Tomato Bacterial Spot

The tomato (Solanum lycopersicum), belonging to the Solanaceae family, is one of the most widely cultivated vegetables in the world, widely used both in raw food and in industry. In Brazil, production stands out due to the diverse climate, different cultivation methods, and the wide variety of cultivars available, serving both the domestic and international markets.

Currently, China leads global tomato production, while Brazil ranks ninth. The 2024 industrial tomato harvest reached 1,7 million tons, a volume that, while stable compared to previous years, keeps the sector alert to market fluctuations and production challenges.

Adverse weather conditions throughout the year directly impacted the crop, delaying seedling transplanting and increasing the incidence of diseases. Tomato plants are particularly susceptible to a wide variety of pathogens, especially when the environment offers favorable conditions for their development. Production losses can range from 10% to 30%, but without effective management practices, this percentage can increase significantly, compromising the entire harvest.

Among the bacterial diseases that affect tomato crops, bacterial spot stands out as one of the most damaging. It causes spots and lesions on leaves and fruit, directly impacting quality and yield. Currently, it is widespread in all major producing regions of Brazil, posing a major threat to the crop, as it can cause significant losses, compromising both the quantity and commercial value of the fruit.

Etiology of the disease

Bacterial leaf spot is caused by four species of the genus xanthomonas (X. vesicatoria, X. euvesicatoria, X. gardneri e X. perforans), the latter being the most prevalent. The presence of multiple species associated with the disease contributes to its widespread dissemination in producing regions. It is known that X. perforans is more aggressive to tomato plants at temperatures around 30°C, while X. gardneri is more severe at average temperatures close to 20°C, being more common in higher altitude areas.

Seeds infected or contaminated by the bacteria are one of the main sources of bacterial spot inoculum in crop fields. The spread of the disease is also favored by volunteer plants from previous cycles (tigueras), staggered cultivation, infected crop residues, and the presence of weeds or other Solanaceae in nearby areas, which act as reservoirs for the pathogen.

In addition to the tomato plant, the pepper, the joá-de-capote (Nicandra physaloides), the black maria (Solanum americanum) and the milkman/wild peanut (Euphorbia heterophylla) can serve as alternative hosts, contributing to the persistence and spread of the bacteria. Studies indicate that X. perforans occurs naturally in the joá-de-capote, while X. gardneri has already been reported in wild peanuts.

Specific symptoms

The bacteria exploits any natural opening or wound in the plant as an entry point, utilizing the water present on the plant's surface to facilitate its penetration into the leaf and fruit tissues. Symptoms of the disease can affect the entire aerial part of the plant, usually beginning on older leaves, with irregular brown spots.

On the leaflets, the initial spots are watery and quickly evolve into necrotic lesions, surrounded by a yellowish halo, characterizing the infection. Lesions caused by Xanthomonas perforans are distinguished from those of other species of the genus by their evolution. As the infection progresses, the central region of the lesions may detach, resulting in perforations, a characteristic symptom of this bacterium.

In fruits, lesions begin as waterlogged areas, which quickly darken and become deep, compromising their quality and viability for sale.

It is common for the initial symptoms of bacterial spot to be confused with those of other important diseases for tomato plants, such as bacterial spot (caused by the bacteria Pseudomonas syringae pv. tomato), black spot (Alternaria Solani) and septoria (Septoria lycopersici).

proper management

For effective bacterial spot control in tomato crops, adopting a combination of management practices is essential. Producers should prioritize prevention, with planting disease-resistant or tolerant tomato cultivars being one of the first and most recommended strategies. Ohio 8245, Heinz 9553, and Yuba are some examples that demonstrate effectiveness.

To ensure phytosanitary quality, seeds and seedlings must be free of bacteria. In the case of seedling production, it is essential that seeds are not taken from fruits showing symptoms of bacterial spot. When seeds come from fields where the bacteria are present, it is recommended to apply a moist heat treatment (56°C for 30 minutes) or a chemical treatment (0,8% acetic acid for 24 hours).

Determining the planting season is an essential factor, and it is advisable to avoid rainy periods, as high humidity favors the proliferation of the disease. Measures such as protected cultivation or drip irrigation are effective strategies to reduce direct exposure of the aerial parts of the plants to water, minimizing the risk of infection.

In areas with a history of bacterial spot, crop rotation, alternating soybeans, corn, and beans, helps reduce the disease by decreasing the sources of bacterial inoculum. In addition, eliminating volunteer plants that may host the pathogen, such as black nightshade, black nightshade, and wild peanut, is essential, as is the removal of crop residues from cultivated areas.

In the context of chemical control, the use of products registered with the Ministry of Agriculture and Livestock (MAPA) can be an effective strategy, especially in areas with a history of the disease or when the first outbreaks are detected. Copper-based products stand out, as well as formulations that combine copper-based products with agricultural antibiotics, such as kasugamycin. However, studies indicate that the continuous use of these pesticides may favor the selection of resistant strains of the pathogen, reducing their effectiveness over time.

As an alternative to antibiotics, resistance inducers have shown promise in disease management. Although they do not directly eliminate pathogens, these compounds activate the plants' natural defenses, hindering tissue colonization. Among the most widely used is acibenzolar-S-methyl (ASM), which reduces the symptoms of bacterial spot.

The use of biological agents is a promising alternative in the integrated management of diseases, with growing demand in different production systems. The genus Bacillus It is one of the most studied for application in biological control. Currently, there are several commercial products based on these bacteria, such as Serenade (Bayer), which contains B. subtilis QST713. These products offer long-lasting and systemic protection against bacterial diseases, being especially relevant in the control of bacterial spot.

By Luana Laurindo de Melo e Tadeu Antônio Fernandes da Silva Júnior, FCAV/Unesp

Article published in issue 157 of Revista Cultivar Hortaliças e Frutas