## 2 VOCs Used in the Diagnosis of Fungi  
Both plants and fungi produce VOCs that enable them to communicate intra- and inter-specifically. By emitting VOCs, plants defend themselves against herbivores and pathogens, compete with other plants, and feed microbial populations. Fungi emit VOCs to communicate or attack each other to settle on other plants without being detected [14]. These VOCs damage the membranes of the host plant, cell wall, and DNA, modify the intracellular redox balance, and increase the loss of electrolytes, reactive oxygen species levels, and lipid membrane peroxidation. For example, 3-methyl-1-butanol and 2-methyl-1-butanol are responsible for membrane lipid peroxidation. Capric acid disrupts the cell membrane, leading to intracellular content leakage. 1-octen-3-ol causes damage in the DNA without mutagenic and clastogenic effects. Finally, decanoic acid reduces intracellular pH and leads to cellular ATP exhaustion [15]. When the host plant's cellular structures, cuticle, enzymes, and defense metabolites are weakened, the fungi can enter, colonize, and spread [16]. [PERSON] et al. [17] measured the emission of VOCs at the beginning of the exponential hyphal growth phase of 43 fungal species, using the 15 best predictors to describe each fungal group with an accuracy of more than 80%. They also found that Zygomycota phyla were characterized by the intense emission of the multicomponent volatile profile, Basidiomycota phyla by 10 different compounds, and Ascomyca phyla by relatively strong emission of only 2 compounds (\(m/z\) 93.092 is not an identified compound and 4-ethyl rescorcinol). Overall, VOCs are synthesized from different pathways, such as mevalonate, shiki-amate, the polyketide biosynthetic, and the fatty acid-derived oxylipin pathway, and have distinct odors. Some fungal VOCs referred to as phytotoxins negatively affect the functioning of plants, such as their growth in the presence of 1-octen-3-ol, trans-2-octenal, 1-hexanol, 3-octanone, 3-methyl-1-butanol, and 2-phenyl ethanol. They also inhibit root and cotyled leaf length due to the effect of 1-octen-3-ol and trans-2-octenal [18]. Conversely, some other fungal VOCs contribute to desirable flavors in the food industry, are identical to produce for the plants, contribute to our understanding of the metabolic status of microorganisms, and potentially can be used as indirect indicators of decay, contamination, and the presence of disease in agricultural products due to alterations in flavors and odors, act as biofilters for use in degrading volatile contaminants, and mediate interactions with other microorganisms in the rhizosphere [19]. Specifically, in the fruit postharvest stage, fungal colonization occurs in three phases: penetration, incubation, and symptom appearance. During the symptom appearance, three lesion grades can be observed on the fruit: the early stage (when the development of the lesion is barely visible), the middle stage (evidenced by lesion expansion and the production of a small number of hyphae on the lesion), and the late stage (when the lesion continues to expand and apparent hyphae and spores are produced on the lesion), which mediate the release of VOCs [20]. Fungal colonization of agricultural products occurs because fungi accelerate the ripening process by promoting ethylene release and increasing the respiratory rate and the enzymatic activity of amylases, lipases, esterases, and proteases. This activity breaks down the fatty acids, amino acids, and sugars, causing their rot. [PERSON] et al. [21] stated that _Penicillium_ fungi secrete approximately 50 enzymes involved in the cell wall degradation of fruit and vegetables, causing the softening of the products and the release of VOCs such as ethers, alcohols, ketones, monoterenes hydrocarbons, and esters. They also observed that in the ripening stage, orange fruit releases very high levels of monoterpene hydrocarbons through the oil glands in the exocarp, favoring their colonization by _Penicillium_.