The Online Gardener's Handbook 2010
Chapter 4: Vegetables
Onion, Leek

Table of Contents

  1. Introduction
  2. Downy Mildew, Purple Blotch, Botrytis Leaf Blight
  3. Leek Moth
  4. Onion Maggots
  5. Thrips
  6. Learn More


In this chapter, a description of various onion and leek pests will be provided along with suggested management options. These management options will not include the use of pesticides. Some biopesticides and certain reduced risk pesticides are still available to the homeowner for controlling weeds and pests in lawns and gardens. For more information, refer to Chapter 2 of this handbook and the Ministry of the Environment's website. For suggestions on managing specific weeds and pests, consult local horticulturalists, Master Gardeners or your local garden supply centre.

Downy Mildew, Purple Blotch, Botrytis Leaf Blight

Downy mildew, purple blotch and Botrytis leaf blight are common fungal diseases, causing leaf spotting and discolouration, particularly on Egyptian tree onions, onions grown from sets, Spanish onions and multiplier bulbs. Under humid conditions, fuzzy fungal growth may be observed. The diseases favour wet conditions.

Management Options

Destroy diseased onion leaves and discarded bulbs at harvest time. Plant in open, sunny locations and avoid overhead watering. Do not apply too much nitrogen. Dry cooking onions properly for winter storage can discourage bulb rot.

Leek Moth

The leek moth, or onion leafminer, is an invasive pest from Europe that damages species in the allium family. In Eastern Ontario and Quebec, leek moth damage has been recorded in garlic, leek and onion fields. The larvae tunnel mines and create pinholes in leaf tissue and scapes, sometimes causing leaf distortion. They occasionally attack the bulb and stem. Damage to garlic cloves may predispose them to diseases. Larvae are yellowish-green with small, greyish spots and a pale brown head. The adult is a small, reddish-brown moth with a white triangular mark on the middle of the folded wings. Adults emerge in the spring when temperatures reach 9.5°C, mate and lay eggs. There are three generations per season, with damage typically increasing as the season progresses.

Management Options

Remove and destroy old and infested leaves, and destroy all plant debris following harvest. Inspect plants for pupae or larvae and destroy them. Early harvesting can help to avoid damage by the last generation of larvae and reduce population build-up. Use of floating row covers can help protect developing plants from leek moth damage.

Onion Maggots

Onion maggots can cause severe damage every year in some areas but not in others. Be guided by local experience. In May, the first-generation flies lay their eggs near onion plants, seldom near leeks. The white maggots attack the lower stem of the plant soon afterwards and tunnel inside, killing many plants. Yellowing of leaves is usually the first sign of a problem, and indicates that maggots are feeding on the roots. Maggots from second generation flies cause less damage during late July and early August and plants are seldom killed. The last generation eggs hatch in mid-September.

Management Options

Plant onion sets (small, dormant bulb) after June 1 to escape most first-generation maggots. Do not sow seeds at this time, as it is too late. Buy sets early and store them in a refrigerator to keep them in good condition until planting time. If all affected onions are collected and destroyed, there will be very few maggots to overwinter. Follow proper garden sanitation procedures.


Thrips cause stippling or streaking on leaves. The small insects are seldom observed but can be found in the centre of the plant or in the folds of the leaf. Spanish onions and green onions are particularly susceptible. On Spanish onions, severe thrips injury promotes secondary bacterial rots. For more information, see the Thrips section of Chapter 1.

Management Options

Thrips may migrate in from weedy areas and grasslands, so try to avoid planting near these areas. A spray of water will help reduce thrips populations. Vigorous plants are more resistant to damage so ensure plants are not stressed. Follow garden sanitation procedures.

Learn More


For more information:
Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 21 July 2005
Last Reviewed: 25 June 2010