Onion black and white

It’s the question you ask when you’ve rooted through the refrigerator and found a carton of strawberries, a few of which have grown a fur coat. It’s the thought that runs through your mind when your kids are starving, you’ve uncapped some yogurt, and you see a few spots of green.

Too moldy?

Most dads of the world will say “It’s fine! Just a little mold! Cut it off and eat it already!”

Germaphobes, on the other hand, will counter with a warning: “In the trash. Now.”

For the first installment of Is This Too Moldy?, we went to a more reliable and reasonable source: Dr. Hassan Gourama, Associate Professor of Food Science at Penn State University.

So, Dr. Gourama—are these foods too moldy?

Photo by Chelsea Kyle

Is this onion too moldy?

“Basically you shouldn’t be eating food with mold on it,” Dr. Gourama warns. But there are some situations where mold can safely be cut off and the remaining food saved. “It depends on the texture,” Dr. Gourama says. Onions being hard textured, they fall into the salvageable camp. “If it’s only a few spores of the mold, you can remove the moldy layers, then wash the onion very well.”

Verdict: Not too moldy! Photo by Chelsea Kyle

Is this cheddar too moldy?

Dr. Gourama has stern warnings about mold and “soft cheese, yogurt, anything that is soft.” With these foods, even if “you remove the surface contamination, if the mold is producing any toxic chemcial, it can easily diffuse into the product, and the mold will not be visible.” This is why any soft food that has visible mold on it should be thrown away—even the parts that don’t appear moldy at all.

But where does cheese fall on the texture scale? Goat cheese is definitely soft. Parmesan is definitely hard. But cheddar? “Hard,” says Dr. Gourama. “You can cut the mold off of cheddar with a clean knife, as long as you cut one inch under the mold.”

Verdict: Not too moldy! Photo by Chelsea Kyle

Is this citrus too moldy?

Okay, we probably didn’t need a doctor’s opinion on this one. But it gave us an opportunity to talk citrus mold in general with Dr. Gourama. If a lemon or orange shows “only small amounts of mold on the peel, and it looks like it’s just on the surface,” you might try removing the mold with a clean cloth dipped in hot water or vinegar. (“Vinegar is a very good antifungal product,” Dr. Gourama says.) You’d then peel the citrus and, if there was no sign of mold on the flesh, proceed. However, any sign of mold on the flesh should be taken seriously. Dr. Gourama would “throw it away.”

Verdict: Too moldy.

Found something in the fridge? Don’t know if it’s too moldy? Hit us up on Facebook or Twitter and we’ll investigate.

Frequently Asked Questions

From cutting an onion to storing onions and everything in-between, these are some of the most commonly asked questions.

Q: How can I reduce tearing when cutting an onion?

A: To reduce tearing when cutting onions, first chill the onions for 30 minutes. Then, cut off the top and peel the outer layers leaving the root end intact. (The root end has the highest concentration of sulphuric compounds that make your eyes tear.)

Q: How many cups of chopped onion will one medium onion yield?

A: One medium onion equals about 1 cup chopped onion.

Learn how to cut an onion here

Q: What is the best way to store onions?

A: Dry bulb onions should be kept in a cool, dry, well ventilated place. Do not store whole onions in plastic bags. Lack of air movement will reduce their storage life. Sweet onions have a higher water content than storage onions, making them more susceptible to bruising, and a shorter shelf life than storage varieties. One way to extend the shelf life of a sweet or high water content onion is to wrap each one in paper towels or newspaper and place them in the refrigerator to keep them cool and dry.

Best practices to store onions here

Q: How do I store whole peeled onions?

A: Whole peeled onions should be properly refrigerated at 40°F or below. (Source: USDA)

Q: After I cut or use part of an onion, how long will it keep?

A: Chopped or sliced onions can be stored in a sealed container in your refrigerator at the proper temperature of 40°F or below for 7 to 10 days (Source: USDA). For pre-cut fresh or frozen products, always use and follow manufactures “use by” dates.

