Garden + Gardening + Gardening Tips & Advice

Onion - Diseases, Pests and Problems

Basic Information

Problem: Onion Root Maggots
Affected Area: Bulb

Description: Small white worms about 1/3" long. Large, winged adult flies are brown or gray, 1/4" long with a humpback appearance. Maggots burrow into onion bulbs destroying the crop. Damage is more serious in wet years.

Control: When needed, foliage can be sprayed with *malathion or diazinon when flies appear, about the time forsythia blooms. * Place Diazinon granules in furrow at planting time. Destroy culls immediately after harvest. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Thrips
Affected Area: Leaf

Description: Thrips are tiny (1/25" long), slender insects, hard to see and often hide in the angles of the leaves (bulb). They feed on underside of leaves turning them a silvery color.

Control: Thrip damage generally does not merit control. * Diazinon or pyrethrum-rotenone combination. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Cutworms
Affected Area: Above ground portion of the plant

Description: Plants chewed off just above ground level. Cutworms are caterpillars that are up to 1 1/2" long and mottled or striped green, brown or gray. When they are disturbed, they roll up in a coil. They usually position themselves at the moisture line in the soil moving up and down according to the water content. If the soil surface is dry, they will be found a couple of inches below the surface where the moisture begins. When newly watered, they will be at the surface.

Control: Put cardboard collar around new transplants to extend 1" to 2" above and below soil level.

Problem: Pink root
Affected Area: Root

Description: Roots rot and take on a pink color. Yields can be severely reduced.

Control: Plant in non-infested soil. Soil can be sterilized with Vapam before planting if problem is severe. Rotate planting location.

Problem: Neck Rot
Affected Area: Bulb

Description: Water-soaked spots appear in neck area, turning yellow. Gray mold brows between bulb scales and bulb deteriorates. Usually appears just before harvest.

Control: Avoid using excessive nitrogen fertilizer and allow tops to mature well before harvest. Avoid injury to bulbs at harvest and dry before placing in storage.

Problem: Pythium Seed Rot and Damping-Off
Affected Area: Disease occurs on the roots

Description: Symptoms include roots becoming soggy and appear gray. Infected seedling usually die while plants infected in later stages do not die. Onions in the later stages of life usually appear slightly yellow and water-soaked.

Control: Control of this disease is done by making sure that the plants are not in too much water. Good drainage and the use of fungicides are recommended.

Problem: Fusarium Damping-Off
Affected Area: The disease affects the roots

Description: Symptoms include moldy, rotting seeds. Roots turn different colors before turning red and then black when as they die. Very slow growth then eventual death of the plant.

Control: Clean seeds should be planted to prevent the disease along with soil rotation. The use of fungicides and fumigants is also recommended.

Problem: Fusarium Basal Plate Rot
Affected Area: The disease affects the roots

Description: Symptoms include the leave turning yellow and curving. The stems may also become a different color ranging from purple to brown. The plants may wilt.

Control: The best way to prevent this disease is to plant resistant onions and garlic. Other ways to help control the disease include cold storage of bulbs and the use of fungicides.

Problem: Pink Root
Affected Area: The disease affects the roots

Description: The disease appears on the roots beginning with a light pink and increasing to a dark pink almost red until they turn purple and begin to decay. The plant will appear to be sick with few leaves and slow growth. Seedlings will die and older plant will be very small and sick looking

Control: Crop rotation is vital to controlling the disease. Using resistant seed along with fumigation also help in controlling.

Problem: Smudge
Affected Area: The disease affects the bulb

Description: Green and black spots on the bottom of the bulb are an indicator of the disease. The spots can form into circular rings. Often confused with smut.

Control: White onions get the disease far more easily than colored onions. When white onions are grown special attention should be given to making sure that the onions are protected from the elements at the time of harvest and stored in a place that promotes quick drying and cool storage just above freezing.

Problem: Soft Rot
Affected Area: The disease affects the bulb and leaves

Description: The leaves will turn white and will begin to wilt. The bulb will turn a yellow/brown color and become very soft. This smells really bad as the rotting gets worse.

Control: Onions should be dried before they are stored and then they should be stored at a temperature just above freezing.

Problem: Sour Skin
Affected Area: The disease affects the bulbs

Description: A few of the inner scales may begin to become slimy and slightly brownish/yellow. These inner scales will begin to rot. The outside and middle of the bulb will remain firm. This can cause the onion to slip apart during harvest.

