Plant Health Problems
There are a number of diseases which are common to all plants considered "bulbs" although their impact and relative importance can vary from genus to genus.
Diseases caused by Fungi:
Leaf scorch, Stagonospora curtsii.
This fungus infects leaves, flower stalks, flower parts, or bulb scales. Affected plant parts are often bent or deformed at the point of infection. Brown spots or blotches with yellow borders develop on diseased tissues. Flower stalks of plants with severe infections often dry up without producing flowers. Since the fungus is probably present in the bulb, infections occur as flower stalks and leaves emerge from the bulb.
Control can be achieved by minimizing moisture on the leaves and flower stalks by careful watering. It is also helpful to provide good ventilation and plenty of light since these will decrease the chance of spreading the fungus. Heavily infected bulbs should probably be discarded.
Bulb rot, Penicillium, Fusarium.
This disease often develops during storage and is frequently associated with mechanical injury or damage from mites. Infection is favored by moist conditions. Infected bulbs have a dry, punky rot and the bulb scales are often covered with the characteristic blue-green (Penicillium) or pink (Fusarium) colored growth of the fungus.
Control is achieved through careful digging to avoid wounding. It is also helpful to control insect and mite pests. Infected bulbs should be destroyed.
Botrytis blight, Botrytis spp.
This is one of the most widespread and common disease of bulbs. Symptoms are variable and can appear as oval or circular spots which are initially reddish-brown and develop pale centers and purplish margins. These spots may run together and rot the entire leaf. These can progress into the stem and cause the stalk to fall over. If the spots dry out, they turn brown or gray. Buds or flowers may turn brown and rot and are often covered with the diagnostic gray, fuzzy growth of the fungus. This disease shows up and spreads rapidly under cool, humid conditions, especially if plants are crowded.
Control involves raking and removing any affected plant parts after the tops are killed in the fall. It is also helpful to avoid overhead irrigation and crowding of the plants. When conditions are favorable, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut are chlorothalonil, thiophanate-methyl, iprodione, and mancozeb. Consult the label for dosage rates and safety precautions.
Diseases caused by Bacteria:
Soft rot, Erwinia carotovora.
Infected plants fail to flower or blossoms fall off before they open. Tops may appear water-soaked and collapse. Infected bulbs have a strong odor and are soft and mushy.
Since this pathogen is highly contagious, all infected bulbs should be discarded. Bulbs should be planted in well-drained soil and watered early in the day. This disease can also be minimized by avoiding overcrowding and wounding during cultivation. Sanitation is also very important. All equipment should be disinfested between use with 10% household bleach, 70% alcohol, or one of the commercially available compounds. It is also helpful to control insects and mites since injuries associated with their activity provide sites for infection.
Aphids, Anuraphis tulipae.
The tulip aphid, Anuraphis tulipae, is perhaps the most common and most troublesome aphid on tulip, and is more apt to cause injury on forcing bulbs than those grown outdoors. It also winters on stored gladiolus corms. Immersing the infested bulbs or roots in 110º F water for 30 minutes will destroy the insects. Among the compounds registered for control of this pest in Connecticut are methoxychlor and malathion. Infested bulbs may be dusted with methoxychlor or dipped in a malathion solution for effective control. Consult the label for dosage rates and safety precautions. Exposure to the treatment should last for several hours before the bulbs are planted or returned to storage. Imidacloprid, applied as a drench, will provide season-long control.
Bulb fly, Merodon equestris.
The maggot of this fly infests the bulbs and ruins them. There is only one maggot in a bulb, and the insect has one generation each year. The larva overwinters in the bulb, and pupation occurs in the spring in the old burrow or nearby in the soil. The adults appear in early summer and lay oval white eggs near the base of the leaves or on exposed portions of the bulbs. The maggot is a yellow or dirty white larva without legs. It is about 3/4" long. The fly is about 1/2" long, black, and banded with yellow or gray, is hairy and resembles a bumblebee.
Destroy all infested bulbs after digging. Three hours of hot-water treatment at 110º F will be helpful in control. Small, stunted and otherwise obviously infested plants may be dug up and burned, thus preventing the spread of the infestation.
Bulb mite, Rhizoglyphus echinopus.
This mite injures nearly all kinds of bulbs. Easter lily plants in greenhouses have been severely injured. The mites breed continuously in greenhouses or wherever the temperature and moisture are sufficiently high. It is possible for 10 or more generations to mature in a year. When the conditions become unfavorable to the mite, such as lack of food or moisture, a resistant stage called hypopus is formed. It is adapted for migration to fresh food supplies.
The control measures are: burn all soft and decayed bulbs, if allowed; store bulbs at about 35º F. Heat treat bulbs before storage (see also bulb fly, above).
These millipedes frequently attack the bulbs in tulip beds, especially in old beds where the bulbs are not reset each year. They eat into the bulbs and the mutilation is followed by decay. Sometimes all the bulbs are destroyed. When abundant, these millipedes also injure other bulbs, strawberry plants, and the roots of various other plants. Tulip bulbs may be dug after flowering, kept cool and dry, and planted in the fall. Dusting the soil surface with diazinon, which is among the compounds registered for control of this pest in Connecticut, will be helpful in managing this pest. Consult the label for dosage rates and safety precautions.
Lesser bulb fly, Eumerus tuberculatus.
This fly is also a pest of onion, but it most severely injures narcissus. It is believed that there are two generations each year. The flies appear in May and June and lay eggs at the base of the leaves. The larvae find their way to the tip of the bulb and then go downward into the interior. As many as 77 larvae have been found in a single bulb. When fully grown, these maggots are between 1/3 and 1/2" long, wrinkled and dirty grayish yellow in color. They pupate in August in the bulb or in the soil near it. Certain larvae overwinter in the bulbs, but these are thought to be the second generation, from which flies emerge the following spring. The fly is about 1/3" long, and has gray wings and a black abdomen marked with three white crescent-shaped bands.
See bulb fly, above, for control.