Providing practical information to help you protect your farm from biosecurity risks

Onion pest threats

 

Onions | Onion pest threats | Pest eradication or control examples

 

The following are some key high priority exotic pest threats for the Australian onion industry as identified through the development of the Industry Biosecurity Plan for the Onions Industry. Any of these pests would have serious consequences should they enter and become established in Australia.

Information about other pests of grain and pests of rice is available from the onions industry page on the Plant Health Australia website.

Implementing biosecurity measures to control endemic pests will go a long way towards preventing exotic pests from entering and becoming established on your farm.

To improve biosecurity measures on your farm, include exotic pests when undertaking routine pest surveillance activities. Ensure that all surveillance activities, for both endemic and exotic pests, are recorded. Visit Records for templates to record surveillance results.

Onion fly

OnionFly1

Adult Delia antiqua (dorsal view). Simon Hinkley & Ken Walker, Museum Victoria, Pest & Diseases Image Library (PaDIL), www.padil.gov.au

  • Onion fly affects onions, shallots, leeks, garlic, chives, Japanese bunching onions and Welsh onions.
  • Adults are about 7mm long, yellowish grey with five dark stripes on the thorax, yellowish wings, black legs and antennae, similar in appearance to a domestic fly.
  • Eggs are dull white with long white stripes
  • Larvae are white maggots which reach 8mm when development is complete.
  • Damages bulbs, leaves, roots, seedling.
  • Look for yellowing and wilting of the host’s outer leaves. Green and apparently healthy leaves will become floppy, and the whole plant may collapse. Later generations of larvae tunnel into the onion bulbs as well as attack the roots.
  • Spread by infested soil and plant material.

Bean fly

Bean fly

Bean fly at various life stages. Onions Australia

  • Bean fly affects onions, leeks, garlic, Brassicaceae species, tomatoes, potatoes and corn.
  • The adults are long and thin with grey bodies.They are about 6mm in length, with black legs and clear wings. Their eyes are large and reddish purple.
  • The eggs are enclosed within a hardened, darkened larval skin, that are oval, red-brown and 8mm long.
  • Larvae are creamy yellow to white 8mm long when fully grown. The body tapers toward the head end but lacks a distinct head and legs.
  • Damages bulbs, seedlings.
  • Look for a borer in the lower leaf and the larvae can
    make corridor-like excursions into the upper leaf. T the mines may be distinguished from other similar flies because the mines of bean fly harbour several larvae.
  • Spread by infested soil and plant material.

Vegetable leaf miner

Vegetable leaf miner

Adult vegetable leaf miners. Pest and Diseases Image Library, Bugwood.org

  • Leaf miner affects a wide range of vegetable and flower crops including onions, eggplant, beans, celery, peas, potatoes and tomatoes.
  • Adult body length is <2mm and the wingspan <2mm. The head (including the antenna and face) is bright yellow. The rear margin of the eye is black. The fore-legs are brownish-yellow and the back legs are brownish-black. The abdomen is largely black with yellow sides. Vegetable leafminers don’t often fly, and in crops showing active mining many flies may be seen walking rapidly over the leaves with only short jerky flights to adjacent leaves. Eggs are very small (<0.5mm long and <0.2mm wide), off-white and slightly translucent. Maggots have no separate head capsule and are transparent when newly hatched. They become yellow-orange in later growth stages and are up to 3mm long.
  • Damages leaves, bulbs, flowers.
  • Mines are usually white with moist black and dried brown areas. They are typically snake-like, tightly coiled. In larger leaves, the mines often form an irregular ‘U’ shape. The frass is deposited in black strips alternately at either side of the mine.
  • Spread by infested planting material.

Onion thrips

Onion thrips

Larvae at base of onion leaf. Whitney Cranshaw, Colorado State University, Bugwood.org

  • Onion thrips affects a wide range of plants including onions, garlic, leeks, shallots, horseradish, daisies, cotton and Cucurbitaceae species.
  • A complete generation requires 3-4 weeks during the summer months. Five to eight generations may occur each year. Adults have a yellow and brown body (<2mm long) with two pairs of hairy wings. They spend the winter in protected sites under plants and debris in onion fields and fly readily when disturbed.
  • Eggs are white to yellow in colour, kidney-bean shaped and microscopic in size. They develop within leaf tissue with one end near the leaf surface.
  • Stage 1 and 2 larvae are <1mm in length and are the active, feeding stages. They are white to pale yellow, with a long, thin body. They resemble adults without wings.
  • Damages leaves, bulbs, flowers.
  • Water loss through the damaged leaf surface may cause stress and reduced plant growth. Onions are most sensitive to thrips injury during the rapid bulb enlargement phase. Fast plant maturity due to thrips injury may shortened the bulb growth period. Following harvest and during storage, thrips may continue to feed on onion bulbs, causing scars that reduce quality and visual appearance of bulbs.
  • Spread through infested plant material. Adults can fly.

