Effects of onion (Allium cepa) plant maturity at harvest and method of topping on bulb quality and incidence of rots in storage

A field study was carried out over two growing seasons to investigate the effects of physiological maturity of onions (Allium cepa L.) at harvest and different topping methods on bulb colour, skin retention, and the incidence of storage rots. Onion plants were lifted at five different stages of maturity from 0 to 4 weeks after 50% leaf collapse (top-down). Foliage was removed from the bulbs (topped) either before or after field-curing. The effects of time of lifting of onion plants and method of topping on the incidence of bulb storage rots were the same in both seasons. Onions that were lifted 3 weeks after 50% top-down and topped before curing had the greatest incidence of rots in store. Increasing harvest maturity increased the mean skin colour score of onions, and decreased markedly the mean number of intact outer skins. The timing of foliage removal had no effect on mean skin colour score, but onions that were topped before curing had slightly more bulb skins than onions topped after curing. Timing of onion lifting to optimise bulb quality appears to be a trade-off between skin retention and colour. These results confirm that traditional method of harvesting onions in New Zealand, where onions are lifted at 60-80% top-down, the bulbs are field-cured, and the foliage is removed after curing, is the simplest method and best compromise to ensure postharvest onion quality and successful storage.