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A long shelf life of onions (Allium cepa L.) is of high importance in the onion industry. Onions are dried and stored in large wooden boxes that are difficult to access. Monitoring temperature and relative humidity during these processes is challenging. Moreover, quality may change in storage without being noticed. Therefore, there is a need to find alternative methods for monitoring and controlling the drying and storage processes of onions and to identify early changes in quality during storage. The potential use of online measurements of temperature and relative humidity (RH) in the vicinity of onions was evaluated during drying and long-term storage of six onion batches (four cultivars and three selections of one of the cultivars) in commercial storage. The batches varied in bulb weight, dry matter content, firmness and disease incidence. The dry matter content and firmness decreased during storage, while the respiration rate and incidences of individual and total disease increased. Two of the batches had low storability with high disease incidences and high average temperatures and variations in the RH. The results showed that tracking the temperature and RH in the vicinity of the onions is a promising tool for improving the drying and storage processes in commercial storage and for identifying onion batches with reduced storability early in storage., Competing Interests: The authors have declared that no competing interests exist.

Basal rot is a common onion disease and is mainly caused by Fusarium oxysporum f. sp. cepae and Fusarium proliferatum. To study the possibility of using volatile organic compounds (VOCs) as biomarkers for these fungi, pathogenic isolates of F. oxysporum and F. proliferatum from onions were cultivated in onion medium and VOCs were measured by solid phase microextraction (SPME). Forty-two compounds were detected, and thirty of these compounds were highly related to fungal metabolic activity. Allyl mercaptan was specific to F. oxysporum isolate Fox006. Analysis of the VOCs showed significant differences between the two species and among different isolates within the same species. Sixteen of the VOCs showed were highly positively correlated with the fungal biomass estimated by real-time polymerase chain reaction (PCR). Ethanol, ethyl formate, ethyl acetate, 2-methyl-1-propanol, methyl thioacetate, n-propyl acetate and 3-methyl-1-butanol are volatile metabolites that were potential indicators of Fusarium growth on onions., (Copyright © 2018 British Mycological Society. Published by Elsevier Ltd. All rights reserved.)

Highlights • Artificial inoculation of onion bulbs causes basal rot. • Pathogenic infection increases emission of volatile organic compounds. • Volatile organic compounds correlate positively with the extent of infection. • 1-Propanethiol, methyl propyl sulfide and styrene are markers of fungal infection. Abstract Diseases develop during the storage of onions. To minimize losses, new methods are needed to identify diseased bulbs early in storage. Volatile organic compounds (VOCs), the respiration rate, weight loss, and the dry matter content were investigated for 1–7 weeks post inoculation of bulbs with water (control) and two strains (Fox006 or Fox260) of Fusarium oxysporum f. sp. cepae. Photos, multispectral image analysis, and real-time polymerase chain reaction (PCR) showed no infection in the control onions, weak pathogenic infection in Fox006-onions, and strong pathogenic infection in Fox260-onions at week 7 post inoculation. Infected bulbs exhibited increased respiration rate, increased VOC emission rate, and increased weight loss. The control and Fox006-onions did not respond to inoculation and had similar reaction pattern. Forty-three different VOCs were measured, of which 17 compounds had sulfur in their chemical structure. 1-Propanethiol, methyl propyl sulfide, and styrene were emitted in high concentrations and were positively correlated with the extent of infection (r = 0.82 – 0.89). Therefore, these compounds were the most promising volatile markers of Fusarium basal rot infection. For the first time, we show that the extent of fungal infection determined by real-time PCR in onion bulbs is related with VOC emission. [ABSTRACT FROM AUTHOR]