Chrysanthemum (Chrysanthemum, Dendranthema, Leucanthemum)
Plant
Health Problems
Diseases caused by Fungi:
Leaf spots, Septoria sp.
Symptoms first appear as yellowish areas which turn brown to
black. Spots often appear on lower leaves first and can coalesce
until the entire leaf turns brown-black and dies. The leaves
appear to die from the bottom of the plant up the stem. Symptoms
are easily confused with those associated with the foliar
nematode.
Efforts to maximize plant vigor by fertilizing and watering are
helpful. However, watering should be done early in the day to
give the foliage a chance to dry before nighttime. It is also
helpful to pick and remove symptomatic leaves as soon as they
develop. Although not usually necessary, applications of
fungicides can be made when new growth emerges in the spring.
Among the compounds registered for use in Connecticut are
chlorothalonil and thiophanate-methyl. Consult the label for
dosage rates and safety precautions.
Wilts, Verticillium, Fusarium.
Vascular wilts are troublesome diseases of garden chrysanthemums.
Plants may be attacked at any stage; young plants can suddenly
dry up or older plants might develop a pale green color
accompanied by wilting of lower leaves, often first appearing on
one side of the plant. This is frequently followed by a general
wilting and death of the entire plant. Plants may wilt in the
middle of the day and seem to recover at night. Some plants may
show no signs of infection until they come into flower, when they
suddenly collapse. When the stem is cut, a brown discoloration or
streaking may appear in the vascular tissues.
Control of these diseases is difficult since the pathogens are
commonly found in soil. One of the key strategies for control of
vascular wilts is prevention. Therefore, it is important to avoid
planting mums in infested soil. It is also helpful to maximize
plant vigor by good cultural care and watering. Careful handling
of plants will avoid root injury which enables the fungus to
enter the plant. Since repeated use of the same area greatly
increases the amount of disease, rotation is essential. When
available, it is also helpful to use resistant varieties.
Chemical controls are not effective for these fungi.
Brown rust, Puccinia.
Rust pustules start as swellings on the underside of the leaves
which soon break open discharging chocolate-brown powdery spores.
These spores infect chrysanthemums and will spread the disease
through the rest of the planting. The alternate host is not
known.
This disease can be minimized by cleaning up plant refuse in the
fall and by adequate spacing of the plants to promote good air
circulation. Although not usually necessary, applications of
fungicides can be made when new growth emerges in the spring.
Among the compounds registered for use in Connecticut are
mancozeb, maneb, and triadimefon. Consult the label for dosage
rates and safety precautions.
Powdery mildew, Erysiphe.
White powdery spots or patches develop on leaves and
occasionally on stems. Symptoms often first appear on the upper
surfaces of the leaves and are usually most pronounced during
hot, humid weather. Heavily infected leaves turn brown and
shrivel.
Disease can be minimized by avoiding overcrowded spacing of
plants and by carefully picking off affected leaves as soon as
symptoms are evident. Symptomatic leaves can be placed into a
plastic bag in order to avoid spreading the spores of the fungus
to other plants. Use of fungicides is usually not necessary.
However, applications can be made as soon as symptoms are
visible. Among the compounds registered for use in Connecticut
are horticultural oil, sulfur, potassium bicarbonate, and
thiophanate-methyl. Consult the label for dosage rates and safety
precautions.
Gray mold, Botrytis cinerea.
Flowers turn a papery brown and become covered with gray, fuzzy
masses. Senescing flowers are particularly susceptible. Tan to
brown spots with a target-like appearance can also develop on the
leaves. These patches are often associated with flowers which
have dropped onto the leaf surface. This disease is particularly
troublesome during periods of extended cloudy, humid, wet
weather.
Good sanitation practices including grooming the plants and
removing spent or senescing flowers can minimize the potential
for infection. These affected tissues should be carefully removed
and discarded when they are dry. It is also important to avoid
wetting the flowers when watering and crowding plants. Adequate
spacing between the plants can promote good air circulation.
