Evaluation of Microbial Antagonists in Combination with Sodium Bicarbonate for Biological Control of Postharvest Diseases of Plums (Prunus salicina Lindl.)

Plums are stone fruit with a limited postharvest life mainly due to softening and mechanical injuries and diseases that cause market rejection. Fruit losses, due to decay, are estimated to be 5-10% when postharvest fungicides are used; without fungicide treatment, losses may reach 50% or even more. However, the application of synthetic fungicides has been increasingly curtailed because different factors, such as the development of pathogen resistance to many key fungicides, the lack of replacement fungicides, and the negative public perception regarding the safety of pesticides and the consequent restrictions on fungicide use. With this scenario, biological control has emerged as one of the most promising alternatives to chemical fungicides. Thus, the aim of this work was to study the antagonistic action of four yeasts and two bacterias (Cryptococcus laurentii, Trichosporon pullulans, Hanseniaspora uvarum, Pichia guillermondii, Bacillus subtilis and Pseudomonas syringae, respectively) in combination with sodium bicarbonate (SB). Plums (Prunus salicina Lindl. cv. 'Angeleno'), at commercial maturity, were disinfected with 2% sodium hypochlorite for 2 min, washed with tap water and air-dried prior to use. Plums were wounded (5mm deep and 3mm wide) at the equator of each fruit with a sterile nail. Each wound was added with 20 mu l of the treatment suspensions as follows: antagonist (1x10(8) cells mL(-1)), antagonist (1x10(8) cells mL(-1)) + SB (2%) and SB (2%). Fruit treated with sterile distilled water served as control. When the wounds were air-dried, plums were challenge-inoculated with 20 mu l of a conidial suspension of Monilia laxa, Botrytis cinerea, Penicillium expansum, Aspergillus niger, Rhizopus stolonifer and Fusarium oxysporum at 1x10(4) spores mL(-1) respectively, and stored in air at 25 degrees C. The combination of microbial antagonists and SB were capable of reducing the percentage of infected wounds. However, in stand-alone treatments, plums treated with microbial antagonists or with SB, the percentage of infected wounds was very high. During these experiments it was found that antagonists in combination with SB was effective against several of the main postharvest pathogens (F. oxysporum, A. niger, B. cinerea and M. laxa), although certain fungal fruit pathogens (R. stolonifer and P. expansum) were less sensitive to inhibition than biocontrol agents. Thus, the combination of microbial antagonists and SB could be an alternative to chemicals for the control of postharvest rot on plums.