Survey of Fusarium species on yellow onion (Allium cepa) on Öland

Abstract: It has been observed by both onion producers and a plant protection advisor on Öland (an island off the east coast of Sweden) that basal rot is the largest contributory factor to reduced onion quality and yield. Basal rot is mainly caused by species of Fusarium fungi. The aim of this study was to: a) investigate which species of Fusarium that can be found in onion produced on Öland, b) describe the symptoms caused by the different Fusarium fungi found and c) explore, through interviews with the onion producers on Öland, the mechanisms that may be involved in the observed increase in basal rot.Onion bulbs (Allium cepa) were sampled on two occasions. In total 181 onions from 11 different fields were analysed. In addition, eight onion producers were interviewed. Tissue from the sampled onion bulbs were placed on water agar for fungal growth. The fungal isolates were sub-cultured, scraped and subsequently, DNA was extracted. The identification of the fungal species was performed with PCR amplification using species specific primers as well as amplification and sequencing of the ribosomal DNA segments of translation elongation factor (TEF) and internal transcribed spacer (ITS). The sequences were compared to reference species in order to identify any species of non-Fusarium, as well as Fusarium fungi present.The result from the first sampling was that 63.3% of the onions were infected by F. oxysporum, 22.4% by F. redolens and 2.0% by Fusarium. sp., F. oxysporum and Trametes sp. co-occured in 2.0% of the onions and another 2.0% of the onions had a double infection of F. oxysporum and F. culmorum. The result from the second sampling showed that 18.9% of the onions were infected with F. oxysporum, 3.0% had a double infection of F. oxysporum and Penicillium sp., 0.8% had a double infection of F. oxysporum and Sclerotium cepivorum and 0.8% was infected with both F. oxysporum and bacteria.The symptoms observed on the sampled onions in this study were basal rot starting at the basal plate and spreading up in the scales, resulting in discoloured and watery bulb tissue. This study observed that 35.7% of all onion bulbs determined to be infected with F. oxysporum were symptomless, both at the time of sampling and analysis.This study suggests that the observed decline in onion quality and yield may be a result of shorter crop rotation periods, accumulation of chlamydospores in the soil and possibly the planting of infected bulb sets.