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 <i>Botrytis</i> spp., including <i>B. allii</i>, 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 <i>B. allii</i> conidia 103 days after sowing. The incidence of <i>B. allii</i> 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 <i>B. allii</i> in leaves from fungicide treatments ranged from 0 to 10%, significantly lower (<i>P</i> < 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 (<i>P</i> < 0.05) lower than that of non-treated plots (63.4%). The incidence of <i>B. allii</i> 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, <i>P</i> < 0.01), except prior to application of inoculum.