## 2 Results
### In Vitro Analyses of the Antifungal Activity of Pure VOCs against V. pyri  
Thirteen pure compounds including all the Ketones, Esters (except for S-methyl 3-methylbutanetthioate, which we could not purchase), Acids and Aldehydes (Table 1) were obtained and tested to gauge their ability to inhibit _V. pyri_ growth. These compounds suppressed _V. pyri_ to varying degrees at a concentration of 250 mL L\({}^{-1}\) (Figure 8A,B). Of the tested substances, hexanoic acid, ibvererin, and 2-methylvaleraldehyde displayed the highest levels of inhibitory activity such that they were able to fully suppress _V. pyri_ mycelial growth (Figure 8A,B). Methyl isobutyl ketone additionally inhibited _V. pyri_ growth by 56.85%, while the inhibitory activity of butanoic acid, 2-methyl-, ethyl ester, butanoic acid, 3-methyl-, ethyl ester, methyl methoxyacetate, and methyl isobutyl ketone was relatively weak (35.14%, 30.91%, 27.82%, and 25.14%) (Figure 8B). Hydroxyacetone, 3-methylbutanone, guanidinopropionic acid, bis-(4-fluorophenyl)-methananone, 4-methylphenopentone failed to exert significant antifungal activity against _V. pyri_ (Figure 8B). The EC\({}_{50}\) values for the three strongest antifungal compounds (hexanoic acid, 2-methylvaleraldehyde, and ibvererin) against _V. pyri_ were 6.65, 74.07, and 0.30 \(\mu\)L L\({}^{-1}\), respectively (Supplementary Table S2).  
\begin{table}
\begin{tabular}{l l l l l l} \hline
**Family** & **Compound** & **CAS** & **Molecular Formula** & **RT (min)** & **Area (\%)** \\ \hline Alkanes & Octane, 3-ethyl-2,7-dimethyl- & 62183-55-5 & C\({}_{12}\)H\({}_{26}\) & 14.56 & 3.62 \\  & Decane, 6-ethyl-2-methyl- & 62108-21-8 & C\({}_{13}\)H\({}_{28}\) & 20.23 & 12.64 \\  & Octane, 2,4,6-trimethyl- & 62016-37-9 & C\({}_{11}\)H\({}_{24}\) & 25.03 & 7.01 \\ \hline Ethers & 1-ethoxy-2-propanol & 1569-02-4 & C\({}_{5}\)H\({}_{12}\)O\({}_{2}\) & 13.09 & 95.76 \\  & Tri(propylene glycol) methyl ether & 25498-49-1 & C\({}_{10}\)H\({}_{22}\)O\({}_{4}\) & 41.66 & 5.69 \\ \hline Acids & Guanidinopropionic acid & 353-09-3 & C\({}_{4}\)H\({}_{9}\)N\({}_{3}\)O\({}_{2}\) & 4.02 & 39.04 \\  & Hexanoic acid & 142-62-1 & C\({}_{6}\)H\({}_{12}\)O\({}_{2}\) & 8.62 & 5.15 \\ \hline Aldehydes & 2-methylvaleraldehyde & 123-15-9 & C\({}_{6}\)H\({}_{12}\)O & 12.33 & 25.42 \\ \hline \end{tabular}
\end{table}
Table 1: \(Cont.\)  
Figure 8: The in vitro antifungal activity of pure compounds against _V. pyri_. (**A**) _V. pyri_ colony morphology following fumigation with the indicated pure compounds at a concentration of 250 mL L\({}^{-1}\). The mycelial plug inoculation plate without fumigation of compounds was used as CK. (**B**) Inhibition rates. 1: Hexanoic acid; 2: 2-methylvaleraldehyde; 3: Iberverin; 4: Methyl thiolacetate; 5: Butanoic acid; 2-methyl-; ethyl ester; 6: Butanoic acid; 3-methyl-; ethyl ester; 7: Methyl methacrylate; 8: Methyl isobutyl ketone; 9: Hydroxyacetone; 10: 3-methylbutanone; 11: Guanidinopropionic acid; 12: Bis-(4-fluorophenyl)-methananone; 13: 4-methylphenopentone. Data are means \(\pm\) SD from three biological replicates. Different letters correspond to significant differences at the \(p\) < 0.01 threshold. This experiment was independently repeated three times and yielded similar results.