Seedlings fall over. Cause: Damping-off. Prevent problems by planting in raised beds and presoaking seed in compost tea.
Plant wilts rapidly. Cause: Cutworms. Check for a hole in the stem at or just below the soil line and fat, 1 "-2" long, dull brown or gray caterpillars in the soil near the base of plants. Sprinkle moist bran mixed with BTK on the soil surface in the evening, or add parasitic nematodes to the soil at least a week before planting to control them.
Plant stunted; leaves may be yellow. Causes: Nitrogen deficiency; waterlogged soil; aphids; pink root. Onions need ample nitrogen, deficient plants are pale and grow slowly. Waterlogged soil damages roots and produces the same symptoms. Spray plants and drench roots with fish emulsion or fish-meal tea to alleviate symptoms. Plant in raised beds to improve drainage and add compost before planting to prevent problems.
Aphids are soft-bodied, pale green, black, gray, pink, or white fluffy-coated, sucking insects. Check for them on young leaves. For mild infestations, knock the pests off the plants with a blast of water. Spray plants with insecticidal soap to control them, or with a commercial neem or pyrethrin spray if infestation is severe.
If roots and bulb are pinkish, the plant has pink root. Roots infected with this fungal disease shrivel and die. Destroy infected plants. Prevent problems by ensuring good drainage and adding ample organic matter to the soil. Plant resistant cultivars such as 'Crystal Wax Pickling', 'Early White Supreme', 'Hybrid Big Mac', 'J K Special', 'Tokyo Long White', 'Uno Grande PPR', and 'Voyager',
Plant yellow and wilted. Causes: Fusar-ium bulb rot; onion maggots; lesser bulb flies; wireworms. Onions infected with Fusarium bulb rot have soft necks, and entire bulbs may be soft and brown. Destroy infected plants. Prevent problems by planting cultivars. such as 'Long White Summer Bunching', 'Northern Oak'. 'Sentinel', and 'Valiant', that are somewhat resistant.
Onion maggots feed on roots, killing seedlings and older plants. They also burrow into bulbs, making them unfit for use. Onion maggots are'/«" long, white, and taper to a point at the head. Adults are small gray flies that lay eggs early in the spring. Destroy infested plants. Do not compost them; onion maggots thrive in compost piles. To prevent problems, apply parasitic nematodes to the soil before planting and cover plants with row cover as soon as they come up to prevent flies from laying eggs. Trap maggots by planting a few onions at scattered points around the garden a few weeks before the main planting date. These larger plants attract egg-laying adults, remove and destroy them when infested. 'Egyptian Tree' onions are tolerant of onion maggots. In general, white cultivars are more susceptible to maggot attack than are yellow or red cultivars.
Lesser bulb fly larvae are l/i" long, wrinkled, and yellow-gray. They cause injury similar to onion maggots and are controlled the same way.
Yellow to reddish brown, slender, tough-bodied, segmented worms up to IV2" long feeding on roots and bulbs are wireworms. Adults are dark-colored, elongated click beetles. Apply parasitic nematodes to the soil before planting to control them.
Leaves with white streaks or blotches. Causes: Precipitation damage; thrips. Excessive rain or hail can spot onion leaves. Spots are various sizes, and damage doesn't spread. Maintain good air circulation and make sure soil is well-drained to minimize problems.
If leaf tips are distorted or brown, and leaves are stippled with white, look for onion thrips. Heavy infestations cause plants to wither and turn brown. Adults are tiny, slender, yellow to brown, rapidly moving, winged insects. The larvae can barely be seen with the naked eye. Thrips thrive in hot. dry weather. Trap them with sticky traps hung just above plant level. Try blue, yellow, and white traps to see which work best. Treat plants with insecticidal soap or a commercial pyrethrin spray or dust to control severe infestations.
Leaves with water-soaked or papery, white spots with vertical splits. Cause: Onion leaf blight. Leaf tips turn yellow, then brown. Spray plants with sulfur as soon as you see symptoms if weather is cool and humid. Prevent problems by planting tolerant cultivars such as 'Tokyo Long White'.
Leaves with pale green to brown spots. Causes: Downy mildew; ozone injury. Leaf tips turn yellow, then brown, and may be cov ered with a fuzzy mold. As the disease progresses, spots turn black with a purple, fuzzy mold, and leaves yellow and die. Downy mildew thrives in cool, humid weather and tends to stop spreading when the weather is warm and dry. Spray plants with sulfur to prevent the disease from spreading in cool, wet weather. Don't handle plants when wet to avoid spreading the disease.
Irregular areas with tiny brown flecks are caused by high levels of ozone in the air. Spray plants with seaweed extract and fish emulsion to encourage new growth.
Leaves with sunken, light-colored spots with concentrically ringed, purple centers. Cause: Purple blotch. Spots enlarge and girdle leaves, which wither and fall over. Spray plants with sulfur if disease is present to keep it from spreading. Cool soil makes plants more prone to this fungal disease. Warm soil by covering it with clear plastic for a few weeks before you plant.
Leaves with small reddish orange blisters. Cause: Rust. Leaves infected with this fungal disease may turn yellow and die. Bulbs are small. Clean up and dispose of tops to eliminate overwintering spores. One type of rust infects both onions and asparagus, so keep the 2 crops apart to prevent problems.
Leaves with black streaks filled with dark brown powder. Cause: Smut. Young plants are usually affected. Cool soil makes plants more prone to this fungal disease. Warm soil by covering it with clear plastic for a few weeks before you plant. Prevent problems by planting tolerant cultivars such as 'Evergreen Hardy White' and 'Tokyo Long White'.
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