The incidence and control of Botrytis spp. on onion seed in New Zealand : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science with Honours in the University of Canterbury [Lincoln College

Abstract

Botrytis allii is one of several Botrytis species causing diseases of Allium species. It causes the disease known as grey-mould neck rot which is one of the major bulb destroying diseases of the onion crop. It has caused significant losses average 10 per cent in most seasons but under conditions favourable to disease development, bulb losses of 50 per cent or more have been reported by growers. (Maude and Presly, 1977a). Tichelarr (1967) reported similar losses in Holland with averages of 10 to 20 per cent and up to 60 per cent loss of bulbs occurring. Losses caused by B. allii have also been reported in other countries including America and New Zealand.

Two other related species may incite a neck rot of onion bulbs and Walker (1926) gave them the distinctive names of mycelial neck rot (B. byssoidea) and small sclerotial neck rot (B. squamosal) in order to differentiate them from grey-mould neck rot (B. allii) . Grey-mould neck rot is the major form of the disease on mature onion bulbs in New Zealadn (Brien et al., 1959) and overseas. Thus neck rot as caused by B. allii has been considered primarily to be a disease of mature bulbs which becomes evident in store, although it has been reported to cause some seedling mortality and some reported to cause some seedling mortality and some reduction of seed set in infected seed crops (Ellerbrock and Lorbeer, 1977). However, losses of bulbs in store are of primary significance because of the high cost of inputs which go into producing and storing each bulb. New Zealand exports mature bulbs and as infected bulbs are not easily identified at the time of harvest, a considerable reduction in the quality of the sample may occur as the disease progresses during transport and storage. Not only will there be a direct loss of bulbs, but the sample may be re-priced because moisture release from rotting bulbs may cause sprouting and a reduction in quality of the sample.

Control of the disease has, in the past been difficult and expensive. Prior to 1970, control largely involved the manipulation of post-harvest conditions so as to minimise fungal development in store. However, in 1970, the significance of the fact that the fungus may be seed-borne was realised and the use of benomyl seed-treatments considerably reduced neck rot incidence in store (Maude and Presly, 1977b).

The experiments described in this dissertation were designed to investigate the following: (1) The growth of Botrytis spp. In culture (2) The incidence of B. allii in onion seed-lines produced in New Zealand (3) The transmission of B. allii from seed to seedling (4) The testing of a range of selected fungicides for their potential in controlling B. allii.