White onion or ''
Allium cepa
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onion ...
'' (“sweet onion”) are a cultivar of dry
onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onion ...
which have a distinct light and mild flavour profile. Much like
red onions
Red onions (also known as purple or blue onions in some mainland European countries, though not the UK) are cultivars of the onion ('' Allium cepa''), and have purplish-red skin and white flesh tinged with red. They are most commonly used in c ...
, they have a high sugar and low sulphur content, and thus have a relatively short shelf life.
[Bill Jones ] White onions are used in a variety of dishes, such as those of Mexican and European origin. Their uses in dishes often relate to their mild nature, they are often included in dishes to provide a light, fresh and sour taste to dishes and are often added uncooked to dishes such as salads.
Flavour profile and culinary uses
White onions have a distinctively mild flavour palette in comparison to other onion
cultivar
A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture, ...
s such as the
red
Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...
and
yellow
Yellow is the color between green and orange on the spectrum of light. It is evoked by light with a dominant wavelength of roughly 575585 nm. It is a primary color in subtractive color systems, used in painting or color printing. In the R ...
onions. They are distinctly less
pungent
Pungency () refers to the taste of food commonly referred to as spiciness, hotness or heat, found in foods such as chili peppers. Highly pungent tastes may be experienced as unpleasant. The term piquancy () is sometimes applied to foods with a l ...
and have a lighter flavour on account of their relatively high
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
and low
sulphur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
content than other onion types. Because of this mild and easily palatable flavour, white onions are often used
raw
Raw is an adjective usually describing:
* Raw materials, basic materials from which products are manufactured or made
* Raw food, uncooked food
Raw or RAW may also refer to:
Computing and electronics
* .RAW, a proprietary mass spectrometry dat ...
in dishes such as
sandwich
A sandwich is a food typically consisting of vegetables, sliced cheese or meat, placed on or between slices of bread, or more generally any dish wherein bread serves as a container or wrapper for another food type. The sandwich began as a po ...
es and
salad
A salad is a dish consisting of mixed, mostly natural ingredients with at least one raw ingredient. They are typically served at room temperature or chilled, though some can be served warm. Condiments and salad dressings, which exist in a va ...
s. When raw white onion is soaked after being finely chopped, it adopts a relatively
sweet
Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, ketones ...
flavour profile as it loses its
astringency.
Due to this flavour profile, white onion is incredibly versatile in its uses for food. Its use is oftentimes central to the construction of salads as it provides a fresh, but not overwhelmingly pungent flavour sensation. It is also widely used as a central ingredient in the making of
stew
A stew is a combination of solid food ingredients that have been cooked in liquid and served in the resultant gravy. A stew needs to have raw ingredients added to the gravy. Ingredients in a stew can include any combination of vegetables and ...
s as its light flavour does not overpower the other elements of the dish, contributing to a cohesive flavour palette. White onion is also extremely useful as a component in
fermented
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
dishes such as
coleslaw
Coleslaw (from the Dutch term ''koolsla'' meaning 'cabbage salad'), also known as cole slaw, or simply as slaw, is a side dish consisting primarily of finely shredded raw cabbage with a salad dressing or condiment, commonly either vinaigrette ...
as its low astringency allows it to adopt a largely sweet flavour in the dish. It also has many uses in dishes such as
salsas
Salsas is a civil parish
In England, a civil parish is a type of administrative parish used for local government. It is a territorial designation which is the lowest tier of local government below districts and counties, or their combined f ...
because of this low astringency.
Diseases affecting white onions
Onion Smudge
The parasitic ‘Onion Smudge’ (“Colletotrichum circinans (Berk.) Voglino”) is a disease which affects crops of onion subspecies internationally, at all stages of development. Agricultural scientist,
James Charles Walker notes that white onions are especially susceptible to infection of onion smudge due to the lack of pigment in its outer shells, as coloured onion subspecies contain a higher concentration of
phenolic acid
Phenolic acids or phenolcarboxylic acids are types of aromatic acid compounds. Included in that class are substances containing a phenolic ring and an organic carboxylic acid function (C6-C1 skeleton). Two important naturally occurring types of ...
in its shells. Onion smudge thrives in damp soil conditions of above 20 degrees Celsius. The fungus can survive in the soil of onion plantations, and thus is extremely dangerous as it can impact the yield and performance of future crops.
