Onions and related crops such as garlic, shallots, chives have similar growing requirements, and naturally they share similar maladies. Problems associated with growing Onions and other Alliums can be categorized into cultural, pest, nutrients and disease
Cultural issues relate to maintenance of the Garden. Proper Sanitation techniques such as removal of plant debris, weeding and so forth. No rocket science here - Try to keep the garden free of any diseased dead or damaged plant materials. Leaving rotting fruits and vegetables lying about is an invitation to unwanted pests and diseases.
If a dead or diseased plant part has to be cut, the microorganism that caused the problem is probably on the tool you just used. Sterilize all tools by washing in soap and water - rubbing alcohol is good too. If you pinch off diseased plant parts, wash hands immediately afterwards.Weeds not only compete with garden plants for resources they are carriers of blight and disease and attract insects. Get rid of them.
Onion Plant Disease
Gray to brown mold develops. The Neck of bulb turns spongy.
Botrytis neck rot or Botrytis Leaf Blight are the likely causes. Fungal diseases. Remove and destroy infected plants. Keep weeds away from the garden where fungal spores may harbor.
It occurs in poorly drained damp areas. The bacteria is spread by garden tools and surface water. Bacterial leaf blight usually occurs only on the upper leaves. Small water-soaked lesions form and eventually coalesce into larger lesions that eventually cover the entire leaf.
There are a number of fungicides registered for treatment of Botrytis Leaf Blight , many are only available only to commercial growers. Products containing Chlorothalonil and Mancozeb are proven to be effective against Botrytis and are available to Home Growers.
Avoid late season applications of nitrogen. Don't irrigate excessively within 2 weeks of lifting onions. Harvest only when the crop is mature, and during dry weather. Provide good ventilation for curing onions before storing.
If you are planning to store Onions grow varieties that are known to store well, such as Candy Hybrid, or its relative Yellow Ebenezer. Good storage onions are low in water and high in sulfur, and should have 3 or more wrapper scales, and the neck will tighten when dried. Stronger flavored onions last longer in storage than the sweet onion varieties. Try storing all your sweet onions no more than two months . If this disease has been a problem in the past you should use resistant varieties.
Stalks wilt, droop and die - Foliage has yellow and/or white spots and grayish purple mold. The probable culprit is Downy mildew, a fungus which attacks onions and related alliums. Its more prevalent in cool, moist weather and rarely occurs in warmer climates.
Downy mildew also produces minute fruiting bodies and spores called sporangia on the surface of the leaves and seed stalks . The masses of spores are at first transparent to grayish, and then become a light violet color.
Leaves become girdled in the region where mildew develops and then collapse. This results in dead leaf tips The dead leaf tissueis rapidly colonized by purple blotch
Downy mildew seldom kills onion plants, but bulb size is reduced. Bulb tissue, may become spongy and the bulb may lack storage quality
Manzate or Dithane are both effective against Downy Mildew.
Powdery Black mold on the Onion itself , sunken lesions. slightly thickened areas on leaves and stems.
Black Mold occurs only sporadically but when it does, it is potentially devestating. It causes damage primarily to stored onions, rendering them unpalatable and unmarketable. The scientific name is 'allium aspergillus' or 'aspergillus niger' but who cares about scientific names - so long as it ain't growing on YOUR onions.
Should your hands come in contact with black mold, a thorough scrubbing with soap and water is advisable, to prevent spreading the fungi.Onions carrying black mold are generally damaged , to be safe, you should discard - nuke- obliterate and destroy any potentially contaminated onions. If you enjoy fungus in your Garden - put them in your compost heap - you'll be sure to have more fun with the same black mold next season.
No particular fungicide has proven to be highly effective. And there's no absolute way to ascertain whether your onions are actually contaminated while still in the ground.
"Research indicates that a good fungicide control program for foliage diseases will reduce the incidence of black mold. Storage and transit temperatures below 55F (12.8C) and as low as 33F (0.6C) are recommended to suppress black mold development. Handling of bulbs to avoid bruising also reduces injury and invasion sites for the fungus." UC Pest Management Guidelines.
Leaves die back from tips, roots turn a pale pink , sometimes light red and purple. Stunted Growth
Pink Root is to blame
Pink root rot symptoms above the ground are often misdiagnosed as nutrient deficiency or drought stress. Pink Root seldom results in plant death. Infection is confined to roots and outer scales of the bulb. Severe infection will reduce bulb size.
Crop rotation and proper cultural practices are the best defense against onion pink root. Fungicides are only moderately effective, and generally not worth the expense for the success rate they have displayed against this disease.
1.] Crop Rotation in your garden planting area
2.] Remove and destroy all plant refuse in the fall and use deep cultivation to bury any remaining refuse.
3.] Do not place diseased plants in the compost heap, as this will only serve to carry bacterial or fungal infestation into the next growing season.
