Pests, Diseases And Disorders Of Carnations
Modern cut flower carnation types are increasingly bred for disease resistance, so choice of variety has a part to
play in the ultimate health of the plant. All members of
the carnation family need a well-ventilated situation with free circulation of air, sharp drainage and
plenty of sun. They don’t like wet feet or damp, humid conditions. Cater to these needs and they
will be well prepared to combat attack.
Dianthus and border carnations are generally easy-care plants and control of pests and disease
poses few problems for home gardeners. The ease with which old garden pinks escape and multiply in
the wild is proof of their tough constitution. Carnations grown in greenhouse conditions need more
attention and should be checked regularly for evidence of pests, particularly on the undersides of
leaves and in the tiny crevices around developing buds.
If you decide intervention is necessary, chemical insecticides act in two different ways. Systemic pesticides, usually applied by spraying leaves and stems, are absorbed by the plant and carried by the sap to
all its living parts. Any insects sucking the sap will
be killed. Systemic insecticides protect the plant for a specified length of time.
Contact insecticides are exactly that. They kill any insect they hit and are applied as sprays, dusts or
smokes. Many of these contain Malathion, which is dangerous to humans. Less potentially damaging
organic insecticides, containing pyrethrum, are available. Organic methods should also be considered.
Care must be taken at all times when dealing with chemical pesticides or
fungicides. Sprays are easily carried by wind, so choose calm
weather and do any necessary spraying in the evening when bees are less active.
Just as bacteria that attack humans have developed resistance to antibiotics, there is a similar risk
of bugs developing resistance to chemical killers. Alternating between the types of chemicals you
use-or between organic methods-will help to prevent this.
The nectar of dianthus attracts ants and when the
plants start to flower, they climb into the open flowers for a feast. If the petals of your carnations fall out and the flower collapses, blame ants-they could be
nibbling on the center tips of the petals. Dusts containing pyrethrum or pirimiphos methyl will control
Aphids (including greenfly)
All gardeners are familiar with greenfly, those tiny insects that swarm onto plants at the height of the
growing season and greedily suck the sap, particularly from juicy young shoots, leaving sticky
honeydew in their wake. They multiply at an alarming rate and like other aphids they look unsightly, their sap-sucking habits
debilitate plants and, most seriously, they often transmit virus from diseased plants to healthy ones.
Virus disease becomes apparent in the form of yellow streaks or spots on the foliage.
If you grow only a few plants it may be possible to control these
annoying insects by physically squashing them on a continuing basis. Systemic pesticides are effective,
but spraying will need to be repeated at regular intervals over summer. Some gardeners report
success controlling aphids with an organic “tea” made from
Garlic planted beside dianthus has been
found to deter some pests, including carnation fly.
Birds, rabbits and mice
Birds will chew on carnations, especially in spring. The plants can be protected by stringing black
thread above them. In rabbit-prone areas, wire-netting fencing, set well into the earth, is the best
protection. Mice can sometimes be a problem in winter, eating plants in greenhouses. Use
mousetraps or the resident cat, so long as it is trained to treat flowerpots with due respect.
Carnation fly, leafminer or carnation maggot
This fly can be a problem on carnations and pinks grown outdoors. It lays its eggs on the foliage of the
carnation. Once hatched, the maggots feed on tissues between the upper and lower skins of the leaves
and create whitish tunnels in their progress towards the main growth, and eventually the central stem of
the plant-hence the name “leafminer.” If the tracks are visible on leaves, cut the leaves off and
preferably burn them. Otherwise, try to extract the
maggot before it can pupate in midsummer, ready to start the next life cycle.
Handpicking is effective where leaves or plants are already mined. Maldison or pirimiphos
methyl based sprays can help where damage is not too advanced. Planting garlic alongside plants may be
an effective deterrent.
Caterpillars, including tortrix moth caterpillar
There is a range of caterpillars with varying habits. Some feed by day, others by night-when they can
be sought by flashlight, picked off and squashed. Some are harder to find, secreting themselves inside
the calyxes of carnations and pinks while they feed on flowers, born or unborn.
The tortrix moth caterpillar is cunning. It’s almost as if nature knew it needed protection
against modern spray chemicals, for the tortrix moth caterpillar has the ability to spin a fine web
and roll the growing tips of the plant around itself as a shield, meanwhile growing plump and long
(3/4 in /2cm) on carnation foliage. The moth is tiny and bright orange in color, an unexpected
metamorphosis from the rather large caterpillar.
