How did just one inner layer of this onion (see photo) turn rotten while the surrounding layers were unaffected?
Ask three experts, get three different answers… We suspect more evidence may be needed to choose between them – Ed
• This is not rot in the pathological sense, but a breakdown of the cells of the onion ring known as internal browning. There is no disease present, and the rest of the onion is safe to eat.
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“This is not a rot in the pathological sense, but a breakdown of the cells of the onion ring”
Internal browning is caused by a lack of calcium in the developing tissue, leading it to collapse. This calcium deficiency is down to the onion experiencing severe drought conditions at some stage during its growth.
Calcium is only sparingly soluble in plant fluids, and although onions are generally drought-resistant, a prolonged dry period, such as that which the UK experienced last year, would have prevented the plant from both taking up calcium and moving it to where it was needed for growth and development. Acidic growing conditions would make the problem worse. As soon as the plant is able to obtain calcium, however, growth proceeds normally.
Internal browning affects many plants, including brassicas, lettuces and apples, and I have even observed similar problems in the flowers of some shrubs. The remedy is to maintain calcium levels in soil by applying lime, and minimising the effects of drought. Under normal growing conditions, an onion crop will require rainfall of 3 millimetres per day to avoid such problems.
J. A. Crofts, Nottingham, UK
• The darker section in this onion is due to a developmental error as the bulb formed.
An onion plant has eight or nine leaves and near the end of the growing season, the bases of the inner leaves swell and store large amounts of starch and some sugars. These swollen leaves then form the bulb or onion. The outer couple of leaves die and dry out to form the thin papery leaves that protect the food store inside.
In this case the dry leaves have formed as usual on the outside, then two or three food store leaves have developed and swollen. But a developmental accident has occurred and the next leaf in has dehydrated to start forming papery outer leaves. Within this there are further food store leaves and the shoot tip with next year’s immature leaves.
This developmental abnormality occurs infrequently, but I have seen it before.
P. Scott, Hove, East Sussex, UK
• Onion bulbs consist of concentric leaf bases, the upper parts of which extend out of the top of the bulb, called the neck, where they are green. Onions can get a disease called neck rot, caused by , which lives on dead leaves while the onion is growing, but becomes pathogenic at the end of the growing season, moving down through the leaves and into the bulb, where it causes them to rot.
The reason that only one layer, or leaf base, in the onion has rotted is because only that particular leaf has a B. allii infection. It has been unable to penetrate the epidermis of the adjacent leaf bases so has not been able to infect them – yet.
Charles Merfield, Canterbury, New Zealand