onions

[josterberry]

I have an allotmentand some of the gardeners onions are covered in a dark powdery substance can you tell me what it is

[Ethel]

Could be chicken manure, bonemeal, bloodmeal or any sort of fertiliser.

Best ask them - gardeners always love to give advice.

[Clanad]

It may well be powdery mildew or downy mildew (no matter, they are both caused by related fungi). It'll be dark or light grey in color and looks.... well, powdery or feathery.
One can buy sulfur based fungus control safe for vegetables or make your own thusly:
1 gallon lukewarm water
7 teaspoons (20 grams) of bicarbonate of soda
1 to 2 ounces (1/4 cup) ordinary liquid dish soap
Mix all ingredients into a good lather, place in a spray bottle and soak the entire plant top. Repeat as neccessary.
Best of luck!