For control, use one of the following insecticides: maldison, acephate, carbaryl, pirimiphos
methyl, permethrin or bifenthrin.
Cuckoo spit or spittlebug (sometimes called froghopper)
Another of the sap sucking variety, this insect is most visible by the frothy deposit it exudes as a
sun umbrella to shield the bug while it carries on its nefarious business. Usually seen in the junction
between stem and leaf, this bug should be treated as for aphids if it becomes a real problem.
Not usually regarded as any kind of pest, earthworms nevertheless are a problem in pot culture
where they have a restricted run. Their prolific casts tend to clog the soil and impede drainage. To
remove them, first allow the soil to dry out, then irrigate the pot with water to which lime has been
added. This causes the worms to come to the surface, where they can be picked off.
Nasty creepy-crawlies these-they chew buds and petals-but you can deal with them effectively and
in an environmentally friendly way. Loosely fill pots with moss, straw,
grass or even crumpled newspaper
and set them out on their sides among the carnation plants for the earwigs to shelter in at night. Dispose
of the critters on a daily basis.
Red spider mite
probably the most threatening enemy of the genus and is especially damaging in greenhouses, where it
spreads quickly if unchecked. Yellow or brownish, rather than red, this spider is so small it can be seen
only with the naked eye once it has formed colonies, when it looks like a type of rust on the
undersides of leaves. It attacks the leaves, puncturing the surface to suck out the contents.
The first indication of the mite’s presence is usually the appearance of pale
blotches on the upper leaf surface. The leaves lose their waxy covering, become
dull, turn yellow and die. Once a plant is depleted of moisture, these ingenious
invaders quickly move on. Unable to fly, they crawl to the top of the dying
plant and start spinning furiously. The aerial “ropes” swing them across to the next healthy plant and
they continue on their destructive passage.
Red spider mites thrive in hot, dry conditions. Shading the greenhouse to reduce temperatures and
raising the level of humidity are two ways of creating conditions less conducive to the mites’ prolific
life cycle. Maximum ventilation is necessary, as humidity can encourage the growth of fungal
infections. Simultaneously, treat the plants with acaricides, spraying the foliage and watering the soil in
the pot. Repeat the spraying every six or seven days to destroy the next cycle of mites. The following ingredients are all effective acaricides: bifenthrin, propargite, dienochlor, abamectin, dicofol.
Slugs and snails
Dianthus foliage and stems are not particularly attractive to these slimy pests but where they are a
problem, tempt them to their deaths with a saucer of stale beer. Alternatively, set out snail bait,
protecting birds and pets by placing the bait in short pieces of broken drainpipe, small jars on their sides,
or similar coverings.
Like aphids, these tiny winged insects are sapsuckers. There are several varieties, but it is the onion thrip that attacks the unopened buds of carnations, leaving the flower to emerge with ugly, pale
blotches where the color pigment has been sucked out.
Thrips breed outdoors in summer and in greenhouses during winter, laying their eggs on the calyx;
the developing nymphs cause the damage. Thrips are especially prevalent during a hot dry period in
summer. Either derris powder (if available), carefully applied to reach into folds in leaf and bud, or a
systemic insecticide, will control this pest.
These pests are a threat to all plants and are especially prevalent in areas of grassland. About 1 in
(2.5 cm) long and orange-yellow in color, they live in the soil for three to five years where they pupate,
eventually appearing as a beetle. They munch plant roots-even burrowing up into the stems of
carnations and dianthus-and cause death. An old-fashioned remedy is to bury pieces of
potato or carrot
just below the soil surface and mark them with sticks. The vegetables become infested with the
worms and are easily removed and destroyed.
Harmful fungi, bacteria and viruses cause disease. Home gardeners who grow their dianthus outdoors
are unlikely to be seriously troubled by disease. Plants grown in greenhouses are more vulnerable
and the results of continuing research on carnation diseases are of much more importance to
commercial growers of perpetual-flowering carnations. In the early years of last century the dread disease of
rust forced some carnation growers out of production. With increased knowledge and improved
growing conditions, this is rarely a problem now.
