Research points to loss of efficiency in fungicides used against diseases in tangerines

Results achieved by Esalq and Fundecitrus corroborate IB research, which identified the problem for the first time

03.02.2021 | 20:59 (UTC -3)
Press Office - APTA

Results of scientific research from the Biological Institute (IB-APTA), of the Department of Agriculture and Supply of the State of São Paulo, which showed the loss of efficiency of the main fungicides used to control Alternaria brown spot in tangerines and their hybrids, were confirmed by studies conducted by the Luiz de Queiroz Higher School of Agriculture (Esalq/USP) and the Citrus Defense Fund (Fundecitrus). The research confirmed the resistance of the fungus that causes the disease to strobilurins and also showed that resistance occurs not only in the Southwest of São Paulo, as indicated by the initial studies by the Biological Institute, but throughout the State of São Paulo, which concentrates the largest national production. of citrus.

Strobilurins were until then the fungicides considered most effective and used in the country to control Alternaria brown spot, a disease that affects tangerines and some of their hybrids, such as tangores and tangelos, causing lesions on leaves, branches, and fruits, causing drought. of branches and falling leaves and fruits. The research results were recently published in the scientific journal Crop Protection.

"This new article includes the preliminary results obtained by IB and also more comprehensive studies conducted by Fundecitrus and Esalq. The research reported in the scientific article also included the use of many isolates of the Alternaria alternata fungus from the Micoteca "Dra . Victoria Rossetti" from IB, highlighting, once again, its importance in conducting many studies in the area of ​​phytopathology", says Eduardo Feichtenberger, researcher at the Institute. A IB mycoteca is the largest collection of fungi and oomycetes in Brazil in the area of ​​citrus diseases. Maintained in Sorocaba, it has been contributing since 1987 to solving important phytosanitary problems in São Paulo and Brazilian agriculture.

What does the new study represent in practice?

In practice, the new study shows that strobilurins are no longer effective in controlling Alternaria brown spot throughout the State of São Paulo, and that they should no longer be included in recommendations for chemical management of the disease in tangerine orchards in São Paulo. and tangors.

"Alternaria brown spot affects the cosmetic appearance of the fruits, reducing their commercial value when destined for the fresh fruit market. It also causes premature fruit drop, reducing plant productivity. When the infection occurs in fruits in the final stage of development, a single injury can cause it to fall prematurely. The producer may have a harvest practically ready, with the expectation of having high profitability when selling the fruits from his orchard, but if the disease occurs during pre-harvest, he may have a good part of his harvest lost in a matter of days", explains the IB researcher. The most affected commercial citrus varieties in Brazil are ‘Murcott’ tangor, ‘Ponkan’ tangerine and ‘Dancy’ tangerine.

The best alternative for the citrus grower would be, according to Feichtenberger, the use of varieties or cultivars of tangerines and citrus hybrids that are resistant or tolerant to the disease, such as the IAC 2019Maria tangerine, developed by the Agronomic Institute (IAC-APTA). Launched in 2018, this cultivar is the first tangerine developed and registered in Brazil that is resistant to the disease, enabling significant reductions in production costs and eliminating all environmental damage resulting from its chemical control.

"However, the replacement of varieties in orchards is subject to commercial restrictions, as tangerines such as 'ponkan' and 'murcott tangor', which are very susceptible to the disease, are also the most commercialized varieties in the country and the preferred of Brazilian consumers", says Feichtenberger.

Another good alternative, according to the researcher, would be the better use of copper-based fungicides in the chemical management of the disease. These fungicides have already been widely used, however, it would be essential to optimize their use, making product doses compatible with the best times and intervals for their application.

"Lower doses of copper can be used in applications as long as the intervals between sprays are also reduced. As copper fungicides only have preventive and contact action, reducing the interval between applications will also help to reduce the period in which organs susceptible areas of the plant are unprotected from the action of the products, especially in the case of new fruits under development", explains the IB researcher.

This recommendation is currently viable, as producers already spray in orchards to control HLB, citrus canker, early blight and other diseases and pests of the crop. "Frequent spraying in orchards to jointly control various diseases and pests is already carried out and its costs have already been absorbed by citrus growers, so this would be a viable alternative", he states.

Sprays made with a low volume of copper mixture will also prevent excess mixture from running off and, consequently, its accumulation in the soil. High levels of copper in soil can be toxic to plant roots.

The producer must also be aware of the use of other disease management measures, such as the use of healthy seedlings in new plantings and replantings; the selection of areas for planting, avoiding those with poor air circulation, such as lowlands, and reserving the highest and most airy areas for the cultivation of susceptible cultivars; carrying out cleaning pruning on the inside of the plant canopy to improve aeration conditions inside the plants, and removing dry branches, which constitute important sources of inoculum (foci of the disease); and maintaining plants in good nutritional and health conditions.

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