Pests and Disease Management in Onion
Black mold Bulb mites Downey mildew Fusarium damping-off Leaf streak and bulb rot Leafminers Onion Maggot Onion yellow dwarf Pink root Purple blotch Pythium seed rot Rust Smut Thrips
Description | Symptoms | Comments | Cause | Management |
Black Mold (Aspergillus niger) | Post-harvest black discoloration at neck, lesions on outer scale | Wash hands after coming into contact with fungus | Fungus | Seeds to be treated with appropriate fungicide to reduce rot in mature bulbs, storing at temperature below 15 degree Celsius reduces mold from spreading, avoid bruising bulbs |
Botrytis leaf blight (Botrytis Squamosa) | Small white lesions with light green halos which may slightly expand as they age | Disease emergence favours high humidity and warm temperatures, older leaves are susceptible to blighting than younger leaves | Fungus | Allow at least 30 cm spacing between plants to promote good air circulation and quick drying of foliage after rain, appropriate fungicide sprays also required |
Downey mildew (Peronospora destructor) | Pale and elongates patches on leaves, leaves turning pale then yellow | Disease emergence favoured by cool temperature and leave wetness | Fungus | Plant in well-draining areas and don’t over crowd plants, avoid infected plant sets, apply appropriate fungicide and destroy infected crops |
Fusarium basal plate rot (Fusarium Oxysporum) | Necrosis begins at leaf tips and move downwards, Wilting plants, infected bulbs may be brown and watery with rot spreading from stem plate to basal leaves | Disease emergence favours moderate to high temperatures | Fungus | Rotate with non-susceptible crops for at least 4 years, plant resistant onion varieties |
Pink Root (Phoma terrestris) | Pink roots which darken and turn purple, roots become transparent and water soaked | Fungus colonizes through root tips, fungus can survive in soil down to a depth of 17.7 inches | Fungus | A rotation of 3 to 6 years is preferred, plant more resistant varieties, fumigation can help reduce the pathogen in the soil |
Purple Blotch (Alternaria porri) | Severely infected foliage may die, large lesions may coalesce and girdle leaf, killing any tissues between the lesions and leaf tip | Disease emergence favoured by wet foliage, with sporulation occurring in night period on high humidity | Fungus | Fungicides are effective at controlling the diseases but should be rotated for optimal control, Cultural controls includes long rotation and the reduction of leaf wetness by planting in well – drained soil and timing irrigation |
Pythium seed rot (Pythium irregular) | Seeds water soaked and decomposing, seedlings that have already emerged prior to infection collapse and die | Disease emergence favours high soil moisture and cool temperatures | Fungus | Can be controlled by minimizing moisture, treat seeds with appropriate fungicides prior to planting |
Fusarium damping-off (Fusarium oxysporum) | Rotting seeds, discoloured root tips which may be pink, tan , yellow , red or black | Fungus survives in soil and emergence is favoured by moist to wet soil | Fungus | Plant only disease free seed, treat seed with fungicide, rotate crops to reduce the levels of pathogen in soil |
White rot (Sclerotium cepivorum) | Older leaves yellowing, stunned growth, fluffy white growth on base of bulb which spreads up bulb to storage leaves | Fungus can survive in soil for 20 years and is one of the most damaging diseases of Allium crops worldwide | Fungus | Fungicides along with cultural methods recommended, treat seeds with hot water prior to planting, use long term rotation with non-allium crops |
Smut (Urocystis colchici) | Dark, thickened areas on cotyledons which may become large and causes leaves to bend downwards, lesion mature and become covered in black powdery fungal masses, plant growth stunted | Smut can persist in soil for many years and is mainly introduced through infected seeds and transplants | Fungus | No resistance to disease known in onion, plant only healthy seeds and transplants – if smut is present in the soil they will not become infected |
Leaf streak and bulb rot (Pseudomonas Viridiflava) | Dark green oval lesions or streaks on leaves, reddish brown discoloration of inner scales, rot developing in ring like pattern | Greatest damage occurs during winter, rapid spread of disease on infected plants is promoted by rainfall | Bacterium | Avoid fertilizing during winter, apply appropriate bactericidal sprays |
Onion yellow dwarf Onion yellow dwarf virus ( OYDV) | Yellow streaks on bases of first leaves, leaves may be flattened or crinkled, flower stalks yellow and twisted | Transmitted by several aphids, including peach aphid, virus is not spread via seed or pollen | Virus | Plant more tolerant varieties, remove infected plants, to certain extent insecticides can be used to control aphids, planting sets or transplants which were produced in area free of virus |
Thrips (Thrips tabaci Frankliniella occidentalis) | Distorted tissues, scarring of leaves, infected plants may have a slivery appearance | Most damaging at the initial stage of bulb development | Insect | Predatory mite, pirate bugs and lacewings are natural enemies, avoid planting onion in close proximity, apply appropriate insecticides, overhead irrigation can helps to reduce thrips |
Rust (Puccinia Porri) | Small white flecks on leaves and stems which develop into circular or elongated orange pustules, severe infestations can cause leaves to yellow and die | Favours high humidity and low rainfall | Fungus | No resistance, apply appropriate protective fungicide |
Leafminers (Lyriomyza spp.) | White and winding trails on leaves, early infestation can cause yield to be reduced | Mature larvae drop from leaves into soil to pupate, entire lifecycle can take less than 2 weeks in warm weather, insect may go through 7 to 10 generations per year | Insects | Check for leafminers before planting, remove plants from the soil after harvest, use appropriate insecticide whenever required. |
Onion Maggot (Delia antiqua) | If infestation occurs during bulbing, bulbs will be deformed and susceptible to storage rots after harvest, stunted or wilting seedlings, adult insect lay eggs around the base of plant and the larvae that emerge are tiny and bore into onion plant | Females can lay hundreds of eggs during their 2 to 4 week life span | Insect | Can be managed by good sanitation and appropriate insecticide. |
Bulb Mites (Rhizoglyphus spp. Tyrophagus spp.) | Stunted plant growth, bulbs rotting, pest is a cream – white, bulbous mite which resemble pearl with legs | Damage by bulb mites allow secondary invasion by other pathogens and causes bulb rots | Arachnid | Don’t plant successive crops of onion or garlic in same location, allow fields to fallow to ensure that any residual organic matter decomposes completely because crop residues harbor mite population |
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