Xanthamonas leaf blight

Xanthomonas leaf blight

Onion plant showing symptoms of Xanthamonas leaf blight. Howard F. Schwartz, Colorado State University, Bugwood.org

  • Xanthamonas affects onions, garlic, leeks, chives, shallots and Welsh onions.
  • Damages leaves.
  • Look for white to tan flecks, light-coloured spots and/or deep wounds in the leaf surrounded by ‘water-soaking’. Wounds quickly get bigger, turning tan to brown with extensive water-soaking. As the disease progresses, wounds merge into dry dead areas of tip dieback. Typically, infection of outer, older leaves leads to plant stunting and undersized bulbs. When conditions are favourable for the disease, all leaves may become infected and plant death may follow.
  • Spread by infested soil and plant material, seed.

Leaf rot and neck rot

Leaf rot and neck rot

Onion showing symptoms of leaf and neck rot. Lindsey du Toit, Washington State University, Bugwood.org

  • Leaf rot and neck rot affect onions, Japanese bunching onions, Welsh onions, leeks and garlic.
  • Damages leaves, seeds.
  • The first symptoms appear as small white wounds on the leaf surrounded by a greenish halo. This early symptom can easily be confused with insect or mechanical damage, or herbicide injury. In later stages, however, the wounds expand and many new wounds appear that cause premature die-back of leaves. Severely affected onion fields develop a withered appearance and small onion bulbs.
  • Spread by infested soil, plant material, wind, and rain.

Leaf blotch

Leaf blotch

Onion leaf showing signs of leaf blotch. Clarissa Balbalian, Mississippi State University, Bugwood.org

  • Leaf blotch affects onions, shallots, chives, garlic and leeks.
  • Damages leaves.
  • Leaf infection results in long wounds that develop parallel to leaf veins. At first, wounds appear as yellow areas but later turn brown. Weak, old leaves are more likely to be colonised by this fungus than healthy leaves. An abundance of brown to olive-brown spores are produced giving affected tissues a dark, velvety appearance. As the disease progresses onion plants begin to die.
  • Spread by infected debris, soil and seed.

Onion smut

Onion smut

Onion plants with symptoms of the onion smut disease caused by onion smut in the field. Howard F. Schwartz, Colorado State University, Bugwood.org

  • Onion smut affects onions, spring onions, leeks and garlic.
  • Damages the whole plant.
  • Occurs as distinctive narrow long dark streaks, usually on the cotyledon or first true leaf, which become thickened. The streaks are evidence of an infection that initially affects the inside of the leaf and later bursts through the leaf surface and releases masses of dark-brown powdery spores. Onion seedlings usually die within a few weeks and plant populations are reduced. Diseased plants that survive are often distorted, develop blisters on green leaves and remain stunted. Yields are reduced. Mature bulbs may, less commonly, have dark pustules.
  • Spread through seedlings, soil, bulbs and as a contaminant on seed.

Read about the eradication of onion smut from Australia

Fusarium basal rot

Fusarium basal rot

Fusarium basal rot of onion bulbs, Onions Australia

  • Fusarium basal rot affects onions, leeks, garlic, and others.
  • Which part of the plant will be damaged?
  • Damages leaves, whole plant.
  • Fusarium basal rot symptoms appear as a pinkish brown rot that becomes covered with a whitish, fluffy fungal growths. Leaf tips yellow, entire leaves wilt beginning with the older outer leaves, scattered plants are stunted, and eventually die. A semi-watery decay progresses from the basal plate upward and secondary invaders (bacteria) cause a watery, foul smelling breakdown.
  • Symptoms may not be visible in the field, but may develop during storage. Bulbs will become spongy or sunken, and will appear brown and watery when cut open. White, light pink or reddish fungal growth may appear on bulbs or within rot cavities.
  • Present in all states except Northern Territory and Tasmania.
  • No movement controls exist for this pest, but it can be prevented from entering your farm through biosecurity measures.
  • Spread through soil, water, transplanted plants and seedlings.

