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Lilies' Pests And Diseases

Lilies seem to be more vulnerable to pests and diseases than many other garden plants. However, with attention and the very careful use of a few modern chemicals most should survive to put up a good show every year. It is unfortunate that chemical pesticides and fungicides are required as many gardeners are beginning to be wary of them, for both safety and environmental reasons. In some cases they can be avoided, for example by killing aphids with fingers and thumbs rather than with a pesticide, but in other cases there really is no alternative if you want to save the plant. A wise gardener will only use chemicals when absolutely necessary and will always follow the instructions on the bottle or packet. Never exceed the given dosage as this will not increase the effectiveness of the chemical; it may well do the reverse and do more damage than the problem the chemical is supposed to control. It will also put more poisonous chemicals into the environment for no good reason.

Viruses

Lily bulbs attacked by virus, the worst disease, display a number of symptoms, to a greater or lesser degree. Normally these include paler streaks in leaves and stems, distorted leaves, buds, flowers or stems and overall failing performance. The same virus may cause the precipitate decline of one lily while another species or cultivar lives with the disease with relative ease. Similarly, a plant with one virus may appear to manage without too much distress but, when attacked by an additional one, will collapse hopelessly. There is no real amateur cure for virus-infected bulbs - affected bulbs should be immediately destroyed before they infect other stock.

Lily longiflorum and Lily formosanum are particularly vulnerable to virus attack, as they show the symptoms quickly and markedly. They can be used to test the presence of virus in other stock by injecting them with material from doubtful cases.

Cucumber mosaic virus
This can cause very streaked foliage color and distorted, brittle leaves and flowers. Leaves may become very pale and plants stunted. It is probably the most obvious and widespread virus, one that also infects many other garden plants such as tulips, dahlias and delphiniums.
Tulip breaking virus
This virus causes the broken color that so excited Dutch tulip growers centuries ago. In lilies it causes mottling of the foliage, the lessening in intensity of darker flowers and sometimes the breaking of the color.
Brown ring virus
This virus was discovered in Lily 'Enchantment'. It causes the scales to be stunted and to be held more loosely. They are marked with brown rings and this tissue perishes. Plants become dwarfed and pale.
Lily symptomless virus
This difficult-to-spot, creeping disorder must be suspected if the plants look less lively, lacking the joie de vivre of their neighbours. They are on the slippery slope to extinction and, if you feel sure that you have discovered cases, they should be helped on their way without delay. Be careful to differentiate between plants that may not be flourishing as well as they might because of other reasons, such as overcrowding, competition from neighboring plants, damage from rabbits or slugs, or distress from drought conditions.

Pests

Lilies are prone to attack by several animal and insect predators, ranging in size from deer and elk to microscopic mites. The presence or absence of a specific predator depends on the region and climate zone in which the plants are grown. One can be assured, however, that predators are never as devastating as diseases such as Botrytis, Fusarium, or virus, which can destroy whole crops or garden plantings.

