Long-term biosanitation by application of Coniothyrium minitans on Sclerotinia sclerotiorum infected crops

The effect of the fungal mycoparasite Coniothyrium minitans applied as a sp ray to crops infected with Sclerotinia sclerotiorum (causal agent of white mold) on contamination of soil with S. sclerotiorum sclerotia was studied i n a 5-year field experiment. Sclerotial survival also was monitored during two subsequent years, when the field was returned to commercial agriculture . In a randomized block design, factorial combinations of four crops and th ree treatments were repeated 10 times. Potato (Solanum tuberosum), bean (Ph aseolus vulgaris), carrot (Daucus carota), and chicory (Cichorium intybus), which are all susceptible to S. sclerotiorum, were grown in rotation. Plot s were treated with C. minitans or Trichoderma spp. or were nontreated (con trol). Crops were rotated in each plot, but treatments were applied to the same plot every year. After 3 years during which it showed no effect on scl erotial survival, the Trichoderma spp. treatment was replaced by a single s pray with C. minitans during the fourth and fifth years of the trial. The e ffect of treatments was monitored in subsequent seasons by counting apothec ia as a measure of surviving S. sclerotiorum sclerotia and scoring disease incidence. Trichoderma spp. did not suppress S. sclerotiorum, but C. minita ns infected at least 90% of S, sclerotiorum sclerotia on treated crops by t he end of the each season. C. minitans lowered the number of apothecia comp ared with the other treatments during the second year after the bean crop. C. minitans reduced the number of apothecia by approximate to 90% when comp ared with the control and Trichoderma spp. treatments and reduced disease i ncidence in the bean crop by 50% during the fifth year of the trial, result ing in a slightly higher yield. In 1993, but not 1994, a single spray with C. minitans was nearly as effective at reducing apothecia as three sprays ( monitored in 1995). The final population size of sclerotia in soil at the e nd of the 7-year period was lower in all C. minitans plots than at the begi nning of the trial, even in plots where two highly susceptible bean crops w ere grown during the period. The results indicate that the mycoparasite C. minitans has the potential to keep contamination of soil with sclerotia low in crop rotations with a high number of crops susceptible to S. sclerotior um.