The maggot of the Onion fly, Anthomyia ceparum, is a terrible thorn in the side of kitchen gardeners. It does not, as a rule, do much damage in wet seasons, because the eggs laid by the fly on the leaves of young Onion plants are washed off and destroyed; but in dry seasons it often decimates the crop, or even ruins it absolutely. Summer sown Onions, and box sown Onions raised in January or February, are never so badly attacked as spring sown plants, and it may be taken as a fact that the stronger the plants the less they are infested. It is a first-rate plan to spray soaparite (see Celery) over young Onions once a fortnight from the middle of April to June, choosing the evening. Little trouble is involved, and if the foliage is well coated with the mixture the fly will not lodge. All sorts of applications have been recommended, prominent among them being soot and salt, but I have found soaparite to be the best of all.
Onions sometimes come to a standstill quite suddenly in summer owing to being affected by a fungus, which causes greyish patches on the leaves, and makes them look as if they had been scorched. If the plants are seriously attacked they make no more growth, and at the first sign of the fungus the plants should be dusted with freshly slacked lime. I may, however., say that I have never known this disease affect Onions growing in thoroughly cultivated, deep soil.