Integrated management does not use chemicals to control orange green mold

The adoption of this model adds sustainability to the citrus production chain and contributes to the UN Sustainable Development Goals (SDG 12)

02.02.2021 | 20:59 (UTC -3)
Christina Tordin

A combination of technologies is the solution presented by Embrapa Environment (SP) to control the fungus that causes green mold on oranges, a disease responsible for the worst losses in the post-harvest phase. Tests proved that the combination of four methods - hydrothermal treatment by spraying and brushing (Thae), ultraviolet C radiation, biocontrol agents and the use of bioactive compounds - managed to obtain performance similar to that of chemical control using fungicides, in a clean and sustainable process of integrated management of the disease.

Brazil is today the largest producer and exporter of orange juice in the world. A significant portion of production is lost due to post-harvest diseases, with the fungus Penicillium digitatum, which causes green mold, the main cause of these losses. Green mold is initially characterized by soft rot on the bark, which takes on a green hue with the production of fungus spores. These spores are easily released when the fruit is moved during storage and transportation, resulting in rapid spread of the disease, contamination and compromise of the entire batch.

“The technology we are proposing meets the increased demand from producers and traders in the citrus sector for effective and alternative methods to the use of fungicides. The sector is dependent on just one chemical fungicide molecule for post-harvest treatment, which is gradually losing its efficiency in disease control. Furthermore, the hydrothermal treatment currently used has not been effective in controlling it. There is also a growing demand from consumers, who are increasingly enlightened, for foods free of agrochemical residues. Our technology is clean and does not leave any toxic residue”, he explains. Daniel Terao, researcher at Embrapa Meio Ambiente.

The adoption of this model adds sustainability to the citrus production chain and contributes to the UN Sustainable Development Goals (SDG 12), which aims to ensure sustainable production and consumption patterns. More specifically, reduce global per capita food waste, at the retail and consumer levels, and also reduce food losses along production and supply chains, including post-harvest losses.

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The method developed by Embrapa favors, in particular, the citrus export sector, since in this segment the time needed for the fruit to reach the consumer's table increases significantly. The orange needs to be healthy, without any symptoms, for approximately 20 days after harvest. Normally, the fungi present in apparently healthy fruit at the time of shipment, in a quiescent manner, manifest symptoms during this long period of storage. Therefore, it is necessary that the fruit receives efficient post-harvest treatment so that it remains healthy. 

Furthermore, there are increasingly restrictive measures imposed by fruit importers regarding the presence of chemical residues. “The post-harvest process that we are presenting does not leave chemical residues”, points out the researcher.

This is a technology with the potential to reduce or even eliminate the use of agrochemicals in the post-harvest treatment of oranges. “The absence of chemical residues in fruits has been increasingly demanded in the demanding international fresh fruit market”, highlights Terao. 

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The method developed by Embrapa maintains sustainability throughout the entire process. In addition to eliminating the disposal of toxic agrochemicals, the technologies used also reduce water consumption and the need for electricity.

In Brazil, the most common hydrothermal treatment is by immersing oranges in heated water at around 52ºC for five minutes. However, in addition to using a large volume of non-renewable hot water and high electricity consumption, it has not shown good results. Furthermore, it accumulates a large amount of debris, organic matter and phytopathogen propagules throughout the day. 

Hydrothermal treatment by spraying and brushing (Thae) is a process that treats the fruits, while they rotate on rolling brushes, with hot water spray at temperatures higher than immersion (from 55ºC to 70ºC), for a short period of time. time (from 10 to 30 seconds), immediately followed by spraying with cold water at 15ºC, to stop the effect of heat.

In studies on Thae, it was found that, using the temperature x time binomial of 55ºC for 30 seconds, there was control of approximately 67% of green mold in Valencia orange, in addition to increasing the shelf life of the treated fruits, prolonging firmness, without altering the flavor of the fruit. 

Terao explains that this treatment provides, in addition to superficial cleaning of the fruit, a spreading of the skin's natural wax, covering and sealing stomata, which serve as sites of penetration and infection by the fungus. 

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The combination of Thae and yeast Candida membranifaciens, followed by irradiation with UV-C light at 1.5 kJ m−2, promoted an increase in disease control, and increased the activity of enzymes related to the natural resistance of the fruit, contributing to reducing the intensity of the disease. This natural resistance is associated with an increase in lignin, phytoalexins and the maintenance of other antimicrobial compounds such as chitinases, which help in the fruit's natural defense against fungal attack.

The strategy to increase the effectiveness of alternative controls is to integrate several methods in order to obtain a performance similar to chemical control, as in addition to compensating for the limitations presented in individual use, it reinforces the additive and synergistic effects that occur between them. Research results have shown that Thae is enhanced when combined with ultraviolet radiation in controlling rot in several species of fruit, such as citrus, mango, apple, papaya, strawberry and cherry.

In the case of orange, Terao explains that the isolated application of three doses of a substance present in the extract of the weed was evaluated (Photo) known as horseweed (Conyza canadensis), spraying 2 mL of the aqueous solution on the orange epidermis. This substance controlled green mold, increasing efficiency gradually with increasing dose, going from 23% at a dose of 10 µg mL-1 to 43% at a dose of 100 µg mL-1. Furthermore, the dose of 100 µg mL-1 delayed the onset of symptoms of the disease by two days. 

It was also observed that when the application of the substance was combined with UV-C radiation, there was an increase in control efficiency. After 15 days of storage, a disease control level of around 70% was found with a dose of 100 µg mL-1, when combined with UV-C radiation. Furthermore, it delayed the onset of green mold symptoms by seven days. 

The complete study by researchers Daniel Terao, Sonia Queiroz, Kátia Nechet and Bernardo Halfeld-Vieira, from Embrapa Meio Ambiente, can be accessed here.

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