Control can also be achieved with the use of fungicide sprays
applied as soon as symptoms are visible. Among the compounds
registered for use in Connecticut are chlorothalonil, mancozeb, copper sulphate
pentahydrate, and thiophanate-methyl. Consult the label for dosage rates and safety
precautions.
Diseases caused by Viruses:
Stunt, viroid.
Symptoms appear as a distinct stunting of individual branches or
the entire plant. Leaves on infected branches may be smaller than
normal and flowers are small and have a washed-out color.
Infected plants often produce flowers earlier than their healthy
counterparts.
Control is very difficult so symptomatic plants should be
promptly removed.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides.
Black to brown V-shaped areas appear in the lower leaves, showing
first at the bottom of the plant and working their way up the
plant. These spots are bounded by veins at first but eventually
may involve the entire leaf. The trouble is caused by microscopic
eelworms which live in and destroy the leaf tissue. They come out
of the leaf when the leaf surface and stems are wet with rain,
dew, or syringing, and swim up the stem to infect the leaves
above. The nematodes overwinter in plant debris in the soil.
This disease can be minimized by reducing leaf moisture and
removal of infected tissues, debris, or plants.
Insect Problems
Aphids, Myzus persicae and Macrosiphoniella
sanborni.
The green peach aphid and the chrysanthemum aphid, Macrosiphoniella
sanborni, often infest chrysanthemum plants in the
greenhouse. They may be managed by spraying with insecticidal
soap or ultra-fine horticultural oil, which are among the
compounds registered for control of this pest in Connecticut.
Aphids on chrysanthemums outdoors can be controlled using
malathion sprays or imidacloprid can be applied as a soil drench
for season-long, systemic control. Consult the label for dosage
rates and safety precautions.
Chrysanthemum gall midge, Rhopalomyia chrysanthemi.
A small slender fly lays eggs in the leaves and tender shoots in
greenhouses and each larva forms a cone-shaped gall. Plants in
the open are not usually injured. Probably there is more than one
generation each year, but it is difficult to estimate the number
because of overlapping of generation. The midge is more abundant
in spring and fall than in other seasons. Bringing only clean
cuttings or plants into the greenhouse can reduce damage from
this pest.
Existing infestations can be managed with sprays of malathion,
which is among the compounds registered for control of this pest
in Connecticut, applied according to label directions.
Chrysanthemum lacebug, Corythucha marmorata.
The adult of this pest is about 1/8" long with sculptured,
lace-like wings. One sure sign of lacebugs is the small dark
spots of feces left on the undersides of leaves. This lacebug
overwinters as adults in protected areas near host plants. Eggs
are laid in groups on the undersides of leaves near veins. There
may be two generations each year, depending on climatic
conditions. Both adults and nymphs suck the sap from the
underside of the leaves, causing a mottling or blanching.
When needed, malathion, insecticidal soap, acephate or ultra-fine
horticultural oil, which are among the compounds registered for
control of this pest in Connecticut, applied the last week in
May, following egg hatching is highly effective. Spray should be
directed from the bottom of the plant upward to ensure thorough
coverage of the lower leaf surfaces. Imidacloprid, applied to the
soil as a systemic, provides season-long control. Consult the
labels for dosage rates and safety precautions.
Chrysanthemum leafminer, Chromatomyia syngenesiae,
Liriomyza trifolii, L. huidobrensis.
These insects overwinter as partially grown larvae inside leaves
(C. syngenesiae), or are transported in commercially
greenhouse-grown plants. The adult C. syngenesiae are gray
flies about 1/12" long, emerge in spring and begin egg
laying. Liriomyza spp. are the same size, but are striking
black and yellow flies. Eggs are laid in the leaves. The leaves
develop narrow serpentine mines. If abundant enough to justify
control, a soil drench of imidacloprid, which is among the
compounds registered for use against this pest in Connecticut,
should kill larvae in the mines. Abamectin, a restricted use
product, is also effective. Organophosphate and carbamate
insecticides tend to have little value against Liriomyza
spp. due to their development of resistance. Consult the label
for dosage rates and safety precautions.
Chrysanthemum thrips, Thrips nigropilosus.
