Dorothy Stratton, of Mosgiel, asks :-
When I was at Auckland University, one of the lecturers was doing research into fungal diseases of onions. Now, 25 years later, I can buy onions, either pre-packed or ones I select myself, and a few days later I have to throw one or two away because they are mouldy. Has any progress been made in this area?
Bob Fullerton, a plant pathologist with Plant and Food Research, responded.
I was one of a group who started working on white rot disease of onions in 1984. That however is not the disease that your enquiry relates to.
There are two common types of mould in onions, Black Mould caused by the fungus Aspergillus niger, and blue mould caused by various species of Penicillium. Black mould tends to be more of a problem in warmer areas and is the most common mould in onions grown in the North Island. Blue mould tends to be found more commonly in cooler climates and is, I understand, relatively common in South Island onions. In each case the spores come from the soil. The bulbs get covered in dust (containing spores) during the lifting, curing and harvesting process. Thereafter if they are stored in relatively humid conditions (or if not properly cured) the moulds develop down between the outer few layers of storage scales.
There is really nothing that can be done to 'control' these disorders. In the case of black mould, keeping onions cool and dry (by forced air ventilation of storage sheds) will help reduce the incidence in warm areas or climates. In the case of blue moulds (Penicillium spp.) probably all that could be done is to ensure the onions are properly cured (or well dried if artificial drying is used). Growers have to be careful with artificial drying because if they keep the onions at high temperature for too long during drying they can actually cause black mould (the warm temperature mould) to develop.
In New Zealand, onions are stored from harvest (usually from December through to March) and released progressively onto the market all year until the next harvest so there is ample time for storage disorders to develop. The incidence of mould in onions in supermarkets or fruit shops will vary according to the source of the onions being stocked, some supplies being worse affected than others. There is no easy way to tell visually whether an onion is mouldy inside though you can gently squeeze the bulb near the neck and if it is a bit softer or spongier than others then it may be affected.
Moulds in bulbs are not regarded by the industry as a major problem and no research is being done to control them.