Control of fruit postharvest diseases: old issues and innovative approaches

Purpose of the review: This review focuses on the issues of fruit postharvest disease control; firstly the appearance of pathogen isolates resistant to common fungicides followed by stricter regulatory policies that impose a reduction in their use. Emphasis is given to the attempts to introduce effective biological control agents (BCAs) able to reduce fruit losses by an improvement in their formulation. Findings: At the beginning of the 1960s, treatments based on chemical fungicides as the main method of reducing postharvest fruit losses obtained satisfactory results. However, the fungicide option for managing postharvest decays is increasingly limited; in addition, the use of fungicides on fruit after harvest is regulated by different food protection agencies. The intense use of fungicides in the postharvest phase has contributed to the appearance of resistant isolates, widespread in packinghouses. Nowadays, fungicide resistance is frequently reported for the main fungal pathogens such as Penicillium, Monilinia, Botrytis, etc. Although the use of synthetic chemical fungicides remains a primary method of controlling postharvest diseases, the global trend appears to be shifting towards reduced use of fungicides, substituting them with alternative methods like biofungicides. However, the great effort made by researchers, documented by many peer-reviewed publications, has been only partially compensated by the appearance on the market of BCAs available for industrial application to control postharvest decays. Limitations: There are several reasons for the limited diffusion of biofungicides, some relate to an intrinsic ineffectiveness of BCAs such as their inconsistency, and the variability of the control under commercial conditions. Some are relate to their formulation which reduces the activity of antagonists with respect to the fresh cells, the high cost of the production, and the regulatory barriers to BCA registration in different countries that do not encourage their dissemination. Directions for further research: The formulation process represents an important step for maintaining a high and stable BCA efficacy. Dehydration of the product and maintenance in a dry environment is preferable to liquid formulations since antagonists can be handled using the normal distribution and storage channels. Unfortunately, not all microorganisms are able to survive drying conditions; they can lose viability during the drying process and storage. In order to avoid undesirable effects, cryoprotecting media have to be assessed during freeze-drying treatments in order to conserve antagonist viability. For example, in the case of Bacillus sp., its sporeforming ability could provide high resistance to extreme environmental conditions, making it a good candidate for developing stable and efficient products. In addition, the introduction of BCAs in wax or oil-based coatings can represent a useful strategy, above all for citrus fruit, papaya or mango that are treated by application of the coating after harvest.