Influence of benzimidazole fungicides on incidence of Botrytis allii infection of onion leaves and subsequent incidence onion neck rot in storage in Tasmania, Australia

Neck rot of onion caused by Botrytis spp., including B. allii, has previously been controlled in Australia with the fungicide Benlate (benomyl). Production of Benlate has recently ceased, therefore a field trial was conducted to examine the efficacy of alternative benzimidazoles fungicides. The trial compared 2 carbendazim fungicides (Marvel and Spin Flo) at 3 rates of application with Benlate applied at commercial rates and to non-treated plots in a randomised complete block design with 4 replicate plots per treatment. Fungicides were applied at 89, 96, 112 and 119 days after sowing. Plots were sprayed with inoculum consisting of a suspension of B. allii conidia 103 days after sowing. The incidence of B. allii infection in leaves was estimated 10 times during the season by collection and incubation of leaf samples. Fifty-six days after inoculum application the mean incidence of B. allii in leaves from fungicide treatments ranged from 0 to 10%, significantly lower (P < 0.05) than that of non-treated plots (28.8%). The mean incidence of neck rot in bulb samples after 3 months of storage ranged from 1.0 to 9.9% in fungicide treatments, significantly (P < 0.05) lower than that of non-treated plots (63.4%). The incidence of B. allii leaf infection in plots sampled at different times during the season and the incidence of neck rot in storage were all significantly correlated (r = 0.42-0.61, P < 0.01), except prior to application of inoculum.