Control: Harvesting at the moment the bulbs reach maturity and quick drying are the best methods in controlling the disease. It is best not to use overhead sprinklers to grow the onions. Try alternative ground sources to get water to the onions and the problem will be solved.

Problem: Stem and Bulb Nematode
Affected Area: This disease affects the leaves and bulb

Description: The leaves of infected plants often develop in a very irregular manner and often have yellow/brown spots. Seedling infested with the disease often die because there leaves are not growing correctly. The bulb usually become very soft and eventually rots completely and the result is a very bad odor.

Control: Crop rotation is one of the best ways to combat the disease. Plant only clean uninfected seed and fumigation may be used to prevent the seeds from being contaminated before the plant begins to grow. The main way to prevent nematodes is to dip the cloves into hot water for a period of time and then in warm tap water.

Problem: Lesion Nematode
Affected Area: The disease affects the roots

Description: This disease causes slow growth because it attacks the roots creating sores and blemishes where extensive root systems once were. The sores turn dark with time. The extent of the damage depends on the amount of time the nematodes spent feeding on the roots.

Control: Dipping the seeds in hot water has been effective in killing the nematodes. The use of fumigants has also been useful in reducing the amount of nematodes in the soil.

Problem: Root-Knot Nematode
Affected Area: roots

Description: Round swells will appear on the roots and the entire root system will be considerably shorter with fewer roots. The roots may also take on a yellowish appearance.

Control: Crop rotation is one of the best ways to control the nematodes.

Problem: Stubby-Root Nematode
Affected Area: The disease affects the roots

Description: The first sign of the nematodes is the lack of growth in the first few weeks. Very few leaves will form. The roots themselves are what?s being attacked and they become very stubby and do not grow very far.

Control: The use of chemicals designed to kill nematodes as well as appropriate fumigants can help control the nematode populations. Flooding the infected area also kills the majority of the nematodes.

Problem: Sting Nematode
Affected Area: The disease affects the roots

Description: Symptoms range from wilting to severe lack of growth. The leaves turn white and my even die. Areas infected also can be very large or very small. The roots will appear very short and stubby with sores at the tips.

Control: Sudden changes in the conditions of the soil will reduce the nematode populations. These changes can be anything from excessive drying to intense flooding. The use of fumigants has also proven effective in controlling the nematodes.

Problem: Garlic Mosaic
Affected Area: The disease affects the leaves

Description: White streaks, striping, and other patterns may develop on the leaves as an indicator of the virus. The growth is often slowed considerably and the bulb size decreases.

Control: The use of virus-free seed and bulbs grown in isolated areas can be used to greatly reduce the risk of infection.

Problem: Aster Yellows
Affected Area: The disease affects the leaves

Description: Yellow steaks or just overall yellowing may occur, beginning on the younger leaves and eventually spreading through the entire plant. The leaves way also become twisted up in each other. The plant sometimes wilts and dies.

Control: Good removal of weeds and use of insecticides will usually control the disease. Planting away from other crops that can be infected is also a good idea.

Problem: Temperature Stress
Affected Area: The damage affects all parts of the plant

Description: High heat and strong sunlight can damage young plants by killing tissue. Older plants may become burned. Cold temperatures may also kill young plants, and if it gets colder than -5?C the bulbs may be damaged and become soggy and gray.

Control: Making sure that the temperature not very intense hot or cold is the only way to control damage.

Problem: Hail and Storm Injury
Affected Area: The damage affects the entire plant

Description: When rain or hail is combined with driving winds the plants are often damaged. The leaves and stalks of the plants are usually damaged by the physical impact and this then weakens the plant and increases the chance of infection by diseases.

Control: It is impossible to control the weather so there are not control measures for this problem.

Problem: Soil Acidity and Alkalinity
Affected Area: Leaf

Description: pH


Problem: Manganese
Affected Area: The damage affects the leaves

Description: The deficiency occurs mainly in alkaline soils. The leaves tend to curl up and become light colored. The bulbs don?t form for a longer period of time and the necks become thick. The leaf tips may also start to burn and the growth may become very slow.

Control: Maintaining a pH of about 5.5 usually can combat most manganese deficiencies.



Cold temperatures after early planting encourages seed stalk development.