Fumonisin production and symptom development in onion (Allium cepa) inoculated with Fusarium proliferatum

Fusarium proliferatum is one of the main pathogens causing Fusarium basal rot of onion, which is a major problem in onion cultivation worldwide. In this work, three Fusarium proliferatum isolates from onion (Allium cepa), two from imported sets and one from a mature bulb grown in Finland, were tested for pathogenicity and mycotoxin production. Symptom development in the inoculated onion bulbs and production of fumonisins, beauvericin, and moniliformin were measured at 1-5 weeks post-inoculation (wpi). Symptoms were observed in all the bulbs inoculated with F. proliferatum, starting as water-soaked lesions and developing into dead brown scales. When colonizing the bulbs, all three F. proliferatum isolates were confirmed by qPCR to express the genes FUM1, required for fumonisin production, and SIX2, encoding a putative virulence factor. Fumonisin B₁ (FB₁) was detected already at 2 wpi in a symptomatic onion inoculated with Fpr047, and at 3 wpi, fumonisin production was confirmed for all three isolates. At 4 wpi, all the isolates had produced quantifiable amounts of FB₁, ranging from 3.9 to 177 µg/kg, and fumonisin B₂, ranging from 3.4 to 89 µg/kg. At 5 wpi, FB₁ was even detected in a symptomless sample. Beauvericin and moniliformin were quantified at 5 wpi in the symptomatic onion tissues with Fpr047 and Fpr049, but not with FUS16163, which was unable to produce moniliformin and produced less beauvericin than the other F. proliferatum isolates also in the rice culture. The results showed that onions infected with F. proliferatum can contain high amounts of fumonisins.