The insect feeds in the developing flowers, causing deformation
and mottling. The adult of this and another damaging species, the
western flower thrips (Frankliniella occidentalis), are
brownish-yellow with feathery wings. The young are pale lemon
yellow and also cause damage with their sucking mouthparts.
Insecticidal soap, malathion, or spinosad, which are among the
compounds registered for use against this pest in Connecticut,
may control thrips. Some populations of western flower thrips are
resistant to all insecticides registered for use by homeowners.
Consult the label for dosage rates and safety precautions.
Cyclamen mite, Phytonemus pallidus.
This translucent, microscopic mite often infests the new leaf and
blossom buds, and can proliferate on spent blossoms.
Characteristic injury includes dwarfed, thickened and wrinkled
leaves. On azalea, dahlia, and other plants with stems, the
distance between internodes becomes drastically shortened.
Infested plants do not bloom, or blossoms may be misshapen.
Cyclamen mite is not easily controlled when mature plants are
badly infested: effective miticides are not available to
homeowners, so a commercial applicator may be required for
effective miticide applications. Effective restricted use
materials are abamectin, dicofol, and endosulfan.
European corn borer, Ostrinia nubilalis.
The second generation of this pest sometimes lays its eggs on
chrysanthemum.
The larvae tunnel in stems, weakening them so they break. The
larva is pale white or gray with black tubercles and is not more
than 1" long when fully grown. Adults have a wingspread of
1" or so and are buff to brown in color. There are usually
two generations annually. Eggs are laid on the underside of
leaves and the larvae tunnel in the stalks and pupate in the
burrows. Second-generation larvae and those of the single
generation corn borer overwinter in stems and pupate in the
spring.
The parasitic wasp Trichogramma has been used as alternative method of control. This tiny wasp attacks the egg masses of the corn borer, and the eggs of other caterpillars, too. Be sure to purchase the insects from a reputable supplier and make sure the strain you purchase is known to be well adapted to attacking corn borer. Bt will not harm the Trichogramma wasps, but other insecticides may.
Fourlined plant bug, Poecilocapsus lineatus.
This bug lays eggs in the soft stems. They hatch about the middle
of May and the young bugs suck the sap from the tender leaves.
They molt five times and when mature, about the middle of June,
they have wings and are nearly 1/3" long. The insect body is
yellow, marked lengthwise on the wings with four black stripes
alternating with three green stripes. The injury to the leaves
consists of sunken areas around the punctures. These areas later
appear as circular transparent spots and finally as circular
holes. This insect injures the new leaves of many different kinds
of annual and perennial plants and shrubs. There is one
generation each year. The nymphs can be managed by spraying with
azadirachtin, ultra-fine horticultural oil, insecticidal soap or
malathion, which are among the compounds registered for control
of this pest in Connecticut. Imidacloprid, applied as a systemic
taken up by the roots, will provide season-long control. Consult
the labels for dosage rates, safety precautions, and preharvest
intervals.
Mexican mealybug, Phenacoccus
gossypii.
Although there are other species of mealybug that attack mums,
the Mexican mealybug is the most injurious. It attacks all stages
of growth, stunting plants by causing distortion of the leaves.
Malathion, insecticidal soap or ultra-fine horticultural oil,
which are among the compounds registered for use against this
pest in Connecticut, should give good control when applied
according to label directions. Ants that tend mealybugs in order
to harvest their honeydew, can be controlled by lightly dusting
the soil surface with chlorpyrifos. Imidacloprid applied as a
systemic to be taken up by the roots will also provide
season-long control. Consult the label for dosage rates and
safety precautions.
Spittlebugs, Philaenus spumarius.
These insects cause a stunting, curling, twisting of the
terminal growth, side branches and infested leaves of hardy mums.
The insects cover themselves with a frothy material, resulting in
the common names "snake-spit" and
"frog-spit." When needed, malathion, which is among the
compounds registered for use against this pest in Connecticut,
applied to the terminals during the spring and early summer
should give adequate control. Imidacloprid applied as a systemic
to be taken up by the roots will also provide season-long
control. Consult the label for dosage rates and safety
precautions.