Long-term survival of Sclerotinia sclerotiorum sclerotia and Verticillium longisporum microsclerotia in soil and the effects of soil depth, soil temperature, and a biocontrol agent

Abstract

This long-term study, conducted over 6 to 12 years in two locations in Germany, investigates the longevity of dormant structures of two significant soil-borne pathogens - Sclerotinia sclerotiorum and Verticillium longisporum - under different conditions in soil in the field, considering exposure duration, soil depth, soil temperature, and a biocontrol agent. When sclerotia of S. sclerotiorum were buried free in soil a significant reduction in viability occurred within the first year, declining to about 20 %, with retrieval rates dropping to 30 %. However, in another experiment when sclerotia were exposed in litter bags, they persisted for more than four years, with viability still above 45 % and retrieval rates reaching up to 64 %, indicating a crucial role of the soil arthropods in degradation Soil depth did not significantly affect longevity, but elevated soil temperatures were associated with higher survival, suggesting that climate warming could exacerbate inoculum persistence. Biocontrol trials with Coniothyrium minitans (Contans®WG) revealed that, without application, sclerotial viability and retrieval rates remained as high as 90 % in the first year, while with its application, these rates dropped to below 4 % within the same time. For V. longisporum, microsclerotial viability declined with a significant drop after one year. Soil depth and soil temperature had no consistent effect on survival of microsclerotia, although an interaction between year and soil depth was noted. Despite overall reductions, a small but viable portion of inoculum from both pathogens persisted until the end of the experiments after 6 to 12 years. These findings highlight the long-term risks posed by these soilborne pathogens and imply that even long crop rotation cycles with non-susceptible crops are not sufficient to completely eradicate these pathogens.