Symptoms, Causes, and Cures
Scan this list of common bulb diseases and pests for any that match damage, insects or other problems you find in your garden. Click on each problem’s tab to find symptoms, causes, and cures.
Before treating any problem, review the steps to controlling pests and diseases using the Integrated Pest Management method [see: Flower and Bulb Pests and Diseases].
If the cure requires application of sprays or other control treatments, read completely and follow exactly the application directions found on the control’s label. Only apply controls that list the specific pest or disease found on your bulbs.
Before treating problems, review the demonstrations of common methods used for treating bulbs with fungal diseases or insect pests [see: Treating Insect Pests on Bulbs and Treating Fungal Diseases in Bulbs].
Bulb Pests and Diseases
Symptom: Curled, twisted, sticky leaves; stunted or deformed blooms; loss of vigor. Sometimes found within tunic covering of lifted bulbs.
Cause: Aphids. Look for clusters of soft-bodied, round, black, gray, green, or yellow insects, 1/16–1/4 inch (1.5–6 mm) long. Found on many plants. Frequently found in combination with ants that milk them for their honeydew secretions.
Cure: Wash off light infestations with a strong stream of water. Release ladybird beetles, lacewings. If ineffective, dust with diatomaceous earth. Spray with a solution of 2–3 T (30–44 ml) of dishwashing liquid per gallon (4 l) of water. Spray with insecticidal soap [see: Treating Insect Pests on Bulbs].
Symptom: Flowers and foliage collapse under heavy fuzz of brown or gray fungal spores.
Cause: Botrytis rot, also known as gray mold, a fungal disease.
Cure: Remove affected blossoms, foliage, or entire plant. Space plants for more air circulation. Reduce or eliminate use of nitrogen fertilizer. Remove mulch. Water early in the day so foliage dries completely before evening.
Symptom: Foliage is eaten and stems are scored or eaten. Black or brown droppings, 1/20 in. (1.2 mm) wide are left on foliage. Moths and butterflies are seen resting on plants.
Cause: Various larvae and caterpillars, wormlike larvae, and loopers. Look at undersides of leaves and along stems for pests during midday.
Cure: Hand pick caterpillars from center leaves of plant. Release parasitic wasps and flies. Spray with Bacillus thuringiensis (BT). Apply horticultural oil.
Symptom: Uprooted plants; foliage eaten to ground level; bulbs and roots eaten, leaving dying stalks and leaves.
Cause: Deer and rodents. Look for hoof and paw prints, burrows, mounds, and tunnels.
Cure: Plant resistant plants. Install fence barriers or cages when planting, including beneath-soil barriers [see Protecting Bulbs from Animal Pests]. Trap and remove rodents. Avoid bonemeal and fish emulsion fertilizers that attract animals by their strong smells.
Symptom: Powdery black or brown dusting on foliage and blossoms. Leaves may drop.
Cause: Leaf spot, a fungal disease. Common in shaded, crowded plantings.
Cure: Remove shading foliage. Increase air circulation. Spray with a fungicide containing sulfur.
Symptom: Stunted plants; white cottony clusters in leaf axils or where leaves meet at bases of plants.
Cause: Mealybugs. Look in the junctions between leaves and stems or at the base of leaf clusters for white or gray, waxy bugs, 1/8-in. (3-mm) long.
Cure: Dab or spray with rubbing alcohol solution diluted 3:1 with water; spray with insecticidal soap; spray with horticultural oil.
Symptom: Streaked and mottled foliage, deformed blooms; stunting; loss of vigor.
Cause: Mosaic virus, an incurable plant disease.
Cure: Remove bulbs and dispose of affected plants, do not compost. Promptly control aphid, spider mite, and thrip infestations, that can spread viral infection. Plant mosaic-virus resistant species and cultivars.
Symptom: Stunted, discolored, or spotted plants with deformed roots, sometimes bearing swollen galls; loss of vigor.
Cause: Soil nematodes, near-microscopic wormlike creatures that live in soil and feed on plant roots and bulbs.
Cure: Release beneficial nematodes. Remove and destroy infested plants. Solarize bed for 3–4 weeks prior to planting by covering soil with clear plastic and allowing sunlight to raise soil temperature to 140°F (60°C). Replant with unrelated species.
Symptom: Light powdery dusting of gray or white on leaves, flowers; deformed new growth; stunting; loss of vigor.
Cause: Powdery mildew, a fungal disease often found when days are hot and humid days and cool nights alternate. Mostly attacks new leaves.
Cure: Remove shading foliage. Improve air circulation. Dust with sulfur. Spray with a solution of baking soda (sodium bicarbonate) and horticultural oil: 1 tablespoon (15 ml) baking soda and 3 tablespoons (44 ml) horticultural oil per gallon [4 l] of water.
Symptom:Chewed leaves and blossoms; silvery mucus trails.
Cause: Slugs and snails. After dark, look for shelled and unshelled mollusks on foliage or soil.
Cure: Remove leaf litter, a common hiding place. Hand pick after dark; use copper foil barriers around beds or containers; dust with diatomaceous earth; use beer-filled traps; use non-toxic baits containing iron phosphate; use bait gel.
Symptom:Leaves develop speckles, wrinkle, turn yellow, and drop. Tiny white webs coat leaf undersides, basal junctions.
Cause: Spider mites. Shake foliage and blossoms over white paper and look for moving red or yellow, spiderlike specks. Thrive in hot, dry conditions.
Cure: Release lady bird beetles. Spray repeatedly with a strong stream of water to rinse off dustlike pests; spray only infested plants with insecticidal soap [see: Treating Insect Pests on Bulbs]. Avoid use of sprays that kill natural spider-mite predators or widespread spraying beyond immediately infested plants.
Symptom: Spotted, sometimes semi-translucent leaves. Frequently found with fungal disease.
Cause: Stem borers. Look on foliage, roots for segmented larvae and caterpillars, 1/2–1-in. (12–25-mm) long. In gardens near corn-farming production, corn borer may infest dahlia and gladiolus.
Cure: Hand pick. apply Bacillus thuringensis (BT) to affected foliage. Remove and destroy infested foliage.
Symptom: Brown-, silver-, or white-speckled leaves. Foliage may feel gummy or be deformed. Blooms are deformed and fail to open.
Cause: Thrips. Shake foliage and blossoms over white paper and look for moving, winged specks. Thrive in hot, dry conditions.
Cure: Release lady bird beetles. Remove and destroy infested foliage. Wash foliage with a strong stream of water. Spray pests on infested plants with insecticidal soap [see: Treating Insect Pests on Bulbs].