Abstract
Bacteria were isolated from internal tissues of surface sterilized healthy tubers of Solanum tuberosum cv. Maris Piper (8 different isolates) and from tubers inoculated with Erwinia carotovora ssp. carotovora showing soft‐rot symptoms (3 different isolates), and identified by fatty acid profiling. Bacillus polymyxa and an Arthrobacter sp. were isolated from both sources, E. carotovora only from the soft‐rotted tubers. The volatile organic compounds (VOCs) generated by tubers inoculated with E. carotovora, B. polymyxa and the Arthrobacter sp. were identified. Inoculated tubers of cv. Maris Piper were incubated under controlled humidity (95% relative humidity) and temperature (10°C) to simulate typical storage conditions. B. polymyxa and Arthrobacter sp. did not cause symptoms, whilst E. carotovora caused limited soft‐rot infections after 4 weeks at the low temperatures typically associated with potatoes in storage. The VOCs released to the headspace around these tubers were collected using an adsorbent system and analysed by Gas Chromatography‐Mass Spectrometry (GC‐MS). Twenty‐two volatiles unique to E. carotovora infection of potato tubers were found, including 10 alkanes, four alkenes, two aldehydes, one sulphide, one ketone, one alcohol, one aromatic, one acid and one heterocyclic compound. B. polymyxa generated three unique volatiles: N,N‐dimethylformamide, 1‐pentadecene and 1‐hexadecane. Only one volatile, 2,3‐dihydrofuran, was unique to the Arthrobacter infection. Production of volatile nitrogen species from E. carotovora‐infected tubers increased with time, whereas none were detected in the headspace above uninfected tubers. Further analysis using a modified GC‐MS method established that ammonia, trimethylamine and several volatile sulphides were evolved from tubers infected by E. carotovora. No specific volatile was useful as a marker associated with any of the three bacterial species but in the case of E. carotovora‐infected potato tubers a significant increase in the volume of compounds evolved was clearly observed. The results are discussed in relation to the use of sensors to detect VOCs evolved from infected tubers in order to provide an early warning system for the control of soft rot in potato stores.
Highlights
The rotting of potato tubers in stores is a major problem for the potato industry
Volatile profiles from potato tubers inoculated with bacteria
E. carotovora Fifty-seven volatiles were identified as being associated with E. carotovora-infected potato tubers at four weeks after inoculation (Table 2)
Summary
The rotting of potato tubers in stores is a major problem for the potato industry. Bacterial soft rot (Erwinia carotovora) is probably the principal cause and the most serious in terms of crop losses. Infection by the late blight pathogen Phytophthora infestans is significant, not just because of the damage it causes per se, but because it appears to provide potential sites for secondary infection by E. carotovora. E. carotovora is strongly pectolytic and, under favourable conditions, can rapidly turn an infected. Tuber into soft, wet, rotten tissue, which carries inoculum to healthy tubers and spreads the infection rapidly throughout a store (Bradbury, 1986). Detection of the disease is vital if it is to be controlled and this could be achieved using vapour sensors that detect volatile organic compounds (VOCs) produced by infection
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