Potential for integrated control of Sclerotinia sclerotiorum in glasshouselettuce using Coniothyrium minitans and reduced fungicide application

All pesticides used in United Kingdom glasshouse lettuce production (six fu ngicides, four insecticides, and one herbicide) were evaluated for their ef fects on Coniothyrium minitans mycelial growth and spore germination in vit ro agar plate tests. Only the fungicides had a significant effect with all three strains of C. minitans tested, being highly sensitive to iprodione (5 0% effective concentration [EC50] 7 to 18 mug a.i. ml(-1)), moderately sens itive to thiram (EC50 52 to 106 mug a.i. ml(-1)), but less sensitive to the remaining fungicides (EC50 over 200 mug a.i. ml(-1)). Subsequently, all pe sticides were assessed for their effect on the ability of C. minitans appli ed as a solid substrate inoculum to infect sclerotia of Sclerotinia sclerot iorum in soil tray tests. Despite weekly applications of pesticides at twic e their recommended concentrations, C. minitans survived in the soil and in fected sclerotia equally in all pesticide-treated and untreated control soi l trays. This demonstrated the importance of assessing pesticide compatibil ity in environmentally relevant tests. Based on these results, solid substr ate inoculum of a standard and an iprodione-tolerant strain of C. minitans were applied individually to S, sclerotiorum-infested soil in a glasshouse before planting lettuce crops. The effect of a single spray application of iprodione on disease control in the C. minitans treatments was assessed. Di sease caused by S. sclerotiorum was significantly reduced by C. minitans an d was enhanced by a single application of iprodione, regardless of whether the biocontrol agent was iprodione-tolerant. In a second experiment, diseas e control achieved by a combination of C. minitans and a single application of iprodione was shown to be equivalent to that of prophylactic sprays wit h iprodione every 2 weeks. The fungicide did not affect the ability of C. m initans to spread into plots where only the fungicide was applied and to in fect sclerotia. These results indicate that integrated control of S. sclero tiorum with soil applications of C. minitans and reduced foliar iprodione a pplications was feasible, did not require a fungicide tolerant isolate, and that suppression of Sclerotinia disease by C. minitans under existing chem ical control regimes has credence.