Avoiding Top Problems of Poinsettias

July 9, 2002 - 10:51

Prevention and early detection of poinsettia problems are critical to growing a high-quality crop. Here, learn how to identify and correct nutritional, physiological and insect- and disease-related maladies.

Poinsettias can, at times, be a problematic crop, but being
aware of the more common nutritional, physiological and insect and
disease-related problems encountered during production can help reduce the
headaches. By watching for potential problems, understanding their causes and
knowing the proper corrective actions to take, one can avoid most of these
issues.

Nutritional and physiological disorders

High pH. High substrate pH can induce nutrient problems in
poinsettias. The recommended pH range in a soilless substrate is 5.8-6.5. Iron
deficiency is the most common problem at a substrate pH above 6.5, at which
point new leaves will exhibit interveinal chlorosis (yellowing). Lower the pH
with an iron sulfate drench or by using an acidic fertilizer. Iron deficiency
can also occur with root death, over-irrigation, poor substrate drainage or
insect damage. Inspecting the roots will help determine the cause of the
problem.

Low Electrical Conductivity. style='font-weight:normal;font-style:normal'> When the root substrate EC is too
low, plants are stunted, and mineral deficiencies occur. Low salts can occur
due to excessive leaching, too many clear-water irrigations between
fertilizations, a broken injector or an improper injector ratio. Deficiency
symptoms such as lower leaf yellowing (nitrogen) or lower leaf speckling
(phosphorus) are common when EC values are below 0.25 mS/cm (1:2 extraction),
0.75 mS/cm (SME extraction) or 1.0 mS/cm (PourThru extraction) during the
active growth stage. Normal fertilizer rates for poinsettias are between 175
and 250 ppm nitrogen.

Calcium Deficiency. Symptoms appear at the top of the plant. Young leaves
may develop variable patterns of chlorosis and distortion such as dwarfing,
strapping or marginal leaf burn. Leaf edges may become necrotic. Typically,
symptoms in young plants occur during periods of overcast weather when the
plant’s ability to uptake calcium is inhibited. Bracts can also develop a
marginal necrosis. Sunny weather, avoiding water-logged substrates, low
humidity and calcium chloride or calcium nitrate foliar sprays will help
prevent the problem.

Magnesium Deficiency. Lower leaves develop interveinal chlorosis; under
advanced conditions, the leaf margins turn necrotic. On younger Á
plants, symptoms appear on the lower leaves. On flowering poinsettias, symptoms
tend to develop on the top half of the plant. One or two applications of
magnesium sulfate (1 lb. per 100 gal. of water) or a magnesium-containing
fertilizer will help prevent the problem.

Molybdenum Deficiency. Symptoms appear as chlorosis (yellowing) of the
recently mature leaves (middle of the plant), rolling of the leaves and leaf
edge burn. The leaf chlorosis of molybdenum deficiency resembles magnesium
deficiency, except that the thin, marginal band of chlorosis is expressed from
the leaf tip to the leaf base. Molybdenum deficiencies can cause distorted
leaves due to the failure of the interveinal areas to expand normally. Monthly
molybdenum applications should be made to poinsettias.

Leaf Distortion. Plants develop distorted or cupped leaves, most often
during the early stages of the crop (within a few weeks of being pinched). Most
poinsettias will outgrow this condition, but shoots with extreme distortion may
not improve. It is unclear what causes this disorder, but thought that rapid
changes in humidity, which can occur early in the morning when the vent fans
come on, may lead to an accumulation of salts along the leaf margins and veins,
resulting in leaf injury. Leaf distortion becomes apparent as these injured
leaves grow and expand.

Excessive Plant Stretch. style='font-weight:normal;font-style:normal'> Plant growth regulators are
excellent tools to control excessive plant stretch. Overdoses can result in
stunted growth of the newly expanding leaves or smaller bract size. Because of
their greater degree of activity, overdoses are more common with the use of
B-Nine + Cycocel tank mixes, Bonzi or Sumagic. Applying the labeled
concentration at the proper time, mixing correctly and using proper application
techniques can help avoid most problems.

