healthy plant, the bacteria enter through stomata, hydathodes, or wounds, reinitiating the infection cycle.
Factors Affecting Disease Development
Temperature – The optimum temperature for disease development is between 77°F and 86°F (25°C to 30°C).
Rainfall – High precipitation promotes disease development due to the rain splash mechanism that spreads the pathogen. Frequent rainfall and over-irrigation further enhance the risk of infection.
Humidity – High humidity and prolonged periods of relative humidity, especially when leaf wetness exceeds 4 hours, significantly favor disease development.
Management strategies for Bacterial spot of tomato and peppers
Cultural practices
Crop rotation – Rotate host plants with non-host crops like cereals and legumes for at least two years to reduce pathogen buildup.
Sanitation – Thoroughly disinfect field equipment using appropriate disinfectants to prevent disease spread.
Treat seeds with disinfectants such as sodium hypochlorite or hot water to minimize bacterial contamination.
Practice proper pruning and maintain adequate plant spacing to improve air circulation and reduce humidity.
Dispose of infected plant debris properly to limit sources of inoculum.
Prune lower leaves to reduce splash dispersal of bacteria.
Resistant varieties
Plant resistant tomato varieties like Arka-Abhed (H-397) and bell pepper varieties such as Nitro S 10 and Sailfish to significantly reduce disease severity.
Keep in mind that resistance may be specific to certain bacterial strains, so selecting cultivars appropriate for the local pathogen population is important.
Chemical control
Chemicals like copper-based bactericides combined with fungicides such as mancozeb are commonly used as preventive treatments.
These sprays create a protective barrier that helps prevent bacterial infection of new plant tissues.
However, Xanthomonas species are developing copper resistance genes, reducing the effectiveness of copper-based treatments in some regions.
Antibiotics such as streptomycin are also used to control the disease.
Excessive use of chemical treatments can be harmful to the environment, so their use should be carefully managed.
Biological control
Beneficial bacteria like Bacillus subtilis and Pseudomonas fluorescens can compete with the pathogen on plant surfaces, limiting its colonization and disease development.
Bacteriophages, viruses that specifically infect bacteria, are also effective in controlling bacterial spot disease.
Integrating cultural, biological, and chemical management practices provides a more effective strategy for controlling bacterial spot in tomatoes and peppers.
References
Jibrin, M. O., Timilsina, S., Minsavage, G. V., et al. (2022). Bacterial spot of tomato and pepper in Africa: Diversity, emergence of T5 race, and management. Frontiers in Microbiology, 13, 835647. https://doi.org/10.3389/fmicb.2022.835647
Plantwise Knowledge Bank. (n.d.). PlantwisePlus Knowledge Bank. Retrieved from https://plantwiseplusknowledgebank.org/doi/full/10.1079/pwkb.species.56981
Agrios, G. N. (2005). Plant diseases caused by prokaryotes: Bacteria and mollicutes. In Elsevier eBooks (pp. 615–703). https://doi.org/10.1016/b978-0-08-047378-9.50018-x
Vegetable Growers News. (2022, April 28). Bacterial leaf spot, copper resistance found in New Jersey tomato and pepper. Retrieved from https://vegetablegrowersnews.com/news/bacterial-leaf-spot-copper-resistance-found-in-new-jersey-tomato-pepper/
University of Kentucky Veggie Scout. (n.d.). Bacterial spot: Scouting guides for vegetable problems. Retrieved from https://veggiescout.ca.uky.edu/bacterial-spot
North Carolina State Extension. (n.d.). Bacterial spot of pepper and tomato. Retrieved from https://content.ces.ncsu.edu/bacterial-spot-of-pepper-and-tomato
University of Wisconsin Extension. (n.d.). Bacterial spot of tomato. Retrieved from https://hort.extension.wisc.edu/articles/bacterial-spot-of-tomato/
University of Minnesota Extension. (n.d.). Bacterial spot of tomato and pepper. Retrieved from https://extension.umn.edu/disease-management/bacterial-spot-tomato-and-pepper
Iowa State University Extension. (n.d.). Bacterial spot on pepper and tomatoes. Retrieved from https://yardandgarden.extension.iastate.edu/encyclopedia/bacterial-spot-pepper-and-tomatoes
Gardner, M. W., & Kendrick, J. (2020). Bacterial spot on tomato. Retrieved from https://www.semanticscholar.org/paper/Bacterial-spot-on-tomato.-Gardner-Kendrick/3c0ec7c5ccb893394498ee1de3862d4420675a33
Potnis, N., Timilsina, S., Strayer, A., Shantharaj, D., Barak, J. D., Paret, M. L., Vallad, G. E., & Jones, J. B. (2015). Bacterial spot of tomato and pepper: Diverse Xanthomonas species with a wide variety of virulence factors posing a worldwide challenge. Molecular Plant Pathology, 16(9), 907–920. https://doi.org/10.1111/mpp.12244
Reddy. (2023, February 11). Bacterial spot management in tomato: Disease symptoms, treatment, chemical, biological, natural, and organic controls. PestsDiseases. Retrieved from https://pestsdiseases.com/bacterial-spot-management-in-tomato-disease-symptoms-treatment-chemical-biological-natural-and-organic-control/
