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Onion (Allium)

Onion (Allium)

Plant Health Problems

Diseases caused by Fungi:

Fusarium basal rot, Fusarium oxysporum f. sp. cepae.
The plant develops a yellowing to the leaf blade which extends down until the whole leaf blade is yellow. Infected plants usually have shallow and stunted root systems and can be pulled up easily. When plants are young, the symptoms may not be noticeable, but if plants are sliced down the middle, a reddish streak can be observed in the base of the bulb.

The fungus is very persistent in soil and very long rotations are recommended. Fungicides are not effective for control. Some cultivars have shown tolerance to the disease. In general, the hybrid cultivars are more susceptible than the open-pollinated cultivars.

Pinkroot,
Phoma terrestris.
Onions with pinkroot are stunted, and the tops wither early. The top symptoms are caused by the rotting away of the roots. Infected roots are pink to red in color before they decay. The disease is caused by a fungus which lives in the soil and is favored by heavy, wet soils. Certain crops such as ladino clover, ryegrass, and corn leave residues in the soil which are injurious to onion roots. This injury makes the onions more susceptible to the pinkroot fungus.

To prevent pinkroot, onions should be planted on well-drained soils, and should not follow certain crops such as ladino clover, ryegrass, or corn. A four-year crop rotation is suggested.

Downy mildew, Peronospora destructor.
This disease shows as yellow spots on the leaves. The fungus which causes this disease appears as a downy, purple growth on the yellow spots following wet weather. The fungus spreads very quickly during wet, warm weather and may destroy whole crops in a few days. The older leaf spots are often invaded by a second fungus which blackens the spots. Fortunately, downy mildew has not been too important in Connecticut in recent years.

Control measures are usually not necessary. However, when weather is favorable for the disease, protectant sprays may be necessary. Among the compounds registered for use in Connecticut are fosetyl-Al, maneb and metalaxyl plus chlorothalonil and copper sprays. Consult the label for dosage rates and safety precautions.

Botrytis leaf blight, Botrytis squamosa.
The disease appears as whitish spots with green halos that can coalesce to cause necrotic areas and tip dieback. The fungus overwinters in culls and dead foliage from previous years and as resting structures called sclerotia.

Botrytis blight can be suppressed by avoiding overhead irrigation and watering in the morning to allow time for leaves and flowers to dry. Proper plant spacing can improve air drainage and rapid leaf drying. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are chlorothalonil, iprodione, and maneb. Consult the label for dosage rates and safety precautions.

Neck rot, Botrytis allii.
This is a dry, brown rot of white onions that appears in storage. The fungus which causes neck rot infects the onions at harvest time and grows in storage. The fungus carries over as hard, black kernels, which may be found in the rotted onions.

To keep the fungus out of the onions, it is best to allow the crop to mature as far as possible before harvest. This makes the neck area small and dry, with less likelihood of infection. The harvested onions should be well dried before storing, and stored in a cool, dry, well-aired space. Control can also be achieved with the use of fungicide sprays applied on the crop while it is still in the field. Among the compounds registered for use in Connecticut are chlorothalonil, iprodione, and maneb. Consult the label for dosage rates and safety precautions.

Smut, Urocystis cepulae.
This disease shows as black, smutty areas on the leaves, and in between the segments of the bulbs of white onions. These areas are filled with the loose, smutty spores of the fungus. Young diseased plants have twisted, curled leaves and may be killed outright.

To control smut, treat seed of white onions with compounds, and make certain sets are not infected. Among the compounds registered for use in Connecticut are maneb and iprodione. Consult the label for dosage rates and safety precautions.

Purple spot, Alternaria porri.
Lesions first appear on leaves as whitish sunken spots that quickly turn brown. As spots enlarge, they develop purple zonate rings.

Long rotations can minimize disease development and severity. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are chlorothalonil, fosetyl-Al, iprodione, and maneb. Consult the label for dosage rates and safety precautions.

Smudge, Colletotrichum circinans.
The smudge fungus causes dry, black, target-spots on the outer scales of white onions. The fungus infects in the field during the growing season.

The disease is not prevalent in Connecticut and control measures are not required. It is best to plant on well-drained soil, and handle the bulbs as for the control of neck rot.

White rot, Sclerotium cepivorum.
Infected plants develop a white mat of the fungus on the base of the plants. The plants turn yellow, wilt, and fall over. A characteristic symptom of white rot is the presence of numerous small, brownish-black bodies called sclerotia that develop inside the base of the stem.

Long rotations of eight to ten years should be practiced. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut is dicloran. Consult the label for dosage rates and safety precautions.

Diseases caused by Bacteria:

Bacterial soft rot, Erwinia carotovora subsp. carotovora, Pseudomonas gladioli pv. allicola, P. cepacia.
This disease is frequently encountered after harvest. The bacteria enter through wounds in the neck or through feeding wounds of the onion maggot. The bulb scales become water-soaked and pale yellow, causing them to slip off the bulb. The entire bulb becomes soft and frequently develops a very foul smell.

Control can be achieved by harvesting the bulbs after the tops have matured and dried down. Exercise care to avoid bruising during the harvesting. Store the onions at 32 to 36 F at 65 to 70% relative humidity.

Insect Problems:

Garden springtail.
This little springtail may occasionally damage young seedling onions.

Lesser bulb fly, Eumerus tuberculatus.
This fly is a pest of onion, but it most severely injures narcissus. It is believed that there are two generations each year. The flies appear in May and June and lay eggs at the base of the leaves. The larvae find their way to the tip of the bulb and then go downward into the interior. As many as 77 larvae have been found in a single bulb. When fully grown, these maggots are between 1/3 and 1/2" long, wrinkled and dirty grayish yellow in color. They pupate in August in the bulb or in the soil near it. Certain larvae overwinter in the bulbs, but these are thought to be the second generation, from which flies emerge the following spring. The fly is about 1/3" long, and has gray wings and a black abdomen marked with three white crescent-shaped bands.

Destroy any infested bulbs after harvesting.

Onion maggot, Delia antiqua.
Small white maggots that feed in the lower part of the stem or bulb may injure onion plants. The flies appear in the fields in spring and lay eggs in the base of the leaf sheath near the ground. The young maggots work their way downward into the stem where they feed. Infested young plants usually die, but in older plants, the maggots burrow into the bulb and decay follows. Several maggots may infest a single bulb. There are usually three generations annually, but the first generation, developing from eggs laid before mid-May is the most damaging. These insects overwinter as pupae in the soil. Adult flies can be diverted from laying eggs around seedlings by planting sprouted, preferably decomposing waste onion bulbs several feet away from the seedlings as a trap crop. Once egg laying is complete, these bulbs should be pulled out and discarded so that the hatching eggs cannot develop further. Onion seedlings can be protected from access by the adult flies using spun-bonded row cover, particularly early in the season. Control with insecticides is difficult due to resistance. If insecticides are used, they must be applied at planting. Once the damage is observed, it is too late for control.

Onion thrips, Thrips tabaci.
Thrips injury is called "white blast" because seriously injured plants turn white due to the feeding on the outer chlorophyll-containing tissue. Thrips are very small insects with narrow fringed wings that are lacking in the nymphs. In feeding, a thrips leaves a whitish chain-like mark on the surface. They hide in the sheaths of the leaves and are difficult to reach with an insecticide. Hot, dry weather increases thrips damage. Some varieties are relatively tolerant of thrips injury, with white onions generally most tolerant, then yellow, then red.