Q: Why do my onions taste bitter after sautéeing?

A: High heat makes onions bitter. When sautéeing onions, always use low or medium heat.

Q: I am worried about having “onion breath.” Is there anything I can do to alleviate this situation?

A: Although onion breath normally comes from eating raw onions, a mild raw onion may cause no odor. Cooked onions leave virtually no odor on the breath. When you are concerned about your breath, use these helpful tips to freshen your breath.

  • Eat a sprig or two of parsley, it’s known as nature’s natural breath sweetener.
  • Rinse your mouth with equal parts of lemon juice and water.
  • Chew a citrus peel.

Q: How do I remove the smell of onions from my hands and/or cooking equipment?

A: Rub your hands or cooking equipment with lemon juice. If your pots or pans are made of aluminum, cast iron, or carbon-steel, rub them with salt instead.

Want to learn some other great uses for onions? Go here.

Q: What should I look for when purchasing onions?

A: When purchasing onions, look for dry outer skins free of spots or blemishes. The onion should be heavy for its size with no scent.

Q: I want to use raw onion. How can I reduce the pungency?

A: To reduce the pungency, sharpness or aftertaste of a raw onion, cut them the way you plan to use them and place into a bowl of ice water to stand for 1 1/2 hours before draining. If time is at a premium, place onions in a strainer or sieve. Run water through onions for at least a minute.

Q: Which color of onion (yellow, red, or white) should I use and does each color taste different

A: Refer to the Color, Flavor, Usage Guide for a chart to help you choose which onion to use in a recipe.

Q: Are onions healthy?

A: Yes. Onions are high in vitamin C and are a good source of fiber and other key nutrients. Onions are fat free and low in calories, yet add abundant flavor to a wide variety of foods.

Garlic Diseases: White Rot


White rot is feared by many garlic growers because it is so difficult to control. Although there are a number of methods to treat white rot once it has occurred, their effectiveness is not reliable. Since any remaining sclerotica can remain dormant in soil for decades, the fungus can render a field uninhabitable for alliums, effectively ending the growth of garlic in that field.

This being the case, prevention is the best way to avoid a white rot infection. Besides carefully monitoring seed and soil introduced to a field, crop rotation of three to four years is essential. It is also advantageous to remove harvest waste from the field and disposing of it by either burning or segregated composting.

Some growers prevent the fungus being introduced by decontaminating their seed stock. Alcohol, bleach and hot water are all used to bathe garlic seed prior to planting. Although this method is usually effective, if the treatment is too prolonged or performed at too high a temperature, the garlic can die.

Once infected, there are a number of organic options for minimizing the he spread of the fungus through a field, although these methods are not always successful. For infections that occur in only small patches, one method is to simply remove the affected plants and the surrounding soil. The diseased material is then disposed of, usually through burning.

If the infection is more widespread, ceasing irrigation of the field during the growing season will dry the fungus out. This method will only minimize the infection, and the remaining sclerotica make the chance of reinfection likely. Conversely, the affected field can also be flooded with water for a number of months, although the practicality and effectiveness of this method are often negligible.


Solarizing is another option for organic growers. Transparent polyethylene is laid over the infected soil during the hottest months, increasing the temperature and killing the white rot. This method is problematic for those growers with larger fields, however, as the soil must be turned over repeatedly down through at least two feet, to ensure as much of the fungus as possible is exposed to the heat. The process is time-consuming and tedious, and often not adequately successful.

Since the fungus is stimulated by allium exudate, a last organic method is to seed a field with allium (preferably garlic) juice solution or powder. The presence of allium material will cause the sclerotica to germinate, but because there is no garlic actually present, the fungus will starve and die. This process can be repeated several times if necessary. Like the previous methods used, this route is time-consuming, and renders the field unusable for at least a season. Even then, there is no guarantee the fungus has been completely eliminated.

