Protective Effects of Saccharomyces cerevisiae Isolates Against Fusarium in Onion Seedlings

Onion (Allium cepa L.) is a major horticultural crop worldwide due to its nutritional and economic importance. In Argentina, San Juan offers optimal agroecological conditions for onion seed production, but fungal diseases, mainly caused by Fusarium species, severely affect seed yield and quality. This study evaluated native enological yeasts as biocontrol agents against Fusarium in onion seedlings. Ten Fusarium isolates obtained from local mother bulbs and seeds were identified as Fusarium annulatum, and their pathogenicity was assessed in seed germination assays. Fifty-five yeast isolates were screened in vitro by dual-culture assays, and the most antagonistic ones were further evaluated by quantifying their populations recovered from onion radicles and their protective effect on seedlings. In pathogenicity tests, F. annulatum isolates reduced the percentage of normal seedlings to 18–38%. Ten Saccharomyces cerevisiae isolates inhibited at least 80% of the F. annulatum isolates in vitro. Among them, S. cerevisiae PB71 and PB78 showed the highest populations recovered from radicles 12 days after seed inoculation. Their protective effect depended on the yeast–pathogen combination: they increased seedling survival by up to 41.7%, with significant protection against three and two F. annulatum isolates, respectively, and they improved several growth parameters in surviving seedlings, particularly root length and biomass in the presence of isolate BC14. These findings identify S. cerevisiae PB71 and PB78 as promising candidates for the sustainable management of Fusarium diseases in onion production.