Insects

Fungus Gnats and Shoreflies. style='font-weight:normal;font-style:normal'> Fungus gnat larvae are one-eighth
of an inch long, white, transparent, legless and have a black head capsule.
Fungus gnat larvae feed on poinsettia roots and may even tunnel into plant
stems, especially newly planted cuttings. This causes plant stunting and
wilting. Fungus gnat larvae feeding provides entry sites for soil-borne
pathogens. Fungus gnat adults are winged, one-eighth of an inch long, with long
legs and antennae. Each wing has a “Y-shaped” vein. Adult
shoreflies resemble houseflies. They are one-eighth of an inch long and deep
black in color with red eyes. Each wing usually has approximately five white or
light-colored spots. The antennae and legs are short. Shoreflies are a nuisance
pest, as the larvae don’t directly feed on plant roots. The lifecycle from
egg to adult for both fungus gnats and shoreflies ranges from 15-28 days,
depending on temperature. Control measures for fungus gnat larvae include
pyriproxyfen (Distance), Cyromazine (Citation), Steinernema feltiae (Nemasys,
Entoneem), Chlorpyrifos (Duraguard) and Bacillus thuringiensis var. israelensis
(Gnatrol). Control measures for fungus gnat adults include bifenthrin
(Talstar), cyfluthrin (Decathlon) and paraffinic oil (Horticultural oil).
Shorefly larval control may be obtained with either pyriproxyfen (Distance) or
cyromazine (Citation).

Western Flower Thrips. Less than one-sixteenth of an inch long, western
flower thrips have piercing-sucking mouthparts that cause direct damage to
poinsettias by feeding on leaf buds before they open. This results in leaf scarring
and new growth distortion, as the remainder of the tissue expands around areas
fed upon by the thrips. Poinsettias are not susceptible to the viruses
transmitted by thrips, including impatiens necrotic spot virus (INSV). Although
not a primary host, thrips will feed on poinsettias if no other food source is
available in the greenhouse. Control measures include spinosad (Conserve),
abamectin (Avid) and methiocarb (Mesurol).

Whiteflies. Whitefly adults are white to slightly yellowish
in color, narrow-shaped and approximately one-sixteenth to one-eighth of an
inch long. Most of the whitefly stages are located on the undersides of
poinsettia leaves. Whitefly “crawlers” hatch and crawl about,
inserting their threadlike mouthparts into the lower leaf surface to feed on
plant fluids. This may result in leaf yellowing, plant stunting, plant wilting
and plant death (if populations are high enough). Whiteflies produce honeydew,
a clear, sticky liquid, during feeding that serves as an excellent medium for
black, sooty mold fungi. Control measures include imidacloprid (Marathon),
pymetrozine (Endeavor), pyriproxyfen (Distance)*, potassium salts of fatty
acids (Insecticidal Soap), Beauveria bassiana (Botanigard/Naturalis),
bifenthrin (Talstar), cyfluthrin (Decathlon), kinoprene (Enstar II) and
pyridaben (Sanmite). (*Distance cannot be used on poinsettias when bracts are
visible.)

Spider Mites. Spider mites are oval-shaped and can be yellow-orange,
green or red. Adult mites have two dark spots on both sides of the abdomen.
Spider mites feed primarily on leaf undersides and remove chlorophyll (green
pigment) with their stylet-like mouthparts. Damaged leaves appear stippled or
“dirty” with small, silvery-gray to yellowish speckles. Webbing may
be present if populations are high. Spider mites prefer warm, dry conditions
with low relative humidity. Lewis mite injury to poinsettias is similar to that
of the Á two-spotted spider mite. Lewis mites are smaller and have
several small black spots on both sides of the body. Control measures include
abamectin (Avid), bifenazate (Floramite), bifenthrin (Talstar), pyridaben
(Sanmite), clofentezine (Ovation), hexythiazox (Hexygon), fenpyroximate (Akari)
and chlorfenapyr (Pylon).

