Electronic Nose Differentiation between Quercus robur Acorns Infected by Pathogenic Oomycetes Phytophthora plurivora and Pythium intermedium

Abstract

Identification of the presence of pathogenic oomycetes in infected plant material proved possible using an electronic nose, giving hope for a tool to assist nurseries and quarantine services. Previously, species of Phytophthora plurivora and Pythium intermedium have been successfully distinguished in germinated acorns of English oak Quercus robur L. Chemical compound analyses performed by HS-SPME/GC-MS (Headspace Solid-Phase Microextraction/Gas Chromatography-Mass Spectrometry) revealed the presence of volatile antifungal molecules produced by oak seedlings belonging to terpenes and alkanes. Compounds characteristic only of Phytophthora plurivora or Pythium intermedium were also found. Methylcarveol occurred when germinated acorns were infected with Pythium, while neophytadiene (isomer 2 and 3) occurred only when infected with Phytophthora. Moreover, isopentanol was found in acorns infected with Phytophthora, while in control, isopentyl vinyl ether was not observed anywhere else. Among the numerous volatile compounds, isopentanol only occurred in acorns infected with Phytophthora and methylcarveol in acorns infected with Pythium.

Keywords: VOC; fungi and biosecurity; odor classification; volatile organic compounds.

Figure 1

Examples of measured samples of three considered categories: Healthy acorn (Control), acorn infected by Phytophthora, acorn infected by Pythium. Tissue necrosis can be distinguished from healthy regions in infected samples.

Figure 2

Example of the sensors’ responses (conductance) during one measurement cycle of a sample of healthy acorns. On the x-axis, the number of individual reads of the sensor resistance is used. The sensor data are collected every 1.2 s. The sensors responses are standardised by the baseline value obtained as the average of the first 100 reads of sensors values when sensors were exposed to the clear air conditions.

Figure 3

Electronic nose device with measured samples of healthy and infected acorns in jars.

Figure 4

Example of all sensors responses collected during one day of the measurements, versus time of the measurement. The sensors’ responses are standardised by the baseline value obtained as the average of sensors values when exposed to the clear air conditions at the beginning of each measurement cycle.

Figure 5

Distribution of measured samples based on principal component analysis transformation of modelling features extracted from sensor response curves. The percentage of variability accounted for by the principal components is indicated in the axis labels. The types of samples measured are represented with different colours and symbols. (a) The three categories of measured samples are plotted. (b) Only the samples infected with oomycetes are shown to illustrate the difference.