Biological control of onion neck rot (Botrytis aclada): Protection of wounds made by leaf topping

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Abstract
The efficacy of fungal antagonists in protecting onions from neck rot caused by Botrytis aclada was investigated. Leaf wounds made by topping of onions during harvest, which are considered as important entrance sites for B. aclada, were treated with conidial suspensions (5 × 107 conidia ml‐1) of antagonists. In field experiments with artificial inoculation with conidia of B. aclada, applications of Trichoderma viride during harvest reduced the incidence of neck rot, assessed after three months’ storage of the onions at 9° C, from 35% to 24% in 1989 and from 28% to 18% in 1990, when onions initially had been stored under favourable conditions for fungal development. Comparable results were obtained with T. hamatum and Gliocladium roseum. A bioassay based on wound treatment of detached onion leaves was developed to select further antagonists. From 40 candidate antagonists tested, 20 strains gave similar or better control than the strain of T. viride applied in the field experiments. Effective antagonists could be found amongst strains of Trichoderma spp. Gliocladium spp. and Penicillium spp. as well as amongst fungi such as Aureobasidium pullulans and yeasts isolated from green leaves of onion or rye.