It has now been proven that fusarium can occur on onion seed. The key question is whether its presence also causes infection in the crop; in other words, is transmission via seed possible? To date, this has never been convincingly demonstrated. Seed companies also indicate that there is no evidence of seed transmission with fusarium, unlike, for example, neck rot. Nevertheless, the debate continues. There is broad support within the sector for further research into fusarium transmission.
The Uireka research program is currently exploring whether it can play a role in follow-up research, and if so, what role. Two questions are being addressed, explains Uireka chairman Gijsbrecht Gunter. "The first question, whether fusarium can occur on onion seed, has already been answered and requires no further research. The second question is whether fusarium can actually be transmitted via seed and subsequently cause disease in the crop." Gunter is referring to so-called transmission, but convincing evidence that fusarium is transmitted via seed is still lacking, he says.
Major concern among onion growers
A group of growers from Flevoland took matters into their own hands. One of them is Pieter Beijen, an arable farmer in Zeewolde (Flevoland). He had his onion seed tested for the presence of fusarium at the end of 2025. The test, conducted by Eurofins, consisted of an analysis based on plating techniques, examining the presence of plant pathogenic fungi and oomycetes. The colonies formed on the nutrient plates were then collected and further identified using a DNA multiscan.
Beijen commissioned several analyses. These studies revealed that various fungi were found on the onion seeds. These included Fusarium spp., Fusarium lactis, Fusarium oxysporum, Fusarium solani, Penicillium spp., and Rhizoctonia solani.
The analysis form shows that Fusarium spp. is a collective term that encompasses both harmful and less harmful species. The best-known and most relevant Fusarium species in agriculture is Fusarium oxysporum, which can be further subdivided into various formae speciales (specifications). The variant known in onion cultivation as the cause of root and bulb rot and which regularly leads to dry rot in storage is Fusarium oxysporum (f.sp. cepae).
However, based on the research conducted, it can only be determined that Fusarium oxysporum is present; the specific species, f. sp. cepae, cannot be identified based on this test. It is difficult to make a definitive statement on this at this time, according to widespread consensus within the sector. According to various advisory organizations, it is unlikely that the Fusarium oxysporum found, without further specification, will actually infect the bulb via the seed in the manner known for Fusarium oxysporum (f. sp. cepae).
Advisory bodies do point out that the presence of fusarium in the seed can lead to problems during emergence, although no targeted research has been conducted on this to date.
André Boot, senior onion specialist at Hazera, refers to the letter Uireka recently distributed about fusarium. At the same time, other studies are circulating within the sector indicating that onions may be susceptible to fusarium species other than Fusarium oxysporum (f.sp. cepae). "However, these findings are not yet widely supported by publicly available research results. As far as we know, and as also mentioned in the relevant letter from Uireka, no convincing demonstration has yet been made anywhere in the world that crop damage is a direct result of fusarium introduced through seed. This, again, is also the official position of the International Seed Federation," says Boot. He also emphasizes that fusarium is a soil-borne fungus.
Uireka weighs research into seed transmission of Fusarium
Uireka has been conducting broad research on fusarium in onions for over nine years, including its occurrence in different soil types. Research into seed transmission hasn't yet been included and is a relatively new topic within the sector, says Gunter. "There are no concrete research proposals yet. First, we need to explore the scientific structure of such research and whether it fits within the available budget."
That budget is limited and can only be spent once, Gunter explains. "The decision is therefore whether this is a chain-wide, structural issue for which Uireka must allocate resources, or whether it primarily falls to other parties. For example, seed companies, growers, or other research institutions."
Underemphasized in this discussion is the fact that the fusarium fungus has multiple host plants and can continue to multiply with excessively narrow crop rotations and increasing weed control problems. If fusarium is also found on onion seeds, this is not the primary source of infection, but it can play a role in the further development of the fungus.