Aphids
The most important pests are aphids. These need to be controlled at all costs. Few insects breed more rapidly, so every measure should be taken to eradicate them on their first appearance. This is crucial if one is growing varieties or species known to be susceptible to virus. The several species of aphids that colonize lilies not only spread virus disease, but they also debilitate the plants physically, causing twisted leaves and distortion of flower buds.
Aphids produce their young viviparously in large numbers. On an infested plant one can often see large female aphids surrounded by scores of tiny offspring. These insects ordinarily move only by crawling, but at certain times they also produce winged migrant offspring that can fly for surprisingly long distances to colonize other plantings.
Systemic insecticides can be used to control aphids. When watered into the soil, the insecticide is taken up by the roots and absorbed through the conducting tissues of the plants. The aphids, being sucking insects, are then killed by the poisoned sap. This class of control is particularly effective in container plantings where the chemical remains concentrated longer. Systemic insecticide sprays should be applied to plantings on a regular basis. They can be combined with a fungicide. The insecticide acephate (e.g., Orthene) has been used quite successfully for aphid control, and malathion, which is sprinkled on the soil and watered in, can protect a plant from aphids for an entire growing season.
A variety of products are available for fumigating greenhouses. These must be used with caution under carefully controlled conditions. Aphids are particularly troublesome in greenhouses, where the environment is optimal for their increase and their natural predators tend to be excluded.
Insecticides available on the market change continually, but all must be used with the utmost care. Always use rubber gloves and protective clothing; many insecticides can be absorbed through the skin and cause neurological damage.
Petroleum oil sprays or mineral oils reduce virus spread in many crops, including potatoes and lilies. The film of oil does not kill the aphids, but it prevents them from transmitting the disease through their stylets by clogging them. Oil sprays are considered to be 60 percent effective in controlling viruses in commercial plantings. The weekly sprays used by most commercial growers are hardly practical for home gardeners, but those with large lily plantings may find the effort worthwhile. Oil spraying should be avoided during the heat of the day, when it may produce leaf scorch and distortion; try to do it just before sundown. A light summer oil, used at about 1 percent dilution, is usually effective. Oils are quite safe to use and can be combined with most insecticides and fungicides.
Any spray can be dangerous to use on tender young seedlings. For this reason, it is better to control aphids, which are very attracted to seedlings, with a granular systemic insecticide such as malathion, which is sprinkled on the soil and watered in. Home gardeners can purchase similar products under such brand names as Dexol or Cole's systemic granules.
If aphids are continually moving into the planting from an outside source, efforts to control them are unlikely to be effective. Hybridizers and other enthusiasts may grow lilies in aphid-proof screen houses; however, aphids can be carried into such structures on clothing or tools, so a spray program should be maintained. Migrant aphids can be inhibited from moving between plants by barriers of other vegetation or gauze; the barrier must exceed the height of the lilies.
Finally, aphids overwinter by laying eggs, which are produced by the migrant phase. It is thus important to destroy any dead plant material that may harbor overwintering eggs.
Bulb mites
The loose structure of lily bulbs makes them susceptible to infestation by pests that live between the scales. Bulb mites (Rhizognyphus echinopus) are troublesome but usually secondary pests; they attack many other bulbs along with lilies.
The adult mites are about the size of a pinhead, rounded, and yellowish white in color, often tinged with pink. In warm climates they are usually present in large numbers, particularly just above the basal plate and between the scales. They attack the roots and basal plate and eventually enter the center of the bulb. The following methods of control may be used against mites:
