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Onion Diseases - An Overview

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

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 diganostic lab.

foliar diseases on onion
Botrytis 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.

Downy 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.

Spores 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.

Plant 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.

Late season 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 (www.nevegetable.org).

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).

During 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.

Maintain 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.
Published: July 2009. Revised April 2015
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