The best defense in the greenhouse is always good hygiene combined with correct cultural
conditions. Always check new plants before you buy them. Introducing diseased plants to healthy stock
is a surefire method of spreading disease. The whole greenhouse should be disinfected once a
year preferably in the fall. Use an antibacterial and anti-fungal disinfectant at the recommended strength
and wash down the glass walls, all the shelves and pots, and don’t forget to also disinfect the tools you use.
- Damping-off problems
- This is one problem amateur gardeners need to be aware of-and it affects not only dianthus. Caused
by a complex of fungi that spreads on the surface of the soil, damping-off attacks seedlings shortly after
they emerge and form their first pair of leaves. Just when you are congratulating yourself on a good
germination rate and admiring a prolific crop of tiny green plants, the fungi can hit. The stem is damaged
at soil level, causing the seedling to collapse and rot. In the space of 24 hours a whole tray of seedlings
can be destroyed. Overcrowding encourages damping-off, as does a high level of humidity. Use a fresh
batch of sterilized seed-raising mix for each planting and avoid overwatering. Spraying the seed mix with
a solution of copper oxychloride or a fungicide containing benomyl before and after sowing is helpful.
New cuttings may also be affected by damping-off.
- Caused by a fungus, this disease shows itself as pale brown lesions on the calyx that progress to the
lower parts of the petals, eventually leading to collapse of the flower. Treat it with a fungicide
- This used to be perhaps the most common disease to attack dianthus. Should you notice the telltale
signs-small, brownish marks on both sides of leaves that split open-it’s time to act. Pick off the
infected leaves, burn them and wash your hands before touching healthy plants. As with mildew,
adequate ventilation will help prevent rust; it likes warm, damp conditions. Treat by spraying with copper oxychloride or dusting with a fungicide containing mancozeb.
- Leaf spot
- There are two forms of this disease: one is marked by small, purple spots with yellow margins that develop
black powdery spores, eventually destroying whole leaves; the second causes brown spots with purple
margins to develop on leaves and stems. Once again, warm, damp conditions are the enemy. Spray
with copper oxychloride or a fungicide containing mancozeb.
- Another disease caused by warm, humid conditions, botrytis causes a gray, powdery mold to form on
flowers. It’s a disease that is easily dispersed, so when disposing of affected flowers, they need to be
handled delicately, to avoid spreading spores. Treatment with benomyl sprays should be an effective control.
- Wilt diseases
- There are several wilt diseases, all of which mainly affect perpetual-flowering carnations grown in
greenhouses. They are invariably fatal, but most new cultivars have been bred for their high
resistance to these and other diseases. Growing plants in pots or “Grobags” resting on straw to prevent
contact with any surrounding soil is an effective way of preventing the incidence of wilt diseases.
- Bacterial wilt causes plants to wilt suddenly, with the stem brownish and slimy.
- Fusarium wilt is a fungal disease. It causes the plant to become stunted and discolored, though the
stems do not become slimy.
- Verticillium wilt (Phialophora cinerescens) is a fungal disease causing leaves to turn straw-colored
and side shoots to twist. Broken stems show brown discoloration.
When any wilt disease appears, it is important to act fast to prevent its spread. Affected plants must
be removed from the greenhouse and burned as quickly as possible. Surrounding soil needs to be
sterilized or discarded, as do any pots in which infected plants are found.
- Stem rot
- Also called root rot, and difficult to identify, this disease causes stem decay just below the soil surface.
The plant suddenly wilts and dies. Avoid infested soil and destroy affected plants. Root rot can be
controlled with drenches of benomyI, but it is rarely a problem provided drainage is good and planting is
not too deep.
- Carnation viruses
- Carnation plants frequently carry viral diseases, although many cultivars may show no symptoms.
Where leaves are severely mottled or streaked, the plants should be destroyed. Eradicating aphids,
which spread viruses, is the best method of control.
- Calyx splitting
- This disorder occurs when the flower is so buxom that as it opens it pushes beyond the natural
containment of the calyx. Extreme fluctuations in temperature or overwatering of a dry plant may cause it
to happen. It is of concern primarily for commercial growers of perpetual carnations, as it spoils their
market value, and for amateurs who plan to exhibit their plants, as blooms with a split calyx are
disqualified. The well-known old garden pink ‘Mrs. Sinkins’ regularly splits her calyxes.
- Curly tip
- Another disorder of perpetual carnations, this very noticeable problem may appear in midwinter. It
causes the growing tips of foliage to curl and distort,
but with increasing light and warmth, the condition disappears.