  • Treat bulbs with hot water at 44C (111F) for one hour.
  • Fumigate dry bulbs with Para dichlorobenzene (the active substance in mothballs) in an airtight container. Spread the fumigant over the bottom of the container, using 4 grams per liter of air space and exposing the bulbs for 12 hours.
  • Dust the bulbs with flowers of sulfur. This is probably the best method for the home gardener.
  • Destroy badly infested bulbs by burning them.
Deer, hares, and rabbits
These herbivores often nibble on young growth, buds, flowers, seedpods, and other plant parts; however, they are seldom seriously destructive. In rural areas gardens can be protected by deer- and rabbit-proof fences, adequately maintained. The presence of a dog often deters deer from invading, and many cats will prey on rabbits.
Chemical controls and repellents, trapping, and shooting offer only temporary solutions, if indeed they are effective at all. Foul-smelling concoctions, both homemade and commercial, are sometimes effective against deer. The material is placed in muslin sacks and suspended around the plantings.
Leatherjackets, wireworms, and millipedes
These groups of underground pests can cause damage and losses in lilies and other crops. All are prevalent in grassland.
  • Leatherjackets are the larvae of the crane fly or daddy-long-legs (Tipulidae). They are sluggish, legless, dull-colored brownish creatures that may reach 3.75 centimeters (1.5 inches) in length.
  • Wireworms are the larvae of click beetles. They are slender, smooth, tough, and wiry creatures, measuring up to 2.5 centimeters (1 inch) in length. They are golden yellow and have six very short legs. These pests burrow into bulbs.
  • Millipedes are dull-colored, sluggish creatures with many legs. They curl up when disturbed.
All these underground pests can be controlled by dusting the ground with benzene hexachloride (BHC) or Bromophos. A nonchemical control involves baiting with slices of potato, carrot, or other root vegetable placed below the surface of the soil. These traps can be skewered on a stick to mark their position and lifted a few days later, when the attached pests can be destroyed.
Lily beetle
Long prevalent in Europe, the lily beetle (Lilioceris lilii) has been reported in eastern North America. The larvae and adult beetles feed on the leaves of lilies and other liliaceous plants, including Convallaria (lily of the valley) and Fritillaria. Both life stages have voracious appetites and soon devour entire plants.
The larva is a humpbacked, dirty yellow grub with a dark head, repulsively covering itself in dark, slimy excrement. The adult is up to 8 millimeters (0.25 inch) long and bright scarlet with black legs and antennae. The eggs are laid on the underside of the foliage. The following controls are effective:
  • Spray plants with contact and systemic insecticides; both are effective.
  • Drench soil with an insecticide such as acephate (e.g., Orthene) to kill the mature larvae that live just under the soil surface in winter. Also, avoid transporting infested soil to other sites.
  • Beware of imported bulbs.
  • Catch adult beetles between the fingers and smash them.
Lily thrips
The adult lily thrip (Liothrips vaneeckii) is very tiny and black in color. The larva is salmon pink and minute. The adults and larvae live out their entire life cycle in the bulb. Feeding seems to be localized at the bases of the scales, where it seriously weakens the bulb, rendering it flabby. This allows the entry of bacteria and fungi, frequently resulting in the bulb rotting away.
The following controls can be used:
  • Treat bulbs with hot water treatment at 44C (111F) for one hour to eradicate the pest.
  • Dust bulbs with benzene hexachloride (BHC).
  • Wash or dip bulbs in a solution of an insecticide.
Lily weevil
The lily weevil (Agasphaerops nigra) is a native of western North America from northern California to Vancouver Island. It has been reported both on native lilies of that region and on cultivated forms of Lilium longiflorum (Easter lily).
The larvae are minute, whitish, legless grubs with chestnut-brown heads. They burrow into the lily stem and bulb. Adult weevils emerge in March and April, feeding on the leaves of plants.
Systemic insecticides are highly effective in controlling weevils.
Nematodes