Onion smudge causes damage to the outer scale leaves of the white onion, damaging the cosmetics of the bulb and ultimately reducing its market value. If the smudge is unnoticed in its early stages and the onion is left untreated with other onions. The parasite has the potential to infect the entire crop and cause the degeneration of the scale leaves, premature sprouting and the creation of conditions in which further storage rot may occur.
Onion White Rot
White Rot (
Sclerotium cepivorum
''Stromatinia cepivora'' is a fungus in the division Ascomycota. It is the teleomorph of ''Sclerotium cepivorum'', the cause of white rot in onions, garlic, and leeks. The infective sclerotia remain viable in the soil for many years and are s ...
) (also known as Allium Root Rot) is a notorious and severe
fungal disease
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
which impacts most members of the
allium
''Allium'' is a genus of monocotyledonous flowering plants that includes hundreds of species, including the cultivated onion, garlic, scallion, shallot, leek, and chives. The generic name ''Allium'' is the Latin word for garlic,Gledhill, Davi ...
family, including
garlic
Garlic (''Allium sativum'') is a species of bulbous flowering plant in the genus ''Allium''. Its close relatives include the onion, shallot, leek, chive, Allium fistulosum, Welsh onion and Allium chinense, Chinese onion. It is native to South A ...
,
leek
The leek is a vegetable, a cultivar of ''Allium ampeloprasum'', the broadleaf wild leek ( syn. ''Allium porrum''). The edible part of the plant is a bundle of leaf sheaths that is sometimes erroneously called a stem or stalk. The genus ''Alli ...
s and
onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onion ...
varieties.
The
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
is unique in that it does not produce
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s of great significance in its lifetime. Rather, it exists in the soil as a hardened fungal mass known as
sclerotia
A sclerotium (; (), is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes. In some higher fungi such as ergot, sclerotia become detached and remain dormant until favor ...
, which remain detached and dormant in the soil until there are favourable conditions and it can attach itself to a host.
Given the resilience of the fungi, it can remain in the soil for over 20 years, rendering many fields unviable for the planting and harvesting of onions.
White Rot can spread throughout a field, or from field to field by means of
flood
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
or
rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
water, agricultural equipment or other organic materials, such as plant scales blown by the wind.
White Rot may be identified initially by the onion's
leaves
A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
, the oldest of which will show yellowing, wilting and leaf dieback, beginning from the base of the allium.
A white fluffy growth (
mycelium
Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...
) may be found at the base of the
bulb
In botany, a bulb is structurally a short stem with fleshy leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food storage organs duri ...
of the onion, which later becomes compacted and the place of formation for the sclerotia.
The fungus thrives in low temperatures and is noticeably dormant during warm temperatures.
Botrytis Leaf Blight
Botrytis Leaf Blight (
Botrytis squamosa) is a fungal disease which mainly affects
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s in the allium family, particularly onions.
The impacts of Botrytis Leaf Blight are mainly seen in the leaves of the plant. After infection, the fungus causes leaf spot (also known as leaf
lesion
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
s), and the
maceration of the foliage tissue of the plant, resulting in dieback and blighting.
The lesions are initially often whitish and approximately 1-5mm long, with a light green halo that appears wet surrounding it.
The lesion then usually becomes sunken and straw-like into the stages of fungal development and may develop a slit in the lesion. Severely affected plants may result in a significantly smaller bulb size and a blighted appearance.
The Botrytis Leaf Blight pathogen survives the
winter
Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...
much like White Rot does, as
sclerotia
A sclerotium (; (), is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes. In some higher fungi such as ergot, sclerotia become detached and remain dormant until favor ...