White rot

White rot

White fluffy bulbs are a key sign of white rot. Onions Australia Poster

  • White rot affects onions, beans, beetroot, capsicum, carrots, Cucurbitaceae species, sweet potato, potatoes and tomatoes.
  • Damages leaves, especially near the soil line.
  • Look for brown to black rot of the stem near the soil line. Plants may be stunted or there will be yellowing and wilting of the leaves. Eventually the leaves will die, beginning with the oldest leaves. During cool weather there may be white, fluffy growth at the base of the stem plate when the leaves are yellowing. On these mats of fluffy fungus, black spores will form that will be about the size of a poppy seed. This disease usually appears on groups of plants in the field that are widely spaced. However, large groups of plants may die suddenly when the fungus is abundant in the soil and conditions are favourable for disease.
  • Present in all states except Western Australia.
  • Restrictions are in place for produce entering Western Australia.
  • Spread by infected plants/material, soil and water.

Read about how white rot became established in Australia

Onion rust

Onion rust

Telia (brown) and uredinia (yellow) on diseased leaf. Bruce Watt, University of Maine, Bugwood.org

 

  • Onion rust affects Allium species including onions, garlic and chives.
  • Damages leaves.
  • Look for rust on leaves which is bright-orange or brownish, circular to long bumps along the veins, followed by the formation of blackish spores. When rust infection is severe the leaves may die.
  • Present in all states except Western Australia.
  • There are restrictions on produce entering Western Australia.
  • Spread by wind, infected plant material such as seed and mother bulbs, and garlic imported for human consumption but planted in backyard gardens.

Iris yellow spot virus

Iris yellow spot virus

Iris yellow spot virus on onion leaf. Ronald D. Gitaitis, University of Georgia, Bugwood.org

  • IYSV affects onion, shallot, leek, garlic, roses, and Rubus species.
  • Damages leaves, roots, whole plant, bulbs.
  • Symptoms of iris yellow spot virus (IYSV) consist of eyespot to diamond-shaped, yellow, light-green or straw-coloured wound(sometimes dead-looking) on the leaves, scape and bulb leaves of onion and other Allium host species. In the early stages of infection, lesions appear as oval, concentric rings. Some green islands can be observed within the necrotic lesions. They usually originate around a thrips feeding point. Infected leaves eventually fall over at the point of infection during the latter part of the growing season. Severely infected fields will age prematurely, and entire areas will turn brown before they collapse.
  • Symptom severity is dependent on host cultivar, timing of infection, overall health of the host at the time of infection, and environmental conditions
  • IYSV does not always kill its host, however, the virus reduces plant vigour, disturbs photosynthesis and reduces bulb size. IYSV infection weakens the plants making them more susceptible to other diseases and pests. IYSV-infected onions grown for seed have reduced seed yield and quality.
  • Found in New South Wales, Victoria, Western Australia and South Australia.
  • There are restrictions for produce entering Tasmania.
  • Carried by Thrips tabaci.

 Onion stunt

Onion stunt

Irregular patches of stunting in the field. Image courtesy of Sue Pederick, SARDI

  • Onion smut affects Allium species and cereal crops.
  • Damages the whole plant.
  • Infection develops as circular to irregular patches varying in size from 1m to 25m or more in diameter. The patches usually have distinct borders between stunted and adjacent healthy plants. Stunted onions are most obvious 6 to 12 weeks after sowing when diseased plants are less than 60 per cent the size of healthy plants. The fungus attacks the roots of young plants, pruning roots and causing rotted root tips (‘spear tipping’) and cortex. Brown melanised mycelium typical of Rhizoctonia can be seen microscopically on the roots and around the basal areas of stunted plants. However, stunted onion seedlings can often appear otherwise healthy, without obvious rotted roots or root lesions.
  • Found in South Australia, Western Australia and Tasmania.
  • No movement controls are in place for this pests, but it can be prevented from entering your farm through biosecurity measures.
  • Spread through infected plants/material, soil and water.

Bulb eelworm

Onion attacked by onion eelworm Ditylenchus dipsaci

Onion attacked by bulb eelworm

  • Bulb eelworm affects a broad range of plants including Allium species, cereal crops, ornamental bubs and potatoes.
  • Damages roots and vegetative organs.
  • Penetration of onion leaves by bulb eelworm causes leaf deformation and leaf swellings or blister-like areas on the surface. The leaves grow in a disorderly fashion, often hang as if wilted and become yellow.
  • Young plants can be killed by high infestations. The inner scales of the bulb are usually more severely attacked than the outer scales. As the season advances bulbs become soft and when cut open show browning of the scales in concentric circles.
  • Present in all states and territories except Western Australia.
  • No movement controls are in place for this pest, but can be prevented from entering your farm through biosecurity measures.
  • Spreads through seed and soil.

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