Many species of nematodes or eelworms inhabit soils everywhere. Some are harmless or even beneficial to plants, but others are destructive. The most harmful to lilies are the root lesion or meadow nematode and the leaf-lesion nematode. These microscopic pests cause serious damage to lily crops in some regions if their populations are not under control.
Nematodes penetrate root tissues, killing cells as they go. They move inside the root, feeding, laying eggs, and destroying additional cells. The roots become soft and flabby, eventually succumbing to infection that moves into the basal plate, turning it into mush.
River water often carries nematodes, which can then enter croplands through irrigation. These pests also host bacteria; some species even carry virus diseases. Nematode infestation causes stunting of growth and can severely reduce commercial production. Crops parasitized by nematodes are seldom uniformly affected.
Foliar nematodes live in the soil. When a suitable host is present, they move through the stem in a surface film of moisture to invade the leaves and flowers.
The following controls are used for nematodes:
  • Keep the foliage as dry as possible to control foliar nematodes by preventing movement of the organisms. Systemic insecticides are very effective.
  • Treat bulbs with hot water at 44C (111F) for one hour.
  • Fumigate soil with methyl bromide, chloropicrin, and metamsodium (e.g., Vapam). This is a highly successful technique in commercial plantings. Steam sterilization of greenhouse soils is very important.
  • Avoid planting lilies continuously in the same site; this prevents harmful nematode populations from building up in the home garden.
  • Apply a granular nematicide such as fenamiphos (e.g., Nemacur) when planting bulbs.
  • Try adding carnivorous nematodes to the soil.
Pheasant and quail
These birds can develop the habit of pecking emerging shoots in very early spring. They may also peck down to destroy bulbs during cold periods when other food is scarce. Control should be restricted to trapping or shooting the birds only when damage is severe. Poison grains are strictly outlawed in most areas and must never be used to control pheasants and quail.
Rodents
Mice and voles often devour lily bulbs, especially when their populations are poorly controlled by animal and bird predators. If moles are active in an area, mice and voles often use the mole tunnels to get access to bulbs. Traps and poison baits are effective controls; in the home garden, a predatory cat or dog can be of great assistance.
Squirrels and chipmunks sometimes learn to prey on garden bulbs. The best control is to plant bulbs some distance from trees, since these pests do not like to venture far from the safety of their homes.
Several species of gophers are serious pests in lily plantings in western North America. They love lily bulbs and can devour great numbers in a season. On a small scale, trapping is also an effective way of controlling gophers.
Slugs and snails
These hermaphroditic mollusks lay clusters of round, white, jelly-like eggs in little niches in the soil. These can be destroyed by frequent cultivation of the top few inches of soil, bringing the eggs to light and exposing them to frost and birds.
Slugs in particular can be a problem both above and below ground, depending on the species present. They find harbors in moist, shaded areas under dead leaves and other plant debris or among low-growing plants. They are particularly prevalent during rainy seasons.
Slugs and snails feed voraciously on lilies when the shoots emerge. Later in the season they can climb the stems, stripping the leaves completely.
To control slugs and snails:
  • Limit habitat around lily plantings by controlling weeds and using mulch instead of ground cover planting.
  • Place bait containing metaldehyde among plantings in the early evening hours. This is crucial at the time when lily shoots are emerging and during damp weather. Bait should be renewed after heavy rain. Liquid bait such as Deadline may be less attractive to pets than pelletized bait. Beer traps have been used successfully by some enthusiasts: simply pour the beer into a shallow dish and place it near the lily plantings in the early evening.
Symphylids
These tiny insects are prevalent in some soils and can be extremely difficult to eradicate. They can damage lily crops severely if their populations get out of control. The tiny creatures are barely visible to the naked eye; they are best diagnosed by submitting a soil sample to an agricultural laboratory. The usual control is soil fumigation.