Read More on Pink Root
Fungicides containing chloropicrin are effective against most strains of Pink Root
Unfortunately it is a restricted use chemical, not available to Home Gardeners
Products containing boscalid are less effective - but readily available.
Small water-soaked lesions on leaves and stalks that rapidly develop white bleached out centers. As the lesions grow they become brown to purple in color occasionally with a red or red/purple border.
The lesion surface may be covered with brown to dark gray masses of spores.
Purple blotch is the probable cause, the lesions often occur along with other diseases in weakened plant tissue and also may be invaded by other fungi.
Purple Blotch progresses in plants weakened by poor care or other pathogens as well as in overly moist conditions from rain or dew. Susceptibility to infection is higher on older leaves when compared to younger leaves and is higher on leaves infested with aphids and onion thrips.
Cultural control strategies include reducing plant density and promoting good field drainage to reduce the duration of leaf wetness. Once disease occurs in a field, long rotations are recommended. Fungicides such as mancozeb can reduce the severity of purple blotch.
Alternating applications of varying fungicides is advisable. Fungicides containing chlorothalonil are the most effective.
Insects that feed on Onions
Leaves fade, yellow and wilt, tips turn brown. Tunnels and cavities in bulb.
Root Maggots and Onion Maggots attack germinating seedlings, they feed on the developing roots and epicotyl. as well as the expanding bulb during later stages of plant development.
They lay their eggs near the plants base during the spring when they emerge from the pupae stage. Eggs hatch and the larva will feed on the onions for up to 3 weeks . They are more prevalent in coastal and cool damp areas.
Maggots will hollow out an entire onion if given the opportunity. Larger bulbs will turn yellowish and rot. Destroy all affected plants. Destroy means just that, destroy em, Nuke em get rid of them but - DO NOT PUT THEM IN THE COMPOST HEAP as this will perpetuate your problem.
Tanglefoot is an organic substance that is sprayed or spread on plant stems and catches anything that attempts to climb up.Yellow Sticky Traps will capture many of the adult flies when they emerge in the spring.Spreading wood ashes and spraying Hot Pepper Wax around the plants to discourage egg laying is helpful
Leaves are distorted and turn gray white and blotchy.
Probably Onion Thrips, they are common during dry warm, weather. Blast thrips with water to wash them away. Insecticidal soap is helpful.
It could also be Aphids or Mites. Aphids are easier to spot than thrips, they aren't in the least bit shy. They are Yellowish-pink to pale green plant lice that suck plant juices. They are a soft-bodied, oval/pear shaped insect and are commonly found on nearly all varieties of plants. See - Aphid Control.
Aphids will frequently leave droppings or secretions which develop a black sooty mold, sometimes mistaken for a plant disease. Spider Mites will leave a tell-tale fine webbing on the underside of leaves.
Seedlings are sheared off at the soil level.
Cut Worms are the culprit. They gray to brown nocturnal grubs that feed at night. Handpicking grubs from the soil around plant bases is helpful, however, they hide in the soil by day and are not easily spotted. Keep the garden free of plant debris.
Bt Bacillius thuringiensis is a bacterium that is known to kill cutworms, and is readily available to home gardeners.
Leaves wilt and yellow - tips die back. Seedlings thicken and are deformed. Mature plants are swollen at the base, stunted and limp.
Probably Nematodes ~ microscopic worms that live in the soil . They enter plant roots and secrete a toxin.
Fish emulsion is believed to help counter nematode toxins, although this has not been scientifically proven. Inter-plant with marigolds. Marigolds are not a magical cure to instantly rid you of nematodes. Marigolds need to be grown at least one season before they are able to fully protect plants from nematodes.
Once infested Predator Nematodes will control the populations of parasitic ones more quickly. Should you add marigolds to the mix, they will also have a negative effect on the beneficial nematodes.
Nutrient Issues in Onion Growing
Onions rapidly bolt to seed. Bulbs are hollow.
More than likely nutrient related. The plants energy is directed toward seed production instead of bulb formation. Pinch off flowers in order to force the plant to devote energy and nutrients into bulb formation instead.
Yellowing Between Leaf Veins
This is a sign of Manganese deficiency
Epsom salts added to the soil will help. Epsom salts is comprised of magnesium ,sulfur and oxygen and in gardening it is used to correct a magnesium or sulfur deficiency in soil. Epsom salt will allow more minerals to be absorbed via the root system, adding to healthier and stronger plants. See - Epsom Salts as Fertilizer
Stunted, twisted and yellow-striped foliage.
This is a sign of Zinc deficiency. Zinc deficiencies are mainly found on sandy soils that are low in organic matter. It occurs more commonly in cold, wet spring weather. A pH imbalance will also inhibit the uptake of zinc and other nutrients.
See - Plant Nutrient Deficiencies