University of São Paulo: Prevention is still the best way to control citrus canker

Brazil is the world’s largest producer of citrus fruits, and the State of São Paulo is the largest producer of citrus fruits in the country, responsible for 78% of orange production, 74% of lemon production and 34% of tangerine production. But the country could have even more wealth in the sector, were it not for the citrus canker.

The drop rate of orange fruits, for example, in the Citrícola Park of São Paulo, in the 2021/2022 harvest, was 21.8%. With citrus canker alone, the loss was R$19 million, equivalent in boxes that were no longer harvested by the disease

Citrus canker is a disease caused by the bacterium Xanthomona citri subsp. citri and specifically attacks citrus orchards such as oranges, lemons, acid limes, tangerines and grapefruit. Originating in Asia, the first detection of the bacterium in Brazil was in the Presidente Prudente region, in the State of São Paulo, in 1957.

Since then, the main way to fight the disease is still prevention, as reported by Franklin Behlau, a researcher at the Citriculture Defense Fund ( Fundecitrus ). In properties free from the disease, the producer must be aware of the transit of machines and the use of equipment, because these utensils can carry the bacteria. For more than 20 years, in the State of São Paulo, it is mandatory to plant seedlings produced in a closed and controlled environment, this contributes to the health of the orchard in general. When possible, the producer can use a selection of tree varieties less susceptible to the disease as a measure to facilitate the management of citrus canker.

Also according to Behlau, the main measures used and recommended are regular applications of copper-based products, especially in spring and summer, due to higher temperatures and incidence of rain, which are conditions that favor the development of the disease. Another important measure, says the researcher, is the control of the citrus leaf miner, a moth that attacks the young leaves of plants, “decreasing the rate of photosynthesis and the development of shoots, causing the affected leaves to dry out and fall off”. The young form of the insect can cause injuries to the fruits and favor the penetration of the bacteria, increasing the incidence of the disease in the orchard”, he says.

New technologies in testing
Due to the high losses in the sector, new technologies are being studied to prevent and fight the disease. “Among the new ways to tackle citrus canker, copper nanoparticles, zinc nanoparticles, silver nanoparticles and also the use of natural products, such as clove oil. In addition to essential oils, gallic acid and protocatechuic acid and their derivatives are being observed for having potent bactericidal action and potent antioxidant action”, comments researcher Maicon Segalla Petrônio, from NPsmart, a nanotechnology startup in the area of ​​agriculture. All tests are carried out from cultivation to post-harvest, for more options for controlling and reducing citrus canker.

Disease identification

José Belasque Júnior – Photo: Fundecitrus/Reproduction
Most of the time, the identification of the disease is simple and visual for those who have experience with the cultivation of fruits. “Brown or brown spots, which start on the lower part of the leaf, progress with the increase in the diameter of the lesion and become rougher. The presence of concentric rings in the lesions also become prominent over time. Lesions can appear on fruits, branches and leaves,” says Behlau.

When there is a need to confirm the disease in citrus trees, there is more than one laboratory method for diagnosis. “There are methods of amplification of nucleic acids, which is PCR, specific for the detection of bacteria, and other simpler methods such as isolation in culture medium, light microscopy or electron microscopy”, informs professor José Belasque Júnior, from the Escola Luiz de Queiroz Superior of Agriculture (ESALQ) at USP, in Piracicaba.

Human health concerns
Professor Belasque Junior explains that there is no risk to people’s health, because the symptoms are restricted to the skin and the beginning of the albedo, which is that white part under the skin of the fruit. The damage is related to the production system, that is, if the producer does not manage this disease well, he will have fruit drop and loss of quality for sale in the fresh market, since the fruit aspect is not so good.

“As a measure to control the disease, fruits with symptoms cannot be marketed in fresh fruit markets, this is to contain the disease, so that this pathogen does not enter areas that are still free. However, this is a measure to contain the disease, so that new areas are not affected and not as a measure so that people do not suffer any harm, because that does not happen”, comments Belasque.