Fungus diseases

Fungi are organisms that live in or on plant tissues, deriving nutrients from the host plant and destroying its cells. As in the case of viruses, multiple fungal diseases affect lilies and tend to be more destructive in combination than singly. The two most serious fungal diseases of lilies are basal rot and Botrytis blight; the former is the more destructive because it attacks the bulb. Other fungal diseases, such as black scale disease, blue mold, Cercosporella blight of foliage, root rots, rust, Sclerotium, and stump rot, are rarely problems for the home gardener. The best policy in the long term is to destroy ruthlessly any sick or suspect plants, no matter what the cause of their symptoms. Attempts to revive unhealthy lilies are usually futile.

Basal rot
The fungus Fusarium oxysporum var. lilii often occurs in combination with another fungus, Cylindrocarpon, especially in the Netherlands. Fusarium is more likely to damage Asiatic lilies, and Cylindrocarpon is more serious in Orientals. Usually Fusarium is the primary pathogen and Cylindrocarpon the secondary one. Cultures isolated from diseased bulbs seldom indicate the presence of only one pathogen; usually there are two or more. Fusarium, however, is the most serious soil-borne disease in North America and should be considered a primary pathogen.
Basal rot is recognized by a chocolate or dark brown rot that extends into the scales from the basal plate. The scales may become detached at the basal plate so that the bulb falls to pieces. The fungus invades the bulb through the roots, the basal plate, and the basal end of the scales. Many lily varieties are very susceptible, and the decay continues rapidly until the bulb disintegrates.
The pathogen is readily disseminated by spores, which can be carried in the soil or on the surfaces of bulbs, tools, agricultural equipment, or packing crates. Released from the debris of the decayed bulbs, the fungus can remain viable in garden soil for at least three years without a host.
The usual symptoms of basal rot in growing plants are premature yellowing of the foliage, stunting, and premature senescence. All are typical reactions to ethylene, a gas produced by the decaying bulb tissues. Infected bulbs tend to produce many new scale bulblets, usually on the severed scales; however, such bulblets form at the infected end of the scale and are thus readily infected in turn. The main bulb is frequently destroyed, but masses of stem bulblets often form.
Fusarium is present in most soils and is most active and destructive when soil temperatures and moisture levels are high, during the summer months. It is prevalent where lilies have been grown for many years. In cool climates in northern areas the disease is less of a problem.
Many lily varieties are highly susceptible, but others show strong resistance, which can vary with climate and soil. The turgid, plump, soft bulbs of certain Asiatic hybrids, such as Lilium cernuum and its hybrids, seem prone to Fusarium bulb rot. There also appears to be a link between the red color in Asiatic hybrids and Fusarium susceptibility: 'Cinnabar', 'Pirate', and 'Scarlet Emperor' are always among the first to suffer. 'Chinook', 'Connecticut King', 'Enchantment', and 'Pollyanna' have shown some resistance.
Gardeners also need to understand more about cultural controls. They can begin by never planting bulbs that show signs of fungus disease. Growers and distributors of lily bulbs must make every effort to ensure that their bulbs are clean. This includes propagating them from clean scales, stem bulblets, bulbils, and seed. When basal rot is detected in a clump or bed of lilies, infected plants should be lifted and destroyed. In the more valuable varieties, some clean, healthy scales and stem bulblets may be identified and saved. In heavily infected sites, replace the soil to a depth of at least 45 centimeters (18 inches). Alternatively, chemical fumigation or sterilization of the soil can be undertaken before replanting. This should be done during a period of high soil temperature.
The best weapon, of course, is preventing infection first, and there are several strategies for this. First, avoid fertilizers high in nitrogen, such as ammonium salts; these promote soft, rapid bulb growth and make bulbs very vulnerable to infection. Organic fertilizers such as manure and garden compost should be well decayed and should never be incorporated deeply into the soil where they will be close to the basal roots. Instead, use well-rotted manures and compost as a mulch; an added benefit is that mulch helps keep the soil cool, which discourages Fusarium.
Second, control soil moisture. Fusarium is always more active in low, wet sites, so lilies should be planted in well-drained positions. Avoid overwatering during the warm summer months.
Third, since acid soils may also aggravate the disease, an application of lime to increase pH may be advisable in some soils. Fourth, avoid mechanical damage during weeding, cleaning up, or transplanting the lilies, and control biting insects such as grubs and nematodes. Any lesion provides easy entry for basal rot organisms.
In areas where Fusarium cannot be controlled in the open ground, lilies may have to be grown in containers, using a soilless mix or otherwise clean soil. Many kinds of lilies are perfectly content in large pots.
The high temperatures necessary for scale propagation favor Fusarium infection, and bulbs can perish in infancy. It is therefore critical to scale only the healthiest bulbs, to wash them thoroughly before scaling, and to dip the scales in a fungicide before incubation.
Lily hybridizers in the commercial field must make greater efforts to breed varieties specifically for Fusarium resistance. This is even more crucial because key chemical controls are becoming unavailable.
Black scale disease
