Bacterial spot is one of the serious bacterial diseases resulting in significant economic losses around the world. It is more prevalent in Tropical and Subtropical regions with warm temperatures and high humidity.
It was reported for the first time in the 1920s. The disease mainly affects the production of tomatoes (Solanum lycopersicum) and peppers (Capsicum sp.). The pathogen thrives well in warm, humid climates, causing water-soaked spots on leaves, stems, and roots and defoliation and yield losses.
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Causal organism of Bacterial spot of tomato and peppers
The disease is caused by mainly four Xanthomonas sp. which includes Xanthomonas euvesicatoria, Xanthomonas perforans, Xanthomonas vesicatoria, Xanthomonas gardneri . These are gram-negative, rod-shaped bacteria belonging to the family Xanthomonadaceae. They are characterized by their yellow pigmentation due to the production of the carotenoid pigment Xanthomonadin which protects against UV light and oxidative stress.
Symptoms of Bacterial spot of tomato and peppers
Symptoms of bacterial spots appear on leaves, stems, and fruits. The extent of symptomatic expression is less consistent than that of other bacterial diseases and depends upon certain factors such as environmental conditions, host susceptibility, and the virulence of the infection. The disease can be distinguished from other bacterial or fungal diseases, as Xanthomonas oozes from infected tissue that can be easily observed under a microscope.
Symptoms on leaves and stems
In tomatoes, during the early stages, water-soaked spots surrounded by a yellow halo are seen on leaves. During advanced stages of infection, these spots become necrotic, brown, or black. During severe infection, massive leaf spotting occurs along with defoliation.
On pepper, irregular spots appear and as the disease advances, they enlarge and become reddish brown with dark margins. The lesions are often angular following the path of veins on leaves.
Dark, elongated lesions are seen on stems and petioles may girdle the plant, temporarily disrupting nutrient supply.
Symptoms on fruits
Water-soaked lesions appear first on both tomatoes and peppers and as they mature, they become enlarged and raised. Lesions may darken and crack, lowering the appearance and market quality of the fruit.
The damaged tissue may serve as a platform for other secondary infections.
The rate of photosynthesis is lowered as leaves are infected and the host is more susceptible to environmental stress.
Disease cycle of Bacterial spot of tomato and peppers
Overwintering
The bacteria overwinter in plant residues of the infected plant debris left in the field after harvest acts as a primary source. Seedborne transmission is also an important potential means of infection. Alternative host plants, such as weeds belonging to the Solanaceae family, also serve as reservoirs for the pathogen. In some instances, bacteria remain in the soil or on contaminated tools, equipment, and greenhouse surfaces. The ability of Xanthomonas to form biofilms on plant surfaces or other substrates also acts as inoculum allowing it to survive in adverse conditions until the next growing season.
Primary Infection
During primary infection, the disease-causing organism establishes itself on new crops. This may happen through viable inoculum that overwinters during the previous infection cycle. Entry of the bacteria into plants is through natural openings, such as stomata and hydathodes, or wounds resulting from mechanical damage, or insect feeding. For successful bacterial colonization, optimum environmental conditions such as warm temperatures (25–30°C) and prolonged leaf wetness from rain, dew, or irrigation must be required. In the host, the bacteria multiply rapidly causing the symptoms, such as water-soaked spots on leaves and fruits.
Colonization and Development of Symptoms
The bacteria after infection multiply in the intercellular spaces inside the host tissue, by secreting enzymes and toxins to break down the plant cell organelles. This would not only help the bacterium in its nutrition but it also suppresses the plant’s defense mechanisms and settles down the invaders. As the infection advances, characteristic symptoms of the disease include becoming more severe on leaves and fruits, necrotic spots turning brownish, and premature leaf fall. Infected tissues may ooze a bacterial exudate that provides an additional source of inoculum for further spread.
Secondary Infection and Spread
During secondary infection, the pathogen spreads from an infected plant to a healthy plant during the growing season. The primary mechanism is rain splash, whereby raindrops or irrigation water splash bacterial cells from lesions onto neighboring plants. Wind-driven rain can also disperse the pathogen over greater distances. The bacteria can spread through contaminated hands, tools, or clothing due to human activities such as pruning, harvesting, or fieldwork. In some instances, insects will act as vectors for localized spread. When introduced to a healthy plant, the bacteria enter through stomata, hydathodes, or wounds, establishing the infection process all over again.
Factors Affecting Disease Development
Temperature– The optimum temperature required for disease development is between 77°-86°F.
Rainfall– As the pathogen is carried via the rain splash mechanism, high precipitation favors disease development. Frequent rainfall and over-irrigation also favor the disease.
Humidity– High humidity and prolonged periods of relative humidity (leaf wetness of more than 4 hours) favor disease development.
Management strategies for Bacterial spot of tomato and peppers
Cultural practices
Crop rotation– The host plant should be rotated with non-host crops such as cereals and legumes for at least two years.
Sanitation– Proper sanitation of field equipment must be done by using disinfectants. Seeds can also be treated with disinfectants such as sodium hypochlorite or hot water to minimize the bacteria.
Proper pruning and spacing must be done to provide good air circulation and infected debris must be disposed of properly. Lower leaves should be pruned to minimize splash dispersal.
Resistant varieties
Resistant varieties of tomatoes such as Arka-Abhed (H-397), and bell pepper varieties such as Nitro S 10, Sailfish, etc. are to be planted which can significantly reduce disease severity. However, resistant varieties may be pathogen-specific so specific cultivars for specific bacterial strains must be chosen.
Chemical control
Chemicals such as copper-based bactericides along with fungicides such as mancozeb are used as preventive measures. These sprays form a protective barrier so that the bacteria cannot infect new tissues. However, Xanthomonas sp. is developing copper resistant gene, so it is becoming less effective in some areas.
Antibiotics such as streptomycin are also used, however excessive use of chemicals can be hazardous to the environment.
Biological control
Several beneficial bacteria such as Bacillus subtilis and Pseudomonas fluorescens could compete with the pathogen on the plant surface which leads to limitations in colonization and disease development. Bacteriophages, the viruses that infect the bacteria are also effective in controlling disease development.
Integration of cultural, biological, and chemical practices is more effective in managing the disease.
References
- Jibrin MO, Timilsina S, Minsavage GV, et al. Bacterial Spot of Tomato and Pepper in Africa: Diversity, Emergence of T5 Race, and Management. Front Microbiol. 2022;13:835647. Published 2022 Apr 18. doi:10.3389/fmicb.2022.835647
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- Bacterial leaf spot, copper resistance found in New Jersey tomato, pepper – Vegetable Growers News. (2022, April 28). Vegetable Growers News. https://vegetablegrowersnews.com/news/bacterial-leaf-spot-copper-resistance-found-in-new-jersey-tomato-pepper/
- Bacterial spot | Scouting guides for problems of vegetables. (n.d.). https://veggiescout.ca.uky.edu/bacterial-spot
- Bacterial spot of Pepper and Tomato | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/bacterial-spot-of-pepper-and-tomato
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- Gardner, M. W., & Kendrick, J. (2020). Bacterial spot on tomato. 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. PestsDiseases. https://pestsdiseases.com/bacterial-spot-management-in-tomato-disease-symptoms-treatment-chemical-biological-natural-and-organic-control/
