Effective, Consistent, and Rapid Noncontact Application Methods for Seedling Basal Stem Infection by Sclerotinia sclerotiorum .

Sclerotinia sclerotiorum (Lib.) de Bary, an economically devastating soilborne fungal pathogen known to cause disease across a wide range of plants, produces long-term inoculum called sclerotia that can germinate either carpogenically by ascospores infecting aboveground plant parts or myceliogenically to infect stem base and roots. Typically, for research purposes, S. sclerotiorum diseases are initiated by direct contact methods, using S. sclerotiorum mycelium agar plugs wrapped around the stem or sclerotia placed directly beneath root mass. However, reproducible noncontact methods leading to basal stem infection are not currently available. Therefore, the objective of this study was to develop effective noncontact protocols that consistently generate basal plant stem infection from S. sclerotiorum in the soil. Using three host plant species (canola, lupin, and lettuce), we determined two methods that reliably produced basal stem infection. The first method, where mycelial agar plugs were positioned just below the soil surface at a distance of 5 mm from each seedling, led to 100% infection in all plants. The second method used pathogen-infested soil by mixing the soil with dry inoculum in the form of a powder prepared from mycelium-colonized organic substrates. Four substrates consistently produced 100% seedling infection at 4 days after inoculation (DAI): wheat bran, wheat grain, red rice, and hulled millet. In contrast, chia, canary, sesame, and ryegrass seed substrates resulted in less than 50% seedling infection at 10 DAI, and infection levels did not progress further. The two soil inoculation methods outlined in this study will enhance future research on the progression of S . sclerotiorum diseases, with the potential to screen disease-resistant host genotypes to basal S . sclerotiorum infection and, in particular, to test the effectiveness of soil applications of fungicides or biocontrol agents against S . sclerotiorum basal infection.