Control of Apple Blue Mold by the Antagonistic Yeast Pichia anomala Strain K: Screening of UV Protectants for Preharvest Application

Lahlali, R., Brostaux, Y., and Jijakli, M. H. 2011. Control of apple blue mold by the antagonistic yeast Pichia anomala strain K: Screening of UV protectants for preharvest application. Plant Dis. 95:311-316. When applied preharvest, antagonistic yeasts that act as biocontrol agents of postharvest fruit diseases must survive the environmental conditions in the field. In particular, UV-B radiation (280 to 320 nm) can greatly reduce their survival and effectiveness. The influence of artificial UV-B radiation on Pichia anomala strain K, an antagonistic yeast with potential for control of postharvest fruit diseases, was evaluated in vitro and in vivo. The in vitro 50 and 90% lethal dose values were 0.89 and 1.6 Kj/m2, respectively, whereas lethal values in vivo were 3.2 and 5.76 Kj/m2, respectively. UV protectants tested in combination with strain K included congo red, tryptophan, riboflavin, lignin, casein, gelatine, folic acid, tyrosine, and four mixtures. Riboflavin, folic acid, and the mixtures 1% folic acid + 0.5% tyrosine + 0.5% riboflavin (formula 2), 0.5% folic acid + 1% tyrosine + 0.5% riboflavin (formula 3), and 0.5% folic acid + 0.5% tyrosine + 1% riboflavin (formula 4) reduced yeast mortality caused by UV-B radiation in petri dish assays. Riboflavin, folic acid, gelatine, lignin, and tyrosine reduced yeast mortality caused by UV-B radiation on apple fruit surfaces. With the exception of lignin and folic acid, none of the compounds or mixtures increased significantly the ability of strain K to control the postharvest pathogen Penicillium expansum on wounded apple fruit. In contrast, casein, gelatine, tyrosine, congo red, riboflavin, and formulas 1 to 4 significantly reduced the effectiveness of strain K. Further investigations are justified to verify a potential benefit of lignin and folic acid for UV protection of strain K in preharvest applications. Fruit diseases caused by postharvest pathogens can result in 25% or more decay (10), depending on the kind of fruit, the pathogen, and storage conditions (1). Such losses are particularly high in developing countries (48) and reflect reductions in the quality and quantity of marketable fruit. Methods for controlling postharvest diseases include careful handling during harvest, cooling of the fruit after harvest, maintaining controlled atmosphere in storage, heat treatment (15), cleaning and disinfecting storage containers and shipping cars, providing ventilation to control relative humidity, and disposing of infected fruit (1). Although these control methods are helpful, they are usually insufficient in preventing fungal infection and, thus, application of fungicides has been the primary means of controlling postharvest diseases caused by fungi (12). The development of resistance in fungal pathogens to fungicides (4,20,38,45,46) and the growing public concern over hazards associated with pesticide application (30,44,49) have resulted in a significant interest in alternative methods for disease control. One alternative method is biological control, which is attractive for the control of postharvest infections because it leaves no chemical residue on the treated fruit (35,46,47). Microorganisms that are antagonistic toward plant-pathogenic fungi and other pests are termed biocontrol agents and include nonpathogenic bacteria, yeast, and fungi. These biocontrol agents are easily cultured in the developing world for use by local specialists (liquid fermentation systems) or farmers themselves using semi-solid cultivation on rice, cassava, wheat bran, or a similar substrate (19). Antagonistic microorganisms that can be produced include Trichoderma viride, Pseudomonas fluorescens, Azospirillum and Phosphobacter spp., and mycorrhizal fungi (39). The yeast Pichia anomala strain K (strain K) was isolated from the surface of apple fruit and was shown to have antagonistic activity against the plant-pathogenic fungi Penicillium expansum and Botrytis cinerea (26). Strain K was reported to be effective against P. expansum, B. cinerea, and other postharvest pathogens of fruit (26,32). Its mechanism of action in controlling B. cinerea on apple includes competition for nutrients and mycoparasitism (14,25,26,33). Several monitoring systems have been developed to track the population dynamics of strain K on apple fruit (9,37). For biological control of postharvest fruit infections, biocontrol agents have been applied alone or in combination with other safety pest and disease control methods before or after harvest (25). The success of preharvest application of yeasts and other microorganisms for biocontrol of postharvest diseases can be greatly affected by temperature, humidity, rain, and UV-B radiation (30). Ippolito and Nigro (24) also reported that the orchard microenvironment can affect the viability of biocontrol agents. The effects of water activity and temperature on strain K have been determined (28), and Lahlali et al. (31) proposed and validated a model for predicting the population density of strain K on apple fruit surface in relation to the microenvironment 48 h after the yeast had been applied in orchards; yeast density on fruit was more affected by relative humidity than by temperature. A formulation of Candida oleophila strain O based on skimmed milk reduced the yeast’s sensitivity to the orchard microenvironment (29). The adverse effect of sunlight on biocontrol agents may require that UV protectants be included in the agent formulation. Such UV protectants include riboflavin, para-aminobenzoic acid, ascorbic acid, folic acid, uric acid, casein, tyrosine, gelatine, and lignin (2,11,15,17,18,21–23,27,42). To the best of our knowledge, there are no reports about the influence of UV-B radiation on the biocontrol agent’s survival when applied preharvest for postharvest disease management. The objectives of this study were to assess the influence of UV-B radiation on the in vitro and in vivo survival of strain K and to evaluate the ability of UV protectants to protect Corresponding authors: M. H. Jijakli, E-mail: [email protected]; and R. Lahlali, E-mail: [email protected] Accepted for publication 28 October 2010. doi:10.1094 / PDIS-04-10-0265 © 2011 The American Phytopathological Society