Non-organic growers have only a slightly easier time destroying the disease. There are currently three fungicides used to treat white rot: tebuconazole, fludioxonil and boscalid. These chemicals can be tilled into the soil in which the garlic is going to be planted, and also applied into the furrows at the time of planting. Unfortunately, even these fungicides are not 100 percent effective, making prevention the best defense against a fungus as intrepid as white rot.

All MOTHER EARTH NEWS community bloggers have agreed to follow our Blogging Best Practices, and they are responsible for the accuracy of their posts. To learn more about the author of this post, click on the byline link at the top of the page.

Spoiled garlic ok to use?

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Ask Lisa: Dusting of black mold on onion? Wash it off

Q: What is the black substance that appears on the outside of onions? It looks like mold but also seems a little like dirt or soot.

Q: What is the black substance that appears on the outside of onions? It looks like mold but also seems a little like dirt or soot.

— R.H., Westerville

A: That black, sooty-looking substance on the skins of onions is indeed a mold, specifically, Aspergillus niger. It is common on onions, both in the field and once dried, bagged and stored.

The fungus typically is found on the outside skins of an onion, and it is harmless, for the most part.

Simply peel away the skins and give the onion a good washing.

Discard the onion if the mold has grown beyond a little dusting on the outside skins.

While not too harmful if ingested, Aspergillus niger can cause sickness, particularly in those with weakened immune systems. It is not nearly as dangerous as its cousin, Aspergillus fumigatus, a common airborne pathogen that can cause severe respiratory illness.

Ask a food or cooking question by writing Ask Lisa at The Dispatch, 62 E. Broad St., P.O. Box 1289, Columbus, OH 43216; calling 614-461-5529; or sending email to [email protected], with “Ask Lisa” in the subject line. Include your name, address and phone number. (Initials are printed on request.)

Green and white mold on dry part of onion only. What to do?

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Vidalia Onion Extension Blog

Photo Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Creative Commons License licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Quick Facts About Black Mold in Onions

There have been some questions about Black Mold in onions the last couple of weeks. I wanted to share with you what we know about the disease, and some possible considerations on management.

For many of you, this may be old news, but I wanted to share some basic facts about the disease as a refresher for myself and those who aren’t familiar with it.

Background/History: Black mold of onions is a fungal disease. The scientific name of this pathogen is Aspergillus niger. It is found around the world, and it can affect many other crops. The spores that cause the disease are everywhere, in the soil and in the air. It usually only poses an issue with onions when conditions are right for it to develop (hot and humid).

Symptoms: The fungus is first detected as an area covered with a black sooty mass (made up of microscopic spores) on the neck or outer scales, where injury, disease, or toppling of the foliage has caused an opening in the skin. Fungal spores may develop between dry, dead outer scales and the first inner fleshy scales of the bulb.

Disease cycle: The Black Mold fungus grows on dead plant and animal tissues and spores are very common in the air and soil everywhere. Infection of onion bulbs can occur in the field or in the packing shed. Infection frequently occurs through injured tissue, such as from cuts, bruises, or damage from other onion diseases. Temperature and humidity play a crucial role in the development of the disease:

  • The minimum temperature for growth of Black Mold is 590F.
  • The optimum temperature for growth of Black Mold is 820 – 930F.
  • Black Mold will not grow in temperatures greater than 1160F
  • Fungal growth is significantly slowed when the relative humidity is below 76%
  • When the relative humidity is above 81%, growth can occur rapidly (3-6 hours).
  • Free moisture must be present on onions for 6 – 12 hours for infection to occur. This can easily happen in the field, and also in the packing shed when onions “sweat” from cooling or temperature changes.