Black scale is caused by the fungus Colletotrichum lilii. The symptoms are shallow, light brown or near-black, irregular lesions on the outer scales of the bulb. The infected tissues shrivel and die, making the bulb very unsightly.
Blue mold
Blue mold is caused by Penicillium molds, with secondary infection by Cylindrocarpon and related fungal species. It occurs only on bulbs in storage and infects mostly their outer scales. It often resembles the blotches on a bruised apple. This problem is particularly prevalent in early dug bulbs that have suffered excessive mechanical injury.
The fungus grows well at low temperatures (its cousins affect fruits in the refrigerator). It can be recognized by chocolate-brown areas on the central or upper parts of the scales. If the rot is left unchecked, the bulbs can be destroyed. Some varieties are more susceptible than others. The disease can become rampant in severely damaged bulbs.
The best control is to avoid excessive bruising of the outer scales during harvesting and processing. If bulbs arrive with typical signs of decay and blue mold, the infected areas can be carefully removed if they are confined to small portions. Drying out and good aeration help. The bulb can then be dusted with a fungicide such as Captan.
Botrytis blight or fire
Botrytis blight is caused by two species of the fungus Botrytis that attack the above-ground parts of the plant. Both B. elliptica and B. cinerea can be present on the same plant, but the former is the more destructive. Botrytis cinerea flourishes in cool temperatures and is more apt to infect leaves, open flowers, and seedpods in cool summer weather and late fall. Botrytis is often considered the most important disease of lilies, especially in "Botrytis climates" such as the warm, moist coastal areas of the Pacific Northwest or the western coast of Britain. In drier, colder climates with low rainfall, it is seldom a problem.
The fungus overwinters as small black sclerotia that formed on the leaves in the previous season. These produce spores, which are blown about by the wind and splashed by rain onto the newly developing foliage in spring. The first signs of Botrytis can be white spots on the leaves; these become teardrop-shaped marks on the upper surface. They are lighter on the margin and darker in the center, so they are often called "pheasant eye" marks. In severe attacks during wet weather and warm, muggy conditions, the spots eventually coalesce, and the whole leaf collapses and decays.
Botrytis does not spread internally through a plant; instead, it continues to appear on new surfaces on the same and neighboring plants as more spores are produced and distributed. In severe cases it enters the stem and the plant collapses. The destruction of foliage can be very rapid. Brown spots appear on open flowers when Botrytis and moisture are present. These are believed to be caused by B. cinerea rather than B. elliptica, but this has not been conclusively demonstrated.
Botrytis is sometimes confused with other problems, including frost damage, sun scorch, hail damage, severe nutrient imbalance, or mechanical injury. To determine whether the problem is Botrytis, examine the spots early in the morning with a hand lens; if tiny, fuzzy strands of fungus are observed standing up like minuscule trees, get ready to spray. These visible signs of the fungus are the fruiting bodies that form conidia spores, the phase that spreads the disease to other parts of the plant and to its neighbors. Injury from hail, frost, or mechanical damage always makes it easier for Botrytis spores to enter the leaf. Spraying is strongly advised soon after an injury occurs.
The disease is not carried by the bulb, which may flower the following year if the infection was not too severe and did not occur early in the season.
There are many strains of Botrytis because both species mutate freely, making control difficult. The gardener must understand the life cycle of the fungus. The spores germinate and enter the leaves through the epidermal stomates. Moisture is essential for the spread of Botrytis. The formation of spores, liberation, and germination all take place within 12 hours; thus 24 hours of wet, moderately warm weather may lead to a considerable outbreak. Prolonged rains, frequent showers, fog, and heavy dew accompanied by warm temperatures, with moisture persisting on the foliage, produce perfect conditions for "fire."
With Botrytis disease, an ounce of prevention is worth a pound of cure. Therefore, early spraying is critical where environmental conditions tend to favor the disease. At the end of the growing season, remove all plant debris, pulling the old stems when they come away easily. Following a severe infection, remove the debris as soon as possible. Pull or cut the stems and rake off as many leaves as possible; all carry the resting sclerotia, which will rise up to bring new infection next spring. Burn the debris if possible; do not incorporate it in a compost pile. An application of fresh mulch is very beneficial at this time. Many growers spray the ground with a copper fungicide during the dormant season, but there is no proof that this kills resting or germinating spores.
Infection can occur at temperatures between 2 and 24C (35 and 75F) and is most likely in mild, moist, or foggy weather. It is advisable to remove spotted leaves when they are still wet in the morning; this can stop or at least inhibit further spread.
Finally, spraying is only effective when the leaves are dry. Pay particular attention to covering the undersides of the leaves, for this is where infection takes place.
Lilies of the Lilium candidum group, which have overwintering foliage, must be watched very carefully. The fungus Botrytis elliptica was first described taxonomically from this lily. Botrytis infects the rosettes during fall and on warm winter days, providing a reservoir of infection for other lilies that emerge in spring.
