Major diseases of tulip in Maryland include tulip fire (Botrytis tulipae), bulb/basal rot (Fusarium oxysporum f. sp. tulipae), bacterial soft rot (Erwinia carotovora), blue mold (Penicillium species), root rot (Pythium species), nematodes, and viruses. Starting with disease-free stock is always the most important step in the growing process. Bulbs should be purchased from a reputable dealer. Most large bulb producers regularly check stock for viruses and other diseases. Inspect bulbs care- fully upon arrival and have suspicious material examined by a lab. The presence of diseases may warrant the rejection of a shipment.
Bulbs may be dusted or dipped in fungicides such as thiram (available under many trade names) or thiabendazole (Mertect), to help control diseases such as Fusarium basal rot and Penicillium blue mold.
Botrytis (Tulip Fire)
Tulip fire is caused by the fungus Botrytis tulipae and is the most common disease of tulips during production and in the landscape. Initial symptoms include light tan patches on tulip leaves. These patches are most noticeable on light-colored varieties. Leaves become flecked with small brown (necrotic) spots.
Spots enlarge and coalesce to form large brown patches and cause blight of entire flowers or leaves. Flowers also frequently become infected and display flecked brown to white spots. On colored petals spots appear white and on white petals they appear brown. Under conditions of high humidity, necrotic spots on leaves, stems, and flowers are accompanied by sporulation of the brown-gray fungus. Further infection occurs very rapidly once the fungus begins sporulation; this is the origin of the name “tulip fire.” Infection can also lead to the collapse of leaves, stems, and flowers. The outer bulb scales may also become infect ed and display yellow to brown, sunken, circular lesions. Small, black sclerotia (resting structures of the fungus) may be found developing on rot- ting leaf, flower, stem, and bulb tissue.
Control: Bulbs should be carefully inspect ed by removing the outer husks of the bulb and discolored or spotted bulbs should be discarded before planting. Plantings should be inspected regularly; early detection and hand-rouging is essential for controlling this disease. While inspecting plants, carry a bag for diseased materials. As soon as the disease is detected in a bed, affected plants or parts should be removed. Remove faded or blighted flowers, blighted leaves, or entire plants infected at the base and place them in the bag. Discard infected parts with the trash or burn them. It is best not to remove material when plants are wet because fungal spores (conidia) could be spread during conditions that favor disease development. Entire plants should be removed including the bulb portion below ground if infection is observed at the base of the plant. Overhead irrigation and crowded plants can increase disease incidence. During rainy springs when conditions favor disease development, applications of fungicides may be useful when new growth emerges in the spring. Fungicides should also be used if infections occurred in the previous year. Compounds currently registered for use in Maryland include carbamic acid, chlorothalonil, copper hydroxide, iprodine, and mancozeb. (Always consult the label for dosage rates and safety precautions.)
Bulb and Root Rots
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.
Phytophthora cactorum and Pythium species
Bulbs develop soft, gray spots with brown borders near the base of the bulb. Shoots may fail to emerge. If bed has a history of Phytophthora or Pythium infection, then chemical or steam-pasteurized potting mix will be required.
Control: Site management is important for controlling bulb and root rot diseases caused by Erwinia, Phytophthora, and Pythium. These pathogens are favored in moist, shaded areas with poor air circulation and poor drainage. These diseases are highly contagious and control requires the removal and destruction of all infected bulbs. Remove and destroy symptomatic foliage and plant debris in the fall after the tops have been killed by frost. Bulbs should be planted in well-drained soil and watered early in the day. These diseases may also be minimized by avoiding overcrowding and wounding during cultivation. Sanitation is also very important. All equipment should be disinfected between use with 10 percent household bleach, 70 percent alcohol, or one of the commercially available compounds. It is also helpful to control insects and mites because injuries associated with their activity provide sites for infection.
Fusarium oxysporum var. tulipae and Penicillium species are two diseases affecting tulip bulbs during storage. These diseases are also associated with mechanical injury or damage from mites. Infection is favored by moist conditions. Infected bulbs have a dry rot and bulb scales are often covered with the characteristic blue-green (Penicillium) or pink (Fusarium) colored growth of the fungus.
Normally, Fusarium infections begin in the field and continue during postharvest storage. There are substantial differences in cultivar susceptibility, but it is important to reduce the number of infected bulbs in storage because of the ethylene they produce. This can lead to flower abortion and abnormalities.
Control: Producers need to dig carefully in order to avoid wounding the bulbs. Wounds provide natural entry points for pathogenic fungi. It is also helpful to control insect and mite pests. Producers and suppliers should cull and destroy infected bulbs prior to storage.
Bulbs in storage must be routinely inspected and the rooms monitored for ethylene. Levels in the storage rooms must not exceed 0.1 part per million. In storage, Penicillium can be controlled with high rates of ventilation and by maintaining a relative humidity of 85 to 90 percent. Tulip bulbs should always be shipped at 63 °F (~17 °C) under a ventilation rate of 150 m3 h-1. With the exception of a 93°F (~34 °C) treatment for early forcing (a process normally completed by the supplier), nonplanted bulbs are never stored above 77 °F or below 32 °F (> 25 °C or < 0 °C).
Stem and bulb nematode (Ditylenchus dipsaci) is the major species affecting tulip. On tulip, infestation is best detected at flowering. The initial symptom is a pale or purplish lesion on one side of the stem immediately below the flower, which bends in the direction of the lesion. The lesion typically increases in size both downward and upward often on to the petals. The bulbs do not show brown rings as with narcissus and hyacinth when cut across. Infestations start at the base of new bulbs. Upon removal of the outer brown scale, soft grayish or brownish patches can be observed on the outer fleshy scale. Nematodes move out of decayed bulbs into the soil where they overwinter. They may then move into healthy bulbs the next spring. Nematodes require lab tests for identification.
Control: No nematicides are currently labeled for use on tulips in Maryland. Stem and bulb nematode is best controlled by prevention. Many areas (especially Europe) have established legislative laws for Ditylenchus dipsaci as a significant quarantine pest. If infected bulbs are found in a shipment, destruction of the entire contents of the shipment may follow. Hot water treatments for tulip bulbs have been quite effective in controlling nematode problems, but this type of treatment jeopardizes bulb vitality. Many growers report that numerous losses of tulip occur when using hot water treatment to control nematodes.
Viruses known to infect tulips include tulip breaking virus, tobacco necrosis virus, tobacco rattle virus, tobacco mosaic virus, and cucumber mosaic virus. Symptoms of infection consist of stripes or streaks of white or another color in the normal colored petals. A classic example of a virus-infected tulip variety is the “Rembrandt tulip.” Modern tulips exhibiting similar streaking characteristics no longer contain the virus. Leaves of virus-infected plants may appear distinctly mottled. Viruses are persistent in the bulbs and are often transmitted from plant to plant by aphids, especially in situations where plants are crowded and heavily infested.
Control: Bulb suppliers regularly inspect and test for viruses during production. Viral diseases are difficult to control so prevention is a major factor for control. Infected plants and bulbs should be removed and destroyed to minimize spread of the virus. Control of the aphid vectors is also important. Weeds can also harbor various viral diseases and other diseases; therefore, it is important to control weeds in and around production areas.