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.
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.
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.
- 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 44°C (111°F) 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.
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.
- 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.
- 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 44°C (111°F) 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.
- 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 44°C (111°F) 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.
- 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.
- 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.
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
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 24°C (35° and
75°F) 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
- 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 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.
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
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.
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.