Selection of volatile markers for rubbery rot in apple fruit caused by Phacidiopycnis washingtonensis

Although a major share of postharvest losses of apples is due to fungal fruit rots, their timely detection is difficult in commercial bulk-storage rooms. Therefore, a method was developed to identify the volatile markers of fruit naturally infected by Phacidiopycnis washingtonensis, a common storage-rot fungus of Northern Europe, and North and South America. Potato dextrose agar, apple juice agar, and fruit of the apple cultivar 'Nicoter' were inoculated with P. washingtonensis. Volatile organic compounds (VOCs) were sampled from the headspace of inoculated and uninoculated agar cultures and fruits using solid-phase micro-extraction and analysed by gas chromatography-mass spectrometry. The number of emitted alcohols and miscellaneous compounds was higher from agar and fruit colonised by P. washingtonensis than from uninoculated controls, whereas more aldehydes and esters were detected in uninoculated samples. These results indicate that the fungus produced alcohols and miscellaneous compounds and consumed aldehydes and esters while growing. The concentration of 37 of the VOCs was higher in the P. washingtonensis inoculated agar compared to the uninoculated agar, and nine of these compounds (3-methyl-1-butanol, 3-methyl-2-buten-1-ol, 2-phenylethanol, acetone, 3-methyl furan, styrene, 1-ethyl-4-methoxybenzene, 4-ethylphenol, and 4-ethyl-2-methoxyphenol) were associated with fungal growth both in vitro and in vivo. Twenty-nine compounds were also detected in higher concentrations in apple fruit naturally infected by P. washingtonensis, indicating that the VOC method has potential as an early warning of storage rot in apples.