Avoid planting lilies in areas with poor air circulation and poor air drainage. An open, breezy area is preferred. Low areas surrounded by shrubs, trees, or buildings can produce a Botrytis trap. Similarly, avoid planting in areas with too much shade where plant surfaces dry slowly. Warm, dry, sunny weather halts the infection. Planting lilies some distance apart also helps control the spread.
Many lilies are highly resistant to Botrytis. Orientals and Aurelians are much less susceptible than Asiatics. Lilium lankongense and its hybrids have shown remarkable resistance; however, L. davidii and some of its hybrids are particularly susceptible. The new tetraploid hybrids, with their thicker epidermis, seem more resistant to both Botrytis and leaf scorch.
There are many growers who never spray their lilies, and in some climates Botrytis may not be much of a problem. Be warned, however, that weather is unpredictable and severe infection can be sudden and amazingly destructive.
Cercosporella blight of foliage
Cercosporella blight is practically unknown in North America, but in parts of Europe is it considered serious. Caused by the fungus Cercosporella inconspicua, it is reported to simulate a powdery mildew infection at its primary stage; in the secondary stage the lesions become brown; in the final stage, a blackened and burned appearance is characteristic.
Root rots
Root rots are associated with poor drainage, lack of soil aeration, and planting in soils that are too finely textured, such as heavy clays. The severity of the problem is related to soil temperature, local fungal flora, and geographical area.
There are several organisms associated with root rots. Cylindrocarpon destructans, Pythium splendens, and Rhizoctonia solani have all been implicated. Injury by root lesion nematodes (Pratylenchus penetrans) , also called meadow nematodes, opens up roots to infection by rots.
Rhizoctonia is a soil-borne fungus and a mild parasite of lily bulbs. The symptoms are dark yellow discolorations around the minute lesions caused by the fungal mycelia. The lesions are numerous and give the scales a yellow tint. Damage to the scales is slight, but the lesions can become entry points for other pathogens. If the bulbs are treated annually in a dip containing quintozene, the Rhizoctonia is eliminated and the bulbs become white. Rhizoctonia is favored by warm temperatures, especially in greenhouses. There are many strains that can become pathogens in lilies.
The best control for root rots is to improve cultural practices, primarily drainage. Overwatering must be avoided at all costs.
Rust
Rust disease, caused by the fungus Uromyces holwayi, is more a novelty than a menace. It is recognized by elliptical rust-colored pustules on the upper surfaces of leaves and occasionally on stems. A rust like condition in certain lilies, particularly the Oriental variety 'Journey's End' has been confused with rust disease. There is no fungal pathogen present; this is a symptom caused by virus infection. It is particularly noticeable when plants are growing under stress, as in forcing under low light conditions.
Southern wilt
Sclerotium disease can be very serious in many vegetable and flower crops, including onions, daffodils, and Dutch iris. It is caused by the fungus Sclerotium delphinii var. rolfsii and is most familiar as southern wilt of delphinium or crown rot of bulbous iris. The fungus produces characteristic round brown or reddish resting bodies, the sclerotia, which resemble turnip or cabbage seeds. The disease attacks the bulbs of actively growing plants during the warm summer months. The warm temperatures cause the sclerotia to germinate and attack plant tissues. Affected lily bulbs develop a white chalky or light brown rot, accompanied by conspicuous white strands (mycelia) of the invading fungus. The white, fanlike patches of coarse mycelia are a sure sign of the disease.
Affected rows of commercial plantings display tell-tale patches of brown plants. The disease spreads quickly under ideal conditions, and at harvest time all the bulbs will have turned to mush. The sclerotia are abundant in the soil around plants dying from the disease and can survive without a host for as much as 10 years.
This disease is seldom carried by commercially produced bulbs. Very few reports have been received of its occurrence in gardens. If sclerotium is detected, lift and wash the healthy bulbs surrounding the infection; remove and destroy infected bulbs and all surrounding soil to a depth of at least 30 centimeters (12 inches). Quintozene has been used as an effective control. A solution of this fungicide can be watered around infected areas to stop further spread. This is done when small pockets of infection occur in commercial plantings.
Southern wilt is native to the southern United States and could become a problem there if lilies are planted in sites previously used for such susceptible plants as delphinium, bulbous iris, or onions. The fungus does not thrive at low temperatures and thus is not much of a threat in northern regions.
Stump rot
Various species of water molds belonging to the genus Phytophthora occasionally invade the crowns of lilies as they emerge from the soil. The affected stems may remain as stumps. The bases of the leaves attached to the stems are destroyed, the leaves wither, and the remains of the crown lie flat on the ground. The browning of stem roots and decay of shoots are other symptoms. Affected lilies are a total loss for the current season, but they usually recover the following year. This condition occurs during cold, wet springs when the rate of growth is slow and mud can wash into the crowns. Planting lilies on ridges in commercial fields and on raised beds in gardens helps control the disease.

