An investigation into the use of fungicides for the control of Leptosphaeria maculans and L. biglobosa in New Zealand Brassica napus ssp. napobrassica crops

Abstract

Three experiments were conducted to assess the efficacy of six fungicides to inhibit mycelial growth and conidial germination of Leptosphaeria maculans and L. biglobosa on swede crops in New Zealand. In experiment one, mycelial discs were grown on agar amended with azoxystrobin, epoxiconazole, carbendazim, fluquinconazole, flusilazole and iprodione at a range of concentrations for 12 days. Experiment two involved placing a 50 μl drop of conidial spore suspension on the amended agar and counting the percentage of spores which germinated after 45 hours. Experiment three involved azoxystrobin, fluquinconazole and epoxiconazole applied at field rate to a susceptible and resistant swede variety inoculated with conidial spore suspensions. Mycelial growth EC50 ranged from 0.152 mg a.i. / L to greater than the maximum concentration applied (3.0 mg a.i. / L). Azoxystrobin provided the greatest control, with epoxiconazole and flusilazole also effectively inhibiting growth of the isolates. Iprodione did not inhibit 50% of the growth at the concentrations tested and was not continued for experiment two. Germination inhibition EC50 values for the fungicides ranged below and above the minimum and maximum concentrations tested. Azoxystrobin provided the greatest conidial germination inhibition (EC50: <0.003 mg a.i. / L), with epoxiconazole, fluquinconazole and flusilazole ranging from 0.028 to 0.224 mg a.i / L EC50 for L. maculans. Carbendazim did not reach 50% inhibition at the concentrations tested. Azoxystrobin, epoxiconazole and fluquinconazole inhibited Leptosphaeria spp. infection of both the resistant and susceptible swede varieties in the pot experiment. This work has provided information on the potential to control both Leptosphaeria maculans and L. biglobosa infection of swede in New Zealand using fungicides, which warrant further testing under field conditions.