|Onions are prone to several different diseases, especially in
wet growing seasons. These often get started on their leaves, and if
severe, can reduce bulb growth and yield. Another way onion
diseases can cause damage is when they infect bulbs later in the
season, which may lead to losses in storage.|
The following is an
overview of onion diseases that are common in the Northeast. Much
of the information was adapted from an excellen fact sheet by H. F. Schwartz, Colorado State University Extension.
Three Common Diseases.
Although they can be hard to tell apart without careful examination,
there are three different onion diseases commonly seen in commercial
fields in the Northeast and other temperate growing areas: botrytis,
downy mildew, and purple blotch. The onions pictured below
may have all three of these foliar diseases. The best way to get a
positive identification of the diseases present is to send sample to a
diseases of onion are caused by several different species of Botrytis,
leading to neck rot, gray mold, or leaf and flower blast of onions and
garlic. Another type of botrytis can infect onion seed heads and cause
brown stain on bulbs.
Botrytis spores kill leaf cells, causing
a small, yellow to white, oval, sunken spot on green foliage, usually
late in the season. Soil-line lesions may also develop. Heavy
infections lead to rapid browning and death (blast) of onion tops,
reducing bulb size. The fungus sporulates on leaf tissue and can then
spread to other plants and fields.
Neck rot symptoms
characteristic of botrytis often appear after bulbs are stored for
several weeks. The fungus grows down through the inner scales and may
partially rot the bulb before external injury appears. Infected scales
become soft, brownish and spongy. Gray mold or thin and
irregularly-shaped black sclerotia (like small hard peas) may form
between scales or, more commonly, at the neck area. The neck area
becomes sunken and the entire bulb can become dried out. Secondary
invasion by soft rot bacteria may cause a watery rot.
mildew of onion is caused by Peronospora descructor. Symptoms appear on
older leaves as oval patches that vary in size and are slightly paler
than the rest of the foliage. With moisture, these areas become covered
with violet-gray fungal strands (mycelium) that contain spores which
can spread to healthy tissue. The infected areas may be violet to
purple, so they are easily confused with the initial oval lesions of
purple blotch. Leaves gradually become pale green and later yellow.
Diseased parts, such as leaf tips, fold over and collapse. Infected
bulbs become soft, shrivelled and watery.
Purple blotch of onion
is caused by the fungus Alternaria porri. It also causes disease in
leek, garlic and chives. Spores germinate on onion leaves and
produce a small, water-soaked spot that turns brown. The oval-shaped
lesion enlarges, becomes purplish, and forms the target spot appearance
that is typical of alternaria on many other crops (like early blight on
tomato). The margin may be surrounded by a yellow zone. During moist
weather, the surface of the lesion may be covered by brown to black
masses of fungal spores.
Lesions may merge or become so numerous
that they kill the leaf. Leaves become yellow then brown, and wilt
downward two to four weeks after initial infection. Purple blotch
infection often follows the small whitish spots caused by Botrytis, or
injury caused by thrips, hail, wind-blown soil, or air pollution.
may be blown or washed down to the neck and infect the outer scales of
bulbs. A yellow to wine-red, semi-watery decay may occur. Diseased
tissue turns brown to black and dries out in the field or, more
commonly, in storage.
Weather conditions influence which
diseases are likely to be most problematic. Dry weather helps
limit all of these diseases. Warm, moist weather after midseason favors
purple blotch. Cool, moist conditions near harvest favor problems with
botrytis and downy mildew.
Management to prevent these diseases.
As with most pests, a multi-pronged approach is the most effective.
During production, these include: crop rotation, sanitation, weed
management, using disease-free seed and transplants, moderate fertility
programs, and if necessary, fungicides.
Follow a three- to
four-year rotation to with Allium crops to prevent these diseases.
Proper sanitation of onion debris, especially culled onions, is very
important. Incorporate all debris into the soil immediately after
harvest. No exposed culls should be present anywhere in the area when
the next crop is planted. Dispose of culls and trash from storage sheds
at landfills or by burying in deep trenches before spring.
only high-quality onion seed and carefully inspect transplants for
signs of contamination. Follow fertility recommendations carefully and
avoid excess N or late applications of nitrogen. Split nitrogen
applications are recommended. Manage weeds so as to allow good air
movement and thus drying of the crop canopy.
applications of labeled fungicides may provide some foliage protection
and reduce neck contamination, especially when conditions are conducive
to infection. See the New England Vegetable Management Guide
Overwintering and Spread.
These fungi survive on organic matter and previously infected debris in
soil, onion cull piles, and dirt or trash in storage sheds.
Spores can spread to onion foliage and bulbs in the field or storage
shed by wind, water splashing, implements and insects or workers.
Harvest, Curing ad Storage.
Use care during lifting and processing to minimize bruising or cutting
of bulbs. Do not irrigate for 10 to 14 before lifting. Discard
thick-necked onions, scallions, rots, doubles, splits, bruised,
sunburned or frozen bulbs.
Dry necks down before topping and
cure bulbs thoroughly before storing. If additional curing or drying of
bulk or crated onions is required, circulate ambient or warm air (90 to
95 degrees) for five to 10 days or more. Storage decay is reduced by
exposing freshly harvested onions to infrared irradiation for six
minutes (6-inch distance from lamps to onions).
storage, promote air circulation by leaving space between crates or
bulked onions and outer walls of the shed. Do not stack onion bins in
direct sunlight before storing or shipping, because translucent scales
may occur or moisture may accumulate at the necks of bulbs.
the storage temperature at 32 to 40 degrees F, and maintain humidity at
65 to 70 percent. Onions will freeze at 30.6 degrees F. Monitor storage
temperatures regularly. Poor ventilation, high humidity and
temperatures greater than 40 degrees F can produce storage rot.
Condensation on onions brought from cold storage into warm, moist air
can increase rot losses during transportation or display.