The Online Gardener's Handbook
Chapter 4: Vegetables
Table of Contents
- Downy Mildew, Purple Blotch, Botrytis Leaf Blight
- Leek Moth
- Onion Maggots
- 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.
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.
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.
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 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.
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
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.