(a small mass of fungi capable of thriving in hostile conditions).
Such sclerotia is responsible for spreading the pathogen and infecting crops as it has been observed to infect leaves and bulbs of
sprouted crops.
Pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s stored on the surface of the soil may produce
asexual spores called
conidia which may further infect crops. Botrytis Leaf Blight thrives in wet and cold temperatures but can still infect outside of these conditions.
Onion Downy Mildew
Onion
Downy Mildew
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of crucif ...
is a disease that is caused by the
oomycete
Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...
Peronospora Destructor.
It is a foliar disease, and causes leaf to become pale green, then tan, brown or yellow and finally collapse.
The fungus is extremely infectious to white onion
crop
A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydroponic ...
s and can often be first observed by patterns of yellowing within a field of onion crops.
Such patterns often correspond to the direction of the
prevailing winds
In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on ...
, enlarging as the disease progresses.
On individual plants, signs of Peronospora Destructor can be visible, as fine, furry growths that have a greyish white colour.
These growths are susceptible to further infection by diseases such as Purple Blotch of Onion or Stemphylium Leaf Blight of Onion, and if infected, can produce purple pigmentation of lesion and dark
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s.
The Peronospora Destructor pathogen only produces spores in live hosts, and releases spores at night, when there are cold temperatures and easily be blown across crops and infect other plants.
Overwintering
Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activi ...
spores of Onion Downy Mildew are known as
oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia, ...
s which can be found in
volunteer
Volunteering is a voluntary act of an individual or group freely giving time and labor for community service. Many volunteers are specifically trained in the areas they work, such as medicine, education, or emergency rescue. Others serve ...
onions,
cull
In biology, culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is the process of removing or segregating animals from a breeding stock based on a specific tr ...
piles and in infected onion bulbs.
The disease can be found worldwide, but favours
humid
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depen ...
climates as it requires moisture for infection of hosts. It is most prevalent in late
Spring
Spring(s) may refer to:
Common uses
* Spring (season)
Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...
and throughout
Summer
Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...
.
Planting of white onions
There are multiple methods of
planting
Sowing is the process of planting seeds. An area or object that has had seeds planted in it will be described as a sowed or sown area.
Plants which are usually sown
Among the major field crops, oats, wheat, and rye are sown, grasses and leg ...
white onions:
* Onion
Transplants
Transplant or Transplantation may refer to:
Sciences
*Transplanting a plant from one location to another
*Organ transplantation, moving an organ from one body to another
*Transplant thought experiment, an experiment similar to Trolley problem
*Tra ...
: a method by which
seedling
A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (embry ...
s started in the current growing period are bought and planted in one's own garden. This method often yields quick bulb growth, but the plants are more vulnerable to disease in maturity.
* Onion Sets: small
bulb
In botany, a bulb is structurally a short stem with fleshy leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food storage organs duri ...
s from the previous harvest are not allowed to mature and are dried. These bulbs are used in the current
season
A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...
, and often grow the quickest out of all planting methods and produce larger than natural bulbs.
* Onion
Seed
A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s: seeds of the white onion that can be planted into sets in late Spring. While this method may have the longest grow time (up to 4 months for mature bulbs), the onions produced are the least susceptible to disease.
Different varieties of onions vary in the time of year at which a mature white onion is cultivated:
* Short-day onions are onions that form bulbs when they are in an environment where they get 10 to 12 hours of
sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
a day.
* Intermediate day onions are onions that will form mature bulbs when they are in an environment where they get 12 to 14 hours of sunlight a day.
* Long-day onions are onions that form bulbs when they are in an environment where they get 14 to 16 hours of sunlight a day.
For most white onion varieties, a
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
pH of 6-7 is optimal. At times when the pH of the soil for onions is below 5.5,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
and
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
supply is inadequate and at a pH of above 6.5,
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
,
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
and
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
are less present in the soil.
References
{{Allium
Onion cultivars