Onions

Allium cepa.

The onion is the most commonly grown vegetable in the world. It is supposed to have originated somewhere in Central Asia around the area where Afghanistan is today. This was long before anyone kept written notes on such things. The popularity of onions is worldwide and onions are used in many different types of cooking to add their unique flavor.

Besides having a special flavor, the onion is nutritionally good food for you.

One serving of onion at 156 g. (5 oz.) has the following nutritional

value:

Calories:          60

Fat:                    0

Sodium:           10 mg.

Protein:              1 g.

Carbohydrate: 14 g.

Dietary Fiber:    3 g.

Onions are considered a cool season plant that will grow well over a wide range of temperatures. Onion seed will germinate best at soil temperatures of  18.3· C. (65· F.) but can still germinate satisfactorily between  7.2· C. (45· F.) and 29.4·C. ( 85·F.).

It will grow best within a temperature range of  12.8· C. (55·F.) to 23.9·C.  (75·F).  Best growth and bulb quality are obtained if the temperature is cool during the early development and warmer at maturity.  A dry atmosphere at harvest is desirable to allow proper curing (drying) of the bulbs. The onion is fairly resistant to frost injury, but not immune. Frost damage can occur at temperatures of  -1.1· C. (30.0· F.)  The root system is shallow, and to grow well, it must receive a regular and uniform water supply. The crop responds well to irrigation.  The formation of the bulb is affected by the amount of sunlight within a day (photo-period) and the genetic codes inherent within the specific variety itself.. The necessary length of day or photo-period varies by variety and type of onion being grown. Onions are divided into a minimum of three types for photo-period classifications.

 Basic Classifications for Onions:

 1. These are long-day onions which require  a photo-period of not less than 14 hours per day during the growing cycle. There is another that requires a photo-period of 16 hours or more. Generally speaking, long-day onions are adapted to latitudes of 36· or greater for satisfactory growth.

 2. Intermediate-day length onions require a photo-period of not less than 12.5 to 13.0 hours and not more than 15 hours per day during the growing cycle. Intermediate onions normally are grown in production areas between the latitudes of 32· to 40·

3. Short-day length onions which are adapted to equatorial regions of the World require a photo-period of not less than 12 hours and not more than 13 hours per day during the growing period. Short-day onion varieties are usually adapted to latitudes of 28· or less.

Maturity or earliness of one variety to another can depend upon the genetic heredity of the onion itself along with the location and photo-period in which a particular variety is planted. The effect of altitude can also interact with a particular photo-period sensitive onion to permit normal growth outside the usual growing region. Generally speaking, onions are developed for specific geographical locations and do not adapt to growing conditions not inherent within a particular variety. The days necessary for maturity of onions can vary from 90 days to over 160 days depending on the many factors that influence the growth cycle. When understanding the specific traits of an onion variety, you must consider cultural practices and environmental conditions along with the other points stated above. The fact that some onion varieties will produce an acceptable bulb outside of normal pre-requisites only makes it more difficult to give the onion a pre-determined rigid set of standards. Each onion variety is unique unto itself. Therefore, field testing with close evaluation of the results is mandatory.

New genetic research carried out by now provides growers with onion varieties adapted to short-day length with tolerance to frost damage and                      pre-mature flowering. These new varieties can be planted in the Fall season and harvested in the Spring or Summer season in areas that are usually classified as intermediate and long-day growing regions.

Growing Conditions and Disease Problems:

Onions should be grown in good fertile soil that has organic matter and is well drained. Soils with warm temperatures that drain poorly or have high mineral content can aid in the promotion of Pink Root Rot [(PRR) =Pyrenochaeta terrestris]. PRR is caused by the soil-borne fungi that can infect onions at any stage of growth. Since the disease is confined to the roots, detection may not be noticed until the bulbs begin to die. The obvious signs of PRR show themselves in the roots by causing them to turn reddish-pink in color, hence the name, Pink Root. The roots then turn brown and black as they dry up and no longer support the growth of the bulb. The leaves turn yellow at the tips and die back. This can cause considerable loss in production.

Most onion varieties have genetic tolerance to PRR. Tolerance or resistance to PRR is subject to a wide range of opinions but the common line of thought is that tolerance or resistance is based on:  First, the severity of the infestation and the genetic tolerance to PRR inherent within the variety and; second, the physical capacity of the infected onion to regenerate new roots in the presence of the disease. The combination of these two factors combined with environmental conditions will define how well a particular onion will survive an attack of PRR.

