Dear Jane

Many thanks for your recent enquiry regarding the prevention and control of rust on onions.

Unfortunately we do not stock any suitable sprays or chemical remedies for this problem, mainly because there are very few, if any, available to the amateur gardener. However, I can offer some growing and general garden husbandry advice which might prevent the disease from making an appearance in your father’s garden.

Onion rust is better known as leek rust but it afflicts all members off the allium family (that’s leeks, onions, garlic, chives and other ornamental plants). It’s a fungal disease which spends its entire lifecycle on leeks or other members of the allium family and is a particular menace during summer and mild autumn periods. Keeping the garden clean and tidy in the following ways will certainly help restrict the disease;

• Remove and burn any infected plants
• Check the garden for any nearby members of the allium family, such as the ornamentals mentioned above, as the harbour the fungal spores overwinter and between cropping
• Remove any rogue or ‘volunteer’ onions, leeks, garlic or chives for the above reason
• Don’t crowd the plants when sowing or planting up your bed; instead, allow a generous spacing so air can circulate freely
• Remove any leaves or plant debris promptly from the crop
• Try and grow varieties with some rust resistance
• Add plenty of potassium to the soil as the rust tends to be more prevalent in crops grown on nitrogen-heavy soils
• If space allows, practice a lengthy 4-5 year crop rotation

I hope this information helps; apologies for not being of more direct assistance on this occasion and I hope your father enjoys a healthy crop of unblemished onions this coming year! 
Martin