Past Research and Findings from UGA:

  • Three on-farm trials were conducted in Toombs and Tattnall 2013 and 2014 to see if fungicides could be used in the field to reduce black mold, and to look at how on farm drying of onions affects the disease. None of the products used significantly reduced the disease. Drying onions also increased the severity and incidence of Black Mold. Read the full report here: https://secure.caes.uga.edu/extension/publications/files/pdf/AP%20114_1.PDF
  • In 2012, UGA conducted a trial looking at post-harvest fungicides to control black mold in Vidalia Onions. These products helped control the disease, but logistics/feasibility/cost could prohibit this practice. See the full report here: https://secure.caes.uga.edu/extension/publications/files/pdf/AP%20109_1.PDF
  • There have also been some questions about different storage types (CA, refrigeration, ozone) and if one is better than the other for black mold. Some evidence would suggest that ozone may help, but I have not been able to find any UGA research to support this. If you know of any research from somewhere else that has been done with storage facilities and black mold, please let me know. This may be something we need to look at in the future.


  • In the field, manage your onions for other diseases. These can weaken the plant and create an entry point on onions for Black Mold.
  • Timeliness! Prompt harvest and post-harvest management of onions is critical, especially as temperatures rise later in the season.
  • Minimize bruising or injury of tissue. This includes during harvesting, transport, pre-grading, grading, and packing. Bruises or injury that are not obvious to the naked eye create entry points for the disease.
  • Minimize “sweating” of onions during cooling/drying process. This creates moisture needed for Black Mold to develop.
  • Temperature and Humidity control are critical: Below 600F and under 76% relative humidity is best.
  • Heated drying of Vidalia Onions is necessary in most situations. Keep in mind, however, that this process often makes Black Mold worse (because we dry at temperatures optimal for development of the disease), especially if the onions already have an established infection.

Sources for this info:

Schwartz, Howard F., Mohan, S. Krishna, et al. “Compendium of Onion and Garlic Diseases.” APS Press, 1995.

Onion Skin Black & White Stock Photos

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Allium cepa

Black Mold: (fungus – Aspergillus niger) Black mold is generally a post-harvest disease, although it may be seen on mature onions in the field. The disease can be recognized by the presence of black powdery spore masses of the fungus on the outer scales. High temperatures (85oF-95oF) and moisture favor disease development. Bulbs should be protected from moisture during harvesting and shipping.

Botrytis Leaf Blight/Blast (fungi – Botrytis allii, B.squamosa, and B. cinerea): Botrytis leaf blight or blast occurs sporadically in Texas, usually early in the season. Several species of Botrytis infect onion. Seedlings may be infected (See Photo). Neck rot is caused by B. allii, leaf fleck is caused by B. cinerea, and leaf blight is caused by B. squamosa. White flecks are found along the length of the leaf (See Photo) and usually have greenish halos. With numerous flecks, the tip of the leaf may die. Non-pathogenic causes of flecks can include: cold rain, sleet, and sandblasting. Fungicides used to control purple blotch will also control Botrytis leaf blight.

Downy Mildew (fungus – Peronospora destructor): Downy mildew is an occasional disease of onion in the Lower Rio Grande Valley of Texas. Symptoms consist of white to light green spots on leaves, which later darken. A fuzzy, gray growth is seen on the leaf surface, particularly during periods of high humidity (See Photo). Lesions enlarge and leaf tissue dies. Lesions may resemble those caused by the purple blotch fungus. Fields should be monitored closely, particularly during prolonged cold, wet weather, when the disease is more likely to occur. Fungicides that are highly effective against downy mildew, such as Ridomil and Aliette, should be applied following the first report of downy mildew in the growing area.

Fusarium Basal Plate Rot (fungus – Fusarium oxysporum f. sp. cepae): The disease develops at the base of the bulb, causing it to become soft. A semi-watery decay progresses from the base of the scales upward (See Photo). The disease may not be noticed until after harvest, when the entire bulb is destroyed. The fungus is soilborne and enters the bulb through wounds, insect injuries or through root scars at the base. High soil temperatures (77oF – 82oF) favor disease development. Losses can be reduced with a 4-year rotation out of onions. Cultivars can also vary in resistance.

Leaf Variegation (Chimera): The leaves have distinct yellow or white longitudinal segments (See Photo). Affected plants occur very infrequently. This is a genetic abnormality.

