## 4 Discussion
2-Undecanone has been previously reported to inhibit the growth of the pathogens _Verticillium dahliae_ and _Fusarium oxysporum_[38]; thus, it is likely that it acts as a defense compound produced by olives in response to pathogen challenge. Besides 2-undecanone, 2-hexanone, 2-heptanone, 2-octanone and limonene might also have a similar role in defending plants from _C. acutatum_ colonization, due to their exclusive production in olives inoculated with _C. acutatum_ or with _A. pullulans + C. acutatum_. In this regard, terpen (that include limonene) constitute one of the largest VOC groups that are assumed to function in plant defense against pathogens [39]. Changes in volatile profiles were also observed over time in the present study, with some VOCs being exclusive to a specific time point, suggesting that microorganisms produce a typical volatile blend depending on their growth and/or interaction stage. We hypothesized that these compounds, which became distinct, may represent signals of the ongoing metabolism derived from the microbial-olive interaction. Accordingly, it is possible that the high variety of VOCs released in the first two days of the assays may be linked to great changes in the metabolism of interacting fungi and olives. Interesting changes in the overall VOC profiles included the dominance of hexanal, 2,2,4-trimethyl-pentane and 2-methyl-1-butanol on days 2, 8 and 16, respectively. These compounds, particularly the aldehyde hexanal and the alcohol 2-methyl-1-butanol, were previously shown to have antifungal proprieties against several postharvest pathogens in fruits and vegetables [14; 40; 41]. To our knowledge, the antifungal potential of 2,2,4-trimethyl-pentane has not yet been analyzed; however, it belongs to the alkane class that is recognized to comprise compounds with the ability to suppress plant pathogens [42]. Multivariate analysis of our results showed a strong relationship among the volatile profiles between the _A. pullulans_ + _C. acutatum_ (Ap + Ca) and _C. acutatum_ treatments, and their clear separation from the _A. pullulans_ treatment, for the first 8 days of the assay. This suggests a greater contribution of _C. acutatum_ when compared to _A. pullulans_ for the VOC blend composition in the Ap + Ca treatment. In addition, the inoculation of olives with Ap + Ca or _C. acutatum_ induced stronger changes in the VOC abundance in relation to the control (olives inoculated with Tween) than the inoculation with _A. pullulans_. Thus, it is likely that metabolic pathways were more changed by the inoculation with Ap + Ca or _C. acutatum_ compared to the inoculation with _A. pullulans_ alone. The VOCs most positively associated with the Ap + Ca and _C. acutatum_ treatments in the first 8 days of the assay included alcohols (2-methyl-1-propanol, 2-methyl-1-butanol, 1-hexanol), ketones (2-hexanone, 2-heptanone), terpen (limonene) and alkanes (2,2,4-trimethyl-pentane), with most of them previously being reported to have antimicrobial activity against a variety of pathogens [5; 14; 39]. Curiously, some of these VOCs, particularly 2-methyl-1-propanol and 2-methyl-1-butanol, were previously identified from _A. pullulans_ and also showed inhibitory activities against an array of pathogens, including _C. acutatum_[14; 19]. The aldehydes hexanal and benzene-etaldehyde and the alcohol [IS-(1,2,2,5,8)]-cyclohexanol, 5-methyl-2-(1-methylethyl) were the VOCs that most discriminated _A. pullulans_ from the other treatments in the present study. The first two molecules are well-known antimicrobials [40; 41], and the last one is a bioactive compound that has been found in several medicinal plant species [43]. At the later stages of the assay (on day 16), a clear discrimination between VOC profiles emitted from the _C. acutatum_, Ap + Ca and _A. pullulans_ treatments was observed. As the antagonism of _A. pullulans_ against _C. acutatum_ was the highest at this stage, we hypothesized that these differences in VOC profiles are important in reducing anthracose development.