No known A. cepa class onion variety has been found to be immune to PRR at this time. Crop rotation and good cultural practice will aid in reducing the degree of PRR found in many soils.

Another disease is Botrytis Leaf Blight. This is also referred to as blast and as neck rot. Control in the early stages of growth is easiest by starting with seed that has been treated with a fungicide prior to planting. Botrytis infects the leaves of onions and can be controlled with several different fungicides. Neck rot [Botrytis alli Munn] is a serious disease that can effect onions at maturity or after harvest while the bulbs are curing or in storage. Neck rot can be controlled by not permitting moisture to collect around the neck of the bulb and by properly curing the onions after harvest. Bulbs can be cured in the field or in controlled storage at temperatures above  32.2· C. (90· F.) for several days.  A “dry onion” is actually composed of approximately 90% water.

The goal is to reduce the excess moisture remaining in the onion leaves and bulb that will cause the bulb to breakdown and decay rapidly after harvest. The natural drying down of the bulb and leaves in the field is easiest, providing the weather is dry.  The bulbs are lifted from the soil and laid down on the dry ground with the leaves covering the bulbs to reduce sunburn. The warmth of the sun will dry the leaves. When they begin to shrivel, the bulbs should be picked up and placed in protected storage. Storage of onions is important since this can provide a grower with the option of delaying his sales for a better market price that can develop after the main crop of the season has already been shipped and sold.

Generally, short-day onions are best when marketed immediately. Some newer hybrids have good or better storage-ability than older varieties, which can be held for up to one month in proper storage conditions.

Fusarium Basal Rot,(Fusarium oxysporium f. sp.cepa ) is common world-wide and can be a serious problem. This disease is more prevalent in areas where onions are grown under high temperature conditions. The infection can be first detected in the basal plate but since the basal plate is hidden in the soil,  you are more likely to see the later symptoms as they appear in the leaf tips which turn yellow. The yellow discoloration then moves downward in the leaf until the entire leaf shrivels, drys up and starts to decay. The bulb will be easy to pull since the roots will be weak and shortened. The roots will appear dark brown, flattened and hollow. Some roots may appear transparent. Fusarium Basal Rot is difficult to distinguish in the early stages from other common diseases. By cutting an infected bulb vertically, a brown discoloration of the stem plate is obvious. This discoloration begins at the outside layer of the basal plate and extends upward within the bulb. One unusual characteristic of bulbs infected with Fusarium Basal Rot is the unpleasant odor in the bulbs. Long crop rotations, good cultural practice to control insects, fungus, correct moisture levels and proper soil preparation are recommended for reducing Fusarium.

Some of the other diseases that can attack onions are: Bacterial Soft Rot (Erwinia carotovora sub. carotovora), Slippery Skin (Pseudomonas allicola pv. allicola), Purple Blotch (Alternaria porri), Downy Mildew (Peronospora destructor), Brown Stain (Botrytis cinerea Pers.), White Rot (Sclerotium cepivorum), Rust (Puccinia porri), Stemphylium Leaf

Blight (Stemphylium vesicarium), Heteroporium Blight, Cerocospora Leaf Spot, Black Stalk Rot (Stemphylium botryosum), Black Mold, Blue Mold Rot, Onion Yellow Dwarf Virus, and Stem or Bulb Nematodes (Ditylenchus dipsaci).

Control measures such as crop rotation, insect control, good field sanitation and the proper use of chemicals should be a standard part of a growers preventive efforts to reduce the incidence of disease(s) and crop loss. Before giving up on the idea of growing onions ever again, please note that of these many diseases, only the following are considered to be of economic importance.

 Stem and Bulb Nematode  

 Downy Mildew

 Onion Yellow Dwarf Virus   

 Purple Blotch (Alternaria porri)

 White Rot    

 Pink Root Rot

 Stemphylium vesicarium  

 Fusarium Basal Plate Rot

 It is possible that not all of these diseases occur in your growing regions. These and other possible diseases may or may not be important in your area depending on the climate, soil conditions, crop rotations and other relevant factors that should be already known to you and the grower. The fact that control measures are available to reduce some of these particular diseases means that growers have the ability to defend a crop. You can focus your attentions on the specific diseases that are known to occur in your onion growing regions.

 Possible seed- borne diseases are:

 Onion Smut*    

 Botrytis*

 Purple Blotch (Alternaria porri)*  

 Stem and Bulb Nematode*

*Each of these pathogens can be reduced by using a fungicide seed treatment prior to planting.

