Novelties in the use of sulfur dioxide (SO2) for the control of gray rot in blueberries

The gray rot caused by the fungus Botrytis cinerea is one of the main causes of rejection for blueberries from Chile. Due mainly to low tolerance (close to 0%) to the presence of decay in the target markets. The symptoms are described as soft and watery fruit consistency, loose skin and eventual superficial sporulation. Also in situations of high severity, due to the advance of the disease between fruits, it is possible to observe B. cinerea nests inside the containers (Figure 1). Different measures of disease control have been established, from key prevention and control strategies with fungicidal application programs during flowering and fruit growth, to post-harvest fungicidal treatments. Within the alternatives of post-harvest treatments in blueberries, in the last seasons the use of sulfur dioxide (SO2) has taken high importance given both by climatic conditions and by distance from the markets; However, the success of the use of the technology will depend on several factors such as the concentration-time of sulfur dioxide used, the time elapsed between harvest and gasification, the quality of the fruit at the time of treatment, and the handling of the cold chain during storage. Failure to consider these factors, rather than a treatment to solve a problem, can become a cause of generation of loss of quality and condition.

Sulfur dioxide (SO2), has been used for more than 50 years in the table grape industry worldwide for the control of gray rot. In table grapes SO2 can be applied to harvest and postharvest with different objectives, depending on the moment and dose used; for the case of blueberries, the same treatment methodologies have been adapted with SO2. The initial gasification is preferably carried out immediately after harvesting, in hermetically sealed gasification chambers, with the objective of eliminating the inoculum of B. cinerea present on the surface of the fruit. Additionally, sulfur dioxide treatment is applied to the container during storage at 0 ° C by means of sulfur dioxide-generating films or papers from sodium metabisulfite. This second application methodology aims to stop the progression of the disease between fruits. In this line, the concentrations reached inside the container are lower than those used in gasification chambers.

Figure 1. Symptoms of gray rot in blueberries. A. Soft, watery rot with presence of superficial sporulation and loose skin. B. Nests of B. cinerea in well.

For the use of SO2 in gasification chambers it is important to have a hermetic gasification container or chamber designed for the volumes of fruit to be gasified routinely, considering that the entire gasification process has a time of 30 to 40 minutes, plus the fruit movement time inside the container. It is also important to bear in mind that the optimal dose for the control of fungal diseases is calculated according to the product between the time of exposure and the variation of the concentration in this time, known as concentration-time (ppm-h). To determine the optimal concentration-time for the control of gray rot, Rivera et al., (2013) studied the effect of different concentrations-times of SO2 in blueberries 'Brigitta' and 'Liberty' gasified at 20 ° C (Figure 2It is possible to observe that a concentration-time between 150 and 200 ppm-h would be sufficient to control the development of gray rot with> 50% effectiveness. In relation to the effect of SO2 on other parameters of quality, the same authors indicate that the use of SO2 can diminish the firmness of the fruit in varieties like 'Liberty', but the importance (if it increases or not the proportion of soft fruits to the touch) and the magnitude of this decrease will depend on the initial quality of the fruit at the time of exposure to SO2, where over-ripe fruit will be more susceptible to softening.

Figure 2. Prevalence of gray rot in blueberry fruits inoculated with B. cinerea and aerated with SO2 time-concentrations from 0 to 400 ppm-h to 20 ° C. After treatment, the fruit was incubated for 15 days at 0 ° C plus 3 days at 20 ° C and> 98% relative humidity, before determining the prevalence of gray rot.

On the other hand, the use of SO2 during refrigerated storage by means of sodium metabisulfite-releasing films has shown an optimum effectiveness for the control of gray rot. The Postharvest Unit of INIA-La Platina together with Quimas SA is evaluating the use of intelligent SO2 releasing containers in a modified atmosphere. As can be seen in Figure 3, the use of smart containers (SmartPac) in wells of blueberries 'Legacy' inoculated with B. cinerea and stored for 45 days at 0 ° C plus 2 days at 20 ° C, allowed to decrease the prevalence of gray rot in an 50% with respect to fruits stored in modified atmosphere (CO2 = 3,7% O2 = 16,8%), without affecting the firmness and loss of fruit weight.

It is important to indicate that when deciding to use SO2 release systems during storage, the success of this technology would imply correct handling of the temperature at 0 ° C, since temperature breaks can be highly detrimental to produce excessive release of SO2 at environment of the fruit, affecting the effectiveness of the technology. Finally, currently the use of sulfur dioxide is not registered for use in blueberries in US markets; however, it is likely that once the product is registered, maximum residue limits similar to table grapes are required, corresponding to 10 mg / Kg of fruit.

Figure 3. Effect on the prevalence of gray rot from the use of AM containers with and without SO2 release film in 'Legacy' blueberries previously inoculated with B. cinerea and stored by 45 from 0 ° C and 45 from 0 ° C plus 2 days from 20 ° C. Bars = Standard error of 3 repetitions.

LQuoted iteratura: Rivera SA, BA Latorre and JP Zoffoli. 2013. Determination of optimal sulfur dioxide time and concentration product for postharvest control of gray mold of blueberry fruit. Postharvest Biol. Technol. 83: 40-46.

Sebastián Rivera Smith, Ing. Agr. M.Sc.
Bruno Defilippi Bruzzone, Ing. Agr. PhD.
Postharvest Unit Institute of Agricultural Research, INIA