Other disease problems

Physiological bulb rot
Physiological bulb rot is a general term for the rapid breakdown of bulbs that is not caused by a pathogen. Winter injury, damage by rodents, and total breakdown of the cells in bulbs harvested too early all fall into this category. The decay develops rapidly; the bulbs assume a wet, glazed appearance and soon become soft and mushy. No primary microorganisms are associated with this condition, but secondary organisms that live on dead tissue soon appear.
Damping-off diseases in seedlings
Damping-off is caused by fungi that grow on the tissues of germinating lily (or other plant) seedlings. The cotyledons, or grass like seed leaves, of epigeal germinators are more susceptible than are the first true leaves of hypogeal germinators. There are several fungi that cause damping-off, particularly species of Pythium, Phytophthora, and Rhizoctonia. The cotyledon or first true leaf simply topples over and decays because the fungus has attacked the tissues near the soil line, and the seedlings quickly die.
A series of strategies is employed to control damping-off. First, commercial and amateur growers should use pasteurized soil mixes or soilless mixes when growing seedlings indoors. Since damping-off fungi are present in all natural soils, they must be destroyed by heating or chemical treatment before the soil is safe to use. Second, growers should use only clean, sterilized pots or other containers for growing seedlings. This can be accomplished by dipping containers in a 10 percent solution of household bleach (sodium hypochlorite). Third, growers should use freely draining soil mixes, and water very sparingly in the early stages of seedling growth. The surface of the soil should be allowed to become dry. Because high humidity and wet soil provide ideal conditions for damping-off, plants should be watered early in the day so that the soil can dry somewhat before nightfall. Fourth, growers should provide seedlings with good air circulation and as much light as possible. Sow the seedlings so that they will germinate in late March or April when conditions are most favorable for their development. Fifth, growers should grow seedlings outdoors in good, clean, well-prepared soil. Damping-off is seldom a problem under conditions of cool temperature and ample light. Finally, growers should use fungicides to control the fungi responsible for damping-off. These can be mixed in solution and watered into the soil. Thiophanate-methyl is widely available and combines several fungicides.

Soil Sterilization

In heavily contaminated soils where lilies have been grown for years, soil fumigation or sterilization may be the only recourse to eliminate pathogens. Soils are sterilized mainly to kill fungi, nematodes, perennial weeds, and weed seeds. The treatment is applied during the summer when soil temperatures are high and moisture levels relatively low; however, the soil must be moist for optimal results.

The chemicals can be either injected or watered into the soil. A tarp or plastic cover is usually necessary to prevent the chemicals from evaporating out of the soil. Most soil fumigants are highly toxic to humans and to living plants and must be used with the utmost care. Chemical-proof protective outer clothing must be worn to keep chemicals from contacting the skin, and a respirator mask to prevent inhaling the fumes. This is a job for professionals with industrial equipment; small growers or home gardeners contemplating this step should hire a contractor who specializes in such work.

The soil must be well tilled before sterilization, free from clods and coarse organic matter. The tools used should also be clean. Time - usually about six weeks - must elapse before bulbs or other plants are set into sterilized soil; during this time the chemicals break down into harmless compounds.

Fasciation

Many lilies sometimes show irregular growth patterns known as fasciation. This has frequently been associated with disease-causing organisms. The condition is usually characterized by flattening of the stems and the production of countless small leaves. These fasciated plants often abort all their flowers, but in some cases (particularly in Oriental lilies) they may produce a multitude of flowers. The stems of such lilies may twist into a spiral and sometimes split open. The condition is most frequently observed with older, larger bulbs, such as those of 'Edith Cecilia' and 'Pink Perfection'. The tendency to fasciate is definitely genetic, and very warm temperatures in spring seem to trigger its expression. No disease is present, however, and the plants usually produce normal growth the following year.

Frost Injury

Late spring frosts can be devastating in northern regions. Frost damage can resemble the symptoms of certain diseases. Of the more widely grown lilies, Chinese trumpets are the most frost -susceptible, followed by Oriental lilies; Asiatic lilies appear to withstand a few degrees of frost more readily.

The emerging young shoots are frequently damaged. When severe frosts occur in late May and June, as may happen in northern or high-altitude gardens, many lilies may be in bud at this time, and the injuries can be quite serious. In preventing frost damage, air drainage is of paramount importance. Plant lilies on slopes or other areas where the air is not stagnant. Avoid low-lying frost pockets.

In gardens subject to late frost, select late-emerging types of lilies to suit the climate. In severely cold areas, avoid planting trumpet lilies.

Sprinkler systems can be helpful in preventing frost damage. Watering has a tempering influence because heat is released as the water droplets freeze; moreover, a thin coating of ice on plants can protect tender foliage by preventing the freezing and rupturing of plant tissues. When a frost is forecast, plants can be covered with tarps or other material such as row-cover fabric. These may offer enough protection if the freeze is not too severe.

After a frost, the most severely damaged parts of the plants can be removed with a sharp knife. The affected plants should be sprayed immediately with a fungicide to forestall Botrytis infection.

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