 Other Production Problems:

Weed control is important in maintaining good onion production. Weedscompete with onions for nutrients and compete for sunlight, water and fertilizer. Good tillage of the soil before planting and during the growing cycle will reduce this competition and provide the onions with non-compacted soil to grow. Be careful not to disturb the onion bulb or damage the roots while removing weeds. Moving the bulb or stressing the root system is referred to in the following paragraph.

Doubling or multiple centers in onions can be caused by several factors, but the basic cause of multiple centers is the result of stress on the plant during the growth cycle. Some onions are more susceptible than others to stress. The definition of stress is any condition that causes the onion to stop growing even for a short period of time. It can depend on the genetic makeup of a particular variety in how well that variety reacts to adverse conditions. There are many factors that can trigger stress and some of these are: direct-seeding, transplanting, temperature changes, either high or low beyond “normal”, too littlewater, too much water, root damage from tillage, etc. The result of stress as understood by the plant is when it senses that it will not be able to successfully complete its normal growth cycle and by internal chemical reactions, stops growing. Once the plant is restored to its “normal” growing conditions, the internal chemical reactions that stopped the growth cycle start again and the plant develops new centers from which new growth begins. The results of stress can be doubling or multiple centers, pre-mature bolting, and splitting.

General Fertilizer Practices:

Average fertilizer rates needed by onions will vary widely based on soil composition, soil history of previous crops and moisture content.  The information below is not a recommendation but offered as a basic point from which to make your own considered decisions. Normal fertilizer applications are for one-third to be applied prior to planting, one-third at the third or forth true leaf stage and the final third just prior to the mid-maturity point in the growing cycle. Late applications of nitrogen (N) will cause the bulbs to be softer than usual. This will shorten the storage life and marketability of the bulbs.  Avoid applying N after the mid-maturity point in the production cycle.

Nitrogen (N) per HA.        112 Kilos

Potassium P2O5 per HA.    85 Kilos

Potash K2O per HA.            85 Kilos

 A complete soil analysis is necessary before making any decisions about a fertilizer program for any particular onion crop. This information is based on averages found in the USA and does not take into account any particular soil type, crop history, or moisture content.

Onions do respond to the foliage applications of micronutrients but should only be applied if testing indicates one or more of these nutrients are limited in the soil composition. Onion is fairly tolerant to Boron.

The response rating given below is based on averages found in the USA and does not take into account any particular soil type, crop history, or moisture content.

 High Response  Medium Response  Low Response

 Manganese         

 Boron

 Copper

 Molybdenum

 Zinc

 Other Classes of Onions:

Another species of onion is Allium fistulosum which is a non-bulbing onion. This class of onion is known under several different common names which are: Bunching onion, Japanese bunching onion and Welsh onion. The species A. fistulosum does not produce a bulb but does develop a long straight neck with a slight bulb at the lower portion which is generally soft.

 A shallot onion is Allium cepa that produces several smaller bulbs that are connected at the basal plate. Leek is classified as Allium ampeloprasum (Porrum group) and is still another category within the Allium class.

 Important Notes for Onion seed Storage and Preservation.

 Seed storage should be in a hermetically sealed container that is kept in a cool, dry location with a temperature never higher than 30· C. (86· F.) and for a maximum time period of not more than nine months.  Seed germination can vary widely depending on storage conditions and a representative sample should be properly tested prior to using seed stored more than 5 months. Always store seed in a dry and cool location and keep it away from exposure to direct sunlight and wide temperature changes. The simplest and easiest storage method for seed, including sealed tins, is to place it in a closed and air conditioned room that will keep the average temperature at 21.1· C. (70· F.) and the humidity below 30%.

 “Seedsmens Rule of Thumb” is: Seed storage should not exceed a combined total of 100.  If the temperature is  86· (30· C.) then the relative humidity cannot be higher than 14% because these add up to 100. The higher the relative humidity, the lower you must keep the average temperature. Examples follow:

 Relative Humidity Temperature in F· Total  Good / Bad

  30%    70·  = 100 = Good

  35%    78·  = 113   = Bad

  45%    82·  = 127  = Bad

 Good storage is good business because it protects the  living product. Remember that seed is alive and that it should be protected  from adverse conditions. It is hard enough surviving in the field, so do not make it any more difficult to survive in the tin.