Effect of inoculum rates and sources of Coniothyrium minitans on control of Sclerotinia sclerotiorum disease in glasshouse lettuce

Coniothyrium minitans isolate Conio grew on both maizemeal-perlite and ground maizemeal-perlite, producing high numbers (1.6x10(7) conidia g(-1) inoculum) of germinable conidia. Coniothyrium minitans isolate Conio applied as a preplanting soil incorporation of maizemeal-perlite inoculum at full application rate (0.6 l m(-2); 10(1)1 colony forming units (cfu) m(-2)) significantly reduced Sclerotinia disease in a sequence of three lettuce crops grown in a glasshouse. No reduction in disease was achieved with any of the reduced rate treatments (10(8) cfu m(-2)) of a range of C. minitans isolates (Conio ground maizemeal-perlite at reduced rate, Conio and IVT1 spore suspensions derived from maizemeal-perlite, IVT1 spore suspension derived from oats and Contans(R) WG spore suspension). After harvest of the second and third crops, C. minitans maizemeal-perlite at full rate reduced the number and viability of sclerotia recovered on the soil surface and increased infection by C. minitans compared with spore suspension and reduced rate maizemeal-perlite inocula. Coniothyrium minitans was recovered from the soil throughout the trial, between 10(5) and 10(7) cfu cm(-3) in maizemeal-perlite inoculum full rate treated plots and 10(1)-10(4) cfu cm(-3) in all other inoculum treated plots. Pot bioassays were set up corresponding to the inoculum used in the glasshouse, with the addition of Conio ground maizemeal-perlite at a rate corresponding to the full rate maizemeal-perlite. Coniothyrium minitans maizemeal-perlite and ground maizemeal-perlite at full rate significantly decreased carpogenic germination, recovery and viability of sclerotia and increased infection of sclerotia by C. minitans in comparison with spore suspension treatments, reflecting results of the glasshouse trials. Additionally, reduced maizemeal-perlite treatment also decreased apothecial production, recovery and viability of sclerotia compared with the spore suspension treatment, despite being applied at similar rates. Simultaneous infection of sclerotia by several isolates of C. minitans was demonstrated. Inoculum level in terms of colony forming units cm(-3) of soil appears to be a key factor in both control of Sclerotinia disease and in reducing apothecial production by sclerotia.