Category : Fungal
Black mold
Aspergillus niger
Close up of black mold on onion scales
Symptoms of black mold on onion bulb
Symptoms of black mold on onion bulb
Symptoms
Post-harvest black discoloration at neck; lesions on outer scales; black streaks under outer dry scales; entire surface of bulb turning black and shriveling
Cause
Fungus
Comments
Management
Treat seeds with appropriate fungicide prior to planting to reduce rot in mature bulbs; avoid bruising bulbs during and after harvest; storing at temeperatures below 15°C (59°F) prevents mold from spreading but it will resume once temperature increases
Botrytis leaf blight
Botrytis squamosa
Lesions on onion leaves caused by Botrytis leaf blight
Lesions on onion leaves caused by Botrytis leaf blight
Lesions on onion leaves caused by Botrytis leaf blight
Symptoms
Small white lesions with light green halos which may expand slightly as they age; in prolonged periods of moisture fungus may develop rapidly and cause leaf blighting
Cause
Fungus
Comments
Management
Plant onions in single rows allowing at least 30 cm between plants to promote good air circulation and quick drying of foliage after rain; time irrigation to allow plants time to dry out sufficiently; apply appropriate fungicide sprays when plants have at least five true leaves and early symptoms of disease
Downy mildew
Peronospora destructor
Symptoms of downy mildew on onion leaves
Symptoms of downy mildew on onion leaves
Symptoms of downy mildew on onion leaves
Symptoms of downy mildew on onion leaves
Symptoms of downy mildew on onion leaves
Symptoms
Pale spots or elongated patches on leaves; gray-purple fuzzy growth on leaf surface; leaves turning pale then yellow; leaf tips collapsing
Cause
Fungus
Comments
Management
Avoid planting infected sets; rotate crops to non-allium species for 3-4 years; plant in well-draining areas and do not overcrowd plants; destroy all infected crop debris; apply appropriate foliar fungicides taking care to apply thoroughly to waxy leaves
Fusarium basal plate rot
Fusarium oxysporum
cross section of onion bulb showing discoloration around basal plate
Symptoms
Curving, yellow or necrotic leaves; necrosis begins at leaf tips and moves downward; wilting plants; infected bulbs may be brown and watery with rot spreading from stem plate to basal leaves; stem plates may have brown discoloration
Cause
Fungus
Comments
Management
Rotate with non-susceptible crops for at least 4 years, plant resistant onion varieties
Fusarium damping-off
Fusarium oxysporum
Onion plant showing wilt associated with Fusarium damping-off
Symptoms
Rotting seeds that are covered in mold; discolored root tips which may be pink, tan, yellow, red or black; slowly growing seedlings which wilt and die
Cause
Fungus
Comments
Management
Plant only disease-free seed; treat seed with fungicide; rotate crops with cereals or grasses to reduce levels of pathogen in soil; steam treatment or fumigation of soil can help reduce levels of Fusarium in the soil
Pink root
Phoma terrestris
Pink root symptoms
Symptomatic onion roots
Pink root symptoms
Symptoms
Light pink roots which darken and turn purple; roots become transparent and water soaked; plant may look like it has a nutrient deficiency; infected seedling may die; stunted plants with undersized, shriveled bulbs
Cause
Fungus
Comments
Management
Disease is most severe when onions are planted continuously or in a 1-year rotation, a rotation of 3-6 years is preferred; plant more resistant varieties; solarization and/or fumigation can help reduce the levels of pathogen in the soil
Purple blotch
Alternaria porri
Advanced symptoms of purple blotch in onion field
Purple blotch symptoms on neck of onion
Symptoms of purple blotch on onion leaves
Symptoms of purple blotch on onion leaves
Symptoms of purple blotch on onion leaves
Symptoms
Small water-soaked lesions lesions on leaves or stalk with white centers; which enlarge to become zonate and brown to purple in color with red or purple margin surrounded by yellow zone; large lesions may coalesce and girdle leaf, killing any tissue between the lesions and the leaf tip; severely infected foliage may die
Cause
Fungus
Comments
Management
Cultural controls include long rotations with non-hosts and the reduction of leaf wetness by planting in well-draining soil and timing irrigation to allow plants to dry adequately during the day; some fungicides are effective at controlling the disease but should be rotated for optimal control
Rust
Puccinia porri
Close-up image of rust pustule on onion leaf
Rust symptoms on onion leaves
Orange rust pustules on onion leaves
Symptoms
Small white flecks on leaves and stems which develop into circular or elongated orange pustules; severe infestations can cause leaves to yellow and die
Cause
Fungus
Comments
Management
No resistance known; use only disease-free seed and plant in well-draining soil; control weeds around crop; apply appropriate protective fungicide
Smut
Urocystis colchici
Onion plants showing symptoms of smut
Stunted onion plants infected with smut
Symptoms
Dark, thickened lareas on cotyledons (seed leaves) which may become large and cause leaves to bend downwards; raised blisters may be present on the base of scales in older plants; lesion mature and become covered in black powdery fungal masses; plant growth stunted; death of plant occurs within 3-4 weeks
Cause
Fungus
Comments
Management
No resistance to disease known in onion; plant onions during periods which promote rapid growth; plant only health sets and transplants - if smut is present in the soil they will not become infected
White rot
Sclerotium cepivorum
Symptoms
Older leaves yellowing; stunted growth; death of all leaves; fluffy white growth on base of bulb which spreads up bulb to storage leaves
Cause
Fungus
Comments
Management
Fungicide treatment may not be effective at controlling white rot under conditions which are favorable to the fungi's development and control may have to rely on cultural methods: avoid transferring soil or plant material between sites; treat seeds with hot water prior to planting; use a long term rotation with non-allium crops; apply appropriate fungicides if available