Aureobasidium pullulans strain Ach1-1 in biocontrol of postharvest diseases of apples: 15 crucial years of research before starting commercial development

Significant losses in harvested fruits are directly attributable to decaying fungi. Biological control using microbial agents including yeasts has been reported among several alternatives to the use of synthetic chemical fungicides for managing postharvest fruit decay. Economic losses caused by postharvest diseases represent one of the main problems of the apple industry worldwide. The major diseases affecting citrus are the "green mold" and "blue mold", caused by Penicillium digitatum and P. italicum, respectively. To control them, synthetic fungicides are the most commonly used method. However, often the emergence of resistant strains occurs and their use is becoming more restricted because of toxic effects and environmental pollution they generate, combined with trade barriers to international markets. The aim of this work was to isolate indigenous killer yeasts Postharvest fungal pathogens are considered the main cause of fresh fruits losses in storage conditions. As a countermeasure yeasts showed to be potential candidates for an efficient biocontrol of postharvest pathogens since they colonize efficiently and steadily wounded and non-wounded plant surfaces even under unfavourable conditions. The aim This led us to select in vivo a potential yeast biocontrol agent able to protect apple fruits in particular, against the fungi Penicillium expansum and Botrytis cinerea in postharvest conditions. The yeast antagonist isolated form the surface of Golden Delicious apple fruit and characterized as Aureobasidium pullulans strain Ach1-1, showed significant ability in controlling blue and grey mold infection diseases. Efficacy results revealed that A. pullulans strain Ach1-1 allowed high protection level (85%) against the two tested pathogens at 25°C and in storage conditions [1]. Many challenges needs to be addressed in order to develop a successful postharvest Biocontrol Agent. From a commercial point of view, a deep understanding of the action mode is essential to develop appropriate formulation, methods of application, and to obtain registration. In this regard, the possible involvement of competition for nutrients in the biocontrol activity of this strain both in vitro and in situ was investigated. The results showed evidence that competition for apple nutrients mainly amino acids and especially Serine, reflects the potential mechanism of strain Ach1-1 biocontrol activity against blue mold infection disease [2]. Special emphasis will be taken to these results. Their implication on the formulation will be further presented. Besides, in an attempt to monitor the population dynamic of A. pullulans strain X on fruit surface, molecular markers and a semi-selective medium method were developed. The random amplified polymorphic DNA (RAPD) technique was applied to a collection of 15 strains of A. pullulans, including the strain Ach1-1. Five specific RAPD fragments were amplified for strain Ach1-1, among them, a fragment of 528 bp specific to this strain was selected, cloned, sequenced, and used to design sequence-characterized amplified region (SCAR) primers. The results showed that the SCAR primers can clearly identify strains Ach1-1 among 14 strains of A. pullulans and among eight yeast strains commonly present on apple fruit surfaces. In addition a semi-selective medium, PDA medium supplemented with euparen, sumico, hygromycin B, streptomycin sulphate, cycloheximide, was developed. These two methods displayed sufficient and high accuracy to monitor strain Ach1-1 populations [3]. Finally, a first trial of biomass production, formulation and evaluation of the formulated yeast for its antagonistic activity at pilot scale was conducted. The results showed high protection level achieved with the formulated A. pullulans strain Ach1-1 towards controlling apple decay caused by Penicillium expansum after 28 and 7 days stored respectively at 5 and 25°C [4]. Based on these crucial steps, Aureobasidium pullulans strain Ach1-1 gained a step stone in fundamental and applied research, leading to a collaboration establishment with Elephant vert group towards its commercial development as potential biocontrol agent for postharvest apple diseases.Développement d’une formulation de biocontrôle à base d’Aureobasidium pullulans Ach1.1 contre des pathogènes de cultures maraîchère et fruitières en pré et post-récolt