Mushy Rot (fungus – Rhizopus spp.): Bulbs have soft areas around the neck. In the neck area, there is a white fuzzy growth with black speckling. This is a post-harvest problem that occurs when onions that are not properly cured or stored are transported at high temperatures.

Neck Rot (fungus – Botrytis allii, Botrytis sp.): This disease is frequently not noticed in the field because damage usually occurs during transit and storage. Diseased tissue at the base of the crown becomes sunken and watersoaked in appearance. A gray fungal growth later forms on the surface (See Photo), which can be followed by other fungi and bacteria, causing decay. Small, black-resting bodies (sclerotia) can sometimes be found on scales. Careful handling of the crop at harvest and prompt drying of onions with heat and air ventilation are the best means of controlling this disease.

Pink Root (fungus – Phoma terrestris): Pink root is a soilborne disease that affects roots. Diseased roots turn pink, shrivel and die (See Photo). As the plant sends out new roots, they also become infected and die. Affected plants do not usually die, although they may develop tip blight. Severe infection will reduce bulb size. The fungus can be introduced to a field by using transplants grown in infested soil. Once a field becomes infested, the fungus remains in the soil for many years. Soil fumigation has been shown to be an effective, but expensive, control measure. Resistant onion cultivars are one management approach, as is a long rotation out of onions.

Powdery mildew (fungus – Leveillula taurica): Powdery mildew occurs rarely in the Lower Rio Grande Valley of Texas. The earliest symptom is a pale discoloration of the leaf. Circular spots with white, powdery growth eventually occur. There are no control recommendations, since the disease is not a serious problem.

Purple Blotch (fungus – Alternaria porri): The fungus usually infects dead or dying leaf tissue. The first symptoms are small, white, sunken lesions. These lesions develop purple centers and enlarge (See Photo). The infection can encompass much of the leaf, leading to the death of tissue above the lesion (See Photo). The disease can be controlled with fungicides. In the Rio Grande Valley of Texas, the need to apply fungicides can be determined by monitoring leaf wetness during the growing season. Leaf wetness occurs as the result of dew, fog or rain. The action threshold is 12 hours of continuous leaf wetness.

Pythium Root Rot (fungus – Pythium sp.): This disease is most serious with young plants growing under conditions of high soil moisture and cool temperatures. Infected roots become water-soaked and flimsy (See Photo). Not all of the roots of the plant become infected (See Photo). The plants will not usually die, but severe infection can result in small bulbs. The loss of a substantial amount of roots will lead to tip dieback. Planting on raised beds will minimize the impact of this disease.

Root Knot: (see Nematode)

Soft Rot (bacteria – Erwinia carotovora subsp. carotovora and other species): Soft rot is one of the more prevalent causes of loss in storage onions. The soft rot bacterium can enter the neck tissues as plants approach maturity. In the field, plants wilt and die (See Photo). As the rot progresses, invaded scales become soft and foul-smelling. Onions with mechanical injuries, sunscald, or bruises are particularly susceptible to bacterial soft rot, especially if they have been held under warm, humid conditions.

Stemphyllium Blight (fungus – Stemphyllium vesicarium): Lesions are initially light yellow to brown and water-soaked. They elongate, often reaching the leaf tips, and become dark brown to black. The disease can become serious following periods of more than 24 hours of rainy weather. Fungicides used to control purple blotch will also control this disease.

Tip Blight (several causes): Infection by several species of fungi infecting leaves or roots can result in tip dieback. There can also be many non-pathogenic causes. These include: overcrowding, insect injury (particularly thrips and leaf miners), drought or salt stress, wind dessication, and occasionally, damage by ozone gas produced by lightning during severe thunderstorms.

Bacterial Blight (bacterium – Xanthomonas axonopodis pv. allii): This disease is not common in Texas. The symptoms are elongated chlorotic areas on one side of the leaf. These areas become sunken, water soaked and necrotic. Eventually, the leaf dies. The pathogen is seed borne, but could spread within a field following rain or overhead irrigation. Disease development is favored by temperatures greater than 68°F. There are no control recommendations.

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.
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.

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