# Endophytic Yeasts for the Biocontrol of _Phycetema vagabunda_ in Apples
## 1 Introduction  
_Phycetema vagabunda_ ([PERSON] (syn. _Norfabruen vagabunda_ [PERSON][PERSON]), the causal agent of bull's-eye rot, has become an important and frequent postharvest disease in apples [1]. It has caused great economic losses in Europe [2], the Pacific Northwest of the United States [3], and Chile [1], and is considered a quarantine disease in some fruit destinations, such as China [4]. In Chile, _P. vagabunda_ was first reported in 2005 [5]. Late-harvest cultivars like 'Cripps Pink' are the most affected varieties, with disease incidences reaching 60%, depending on the season and locality [1]. In organic production, incidences in these cultivars can be as high as 80-90%. Fruit infection occurs in the orchard [2] throughout the season [3] and is favored by abundant rainfall [6]. Infection can become established between peak fall and harvest, increasing susceptibility gradually during fruit development [7]. However, symptoms appear only a few months after harvest (usually 3-4 months in cold storage), when numerous lesions can develop on a single fruit [8]. This is, therefore, a major limitation for the late harvest variety 'Cripps Pink' when apples are held under prolonged cold storage [9].  
Current management practices to control _P. vagabunda_ in Chile include pre- and postharvest treatments with fungicides [10]. However, increasing concerns regarding chemical residues in food [11] have generated interest in developing non-chemical control methods, with biological control being one of the most promising and explored alternatives [12].  
The use of antagonistic microorganisms has been recognized as one of the most promising alternatives to fungicides [11]. Among these microorganisms, antagonistic yeasts have proven to effectively control numerous postharvest diseases in apples [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Nonetheless, no studies have yet been carried out on _P. vagabunda_. Yeasts may be a viable control alternative for this pathogen because they have the ability to colonize the surface of fruits for long periods of time under dry conditions, produce extracellular polysaccharides that enhance their ability to survive, and rapidly use available nutrients. Yeasts are also minimally affected by pesticides [24], do not produce allergenic spores or mycotoxins [25; 26], are genetically stable [19], and have simple nutritional requirements [19; 26]. In addition, endophytic yeasts grow in fruit tissues under the same growth conditions as the pathogen _P. vagabunda_ and, therefore, are already colonizing these tissues, which gives them an advantage over the pathogen [27].  
An effective biocontrol agent requires multiple modes of action to antagonize a pathogen [28]; a combination of different mechanisms thus provides yeast with its antagonistic capacity [29] and reduces the risk of pathogen resistance [30]. The reported modes of action in yeasts include the ability to compete for nutrients and space, oxidative stress tolerance, parasitism, secretion of hydrolytic enzymes, and the ability to produce siderophores, volatile organic compounds, and biofilms, in addition to the induction of resistance [27; 31; 32; 33; 34; 35; 36; 37].  
In this research, we selected a native endophytic yeast for the biocontrol of _P. vagabunda_ in apples. The mechanisms of action of the yeast strains involved in the biocontrol activity against the pathogen were also determined. This research could generate beneficial knowledge regarding the application of yeasts for food production and security.