Standardization of challenge inoculation protocols for artificial infection of Fusarium acutatum and Fusarium falciforme causing basal rot of onion

Fusarium basal rot (FBR) severely reduces onion production by causing substantial pre- and post-harvest losses, resulting in shortage of onion during the lean period. Among many reported Fusarium species causing FBR, the Fusarium acutatum and Fusarium falciforme have become more aggressive with their increasing prevalence. The current study evaluated and compared the five different 'onion bulb inoculation methods' to induce F. acutatum and F. falciforme mediated bulb rotting under controlled laboratory conditions. The methods tested included: (1) cotton swab impregnated with spore suspension, (2) pinprick at the basal plate followed by dipping in spore suspension, (3) a combination of pinprick, dipping, and cotton swab, (4) disk inoculation, and (5) injection of spore suspension. Onion bulb rotting was monitored for 15 days post-inoculation observing mycelial growth, tissue necrosis, affected bulb surface area and affected bulb tissue volume, histopathology, and disease rating. Depending upon effectiveness of methods, the bulb rot affected area ranged from 32 to 70 cm² for F. acutatum and from 64 to 81 cm² for F. falciforme, while the volume of affected bulb tissue ranged from 17 to 55 cm³ for F. acutatum and from 48 to 69 cm³ for F. falciforme. The affected surface area, affected volume, rotting over control and disease rating in Cotton swab method was 48 cm², 31 cm³, 56%, 7 for F. acutatum and 80 cm², 67 cm³, 92%, 9 for F. falciforme, respectively, which indicated the pathogenicity of F. falciforme was higher than that of F. acutatum across all five inoculation methods (Methods 1-5). Further, we observed that F. acutatum caused rot within 3 days of exposure while symptoms in F. falciforme appeared on 5th day, however the extent of the tissue damage was higher. In our findings, the 'cotton swab method' was found best for creating the artificial infection on the onion bulbs followed by 'Pinprick at the basal plate and dipping in spore suspension method' in comparison to other methods. The results of this study would pave the way to onion breeders and pathologists to employ suitable screening method for mass screening of onion genotypes for FBR resistance/tolerance.