Diseases

Botrytis. (Gray Mold) quickly colonizes damaged or senescing plant tissues
and initially appears as brown spots on the leaves and flower bracts. Large
numbers of gray to olivaceous green spores are produced on the infected
tissues. This fungal pathogen thrives under cool temperatures around 68-77°
F. The relative humidity within the plant canopy should be kept below 93
percent throughout the production cycle. Large, tan, sunken cankers may form on
the older stems, girdling the stem. Promptly remove fading flowers and dead
plant tissue from the greenhouse. Fungicides such as chlorothalonil,
fenhexamide and fludioxonil, in conjunction with good cultural practices, can
help prevent this disease. Carefully read and follow label precautions to avoid
damaging flower bracts.

Powdery Mildew. This pathogen can be moved from greenhouse to greenhouse on
infected cuttings or plants. Yellow spots form on the upper surface of leaves
indicating the location of grayish-white fungal colonies on the undersides of
the leaves. Characteristic white, powder-like colonies may also be present on
both leaves and flower bracts. However, it is important to control this disease
before the bracts become infected, as the white fungal patches are not
“erased” by fungicide applications. Frequent and careful scouting
of the crop for powdery mildew symptoms should begin when greenhouse
temperatures begin to drop below a daytime high of approximately 86° F.
Temperatures above this point are inhibitory to the infection process. Powdery
mildew can be controlled with the application of fungicides such as
myclobutanil, kresoxim-methyl, piperalin, triadimefon, triflumizole and
trifloxystrobin when fungal colonies are first observed. Piperalin does have
curative activity, but like all fungicides, it cannot erase the damage already
done.

Pythium Root Rot. This is most likely to show up early in the season,
soon after cuttings are planted. The base of the cutting will appear brown, and
severely infected cuttings will wilt and quickly die. Infected plants are
usually stunted and show signs of wilt during the heat of the day. Infected
roots are dark brown, and the outer cortex of the root is easily slipped off,
leaving a thread-like strand of vascular tissue. Plants that survive until
flowering will often flower prematurely and defoliate. Control of this pathogen
is difficult once infection has begun. Sterile potting mix and disinfestation
of work surfaces will help avoid the contamination and spread of this fungal
pathogen. Fungicides and biological control agents can be applied as
protectants to help manage this disease. Preventative applications of
mefenoxam, propamocarb or etridiazole may be applied.

Rhizoctonia stem rot. An important disease most likely to occur during
propagation, Rhizoctonia stem rot causes small lesions to develop at the point
on the stem that is even with the top of the rooting cube. If the rooting cube
is later transplanted too deeply, it may appear that the lesion is originating
at the roots. Stem lesions have a dry appearance, with a tan center and dark
border. The stem lesion may expand, girdling and quickly killing the cutting.
Web-like hyphae can sometimes be seen at the base of the rooting cube. Leaves
may become infected when they contact the potting mix or bench surface and can
become quickly colonized. The fungal infection can move down the leaf and
infect the stem. Once a cutting strip is infected, the fungus can move through
the entire strip and infect other cuttings. The entire strip should be
discarded if any diseased cuttings are found. Fungicides such as chlorothalonil,
flutolanil and iprodione have been shown to prevent colonization of the rooting
cube by this fungus.

Scab (Sphaceloma poinsettiae). style='font-weight:normal;font-style:normal'> Symptoms of this pathogen appear
as small, circular spots on the leaves. The lesions are raised above the top
surface of the leaf, giving them a blister-like appearance. The center of the
spot is initially white, turning to brown as the fungus begins to produce
spores within the affected leaf tissue. The leaf spots have a reddish-purple
border that is often surrounded by an area (halo) of yellow tissue. Stems may
develop raised cankers that appear white with red pigmentation around the
border. Often before the stem cankers are actually noticed, an abnormal
elongation of shoots (that may also be curled or twisted) is observed. These
affected stems are much taller than the rest of the crop due to a natural,
growth-regulating chemical produced by the fungus. Inspect cuttings for leaf
spots at the time they are received. Keep plant leaves dry and lower the
humidity to control this disease. Routine scouting of the crop for leaf and
stem lesions can help reduce losses through early detection and treatment.
Fungicides can be used as protectants against this disease. style="mso-spacerun: yes">  Axoystrobin, chlorothalonil, mancozeb,
chlorothalonil + thiophanate-methyl and myclobutanil have all been shown to be
effective protectants against poinsettia scab.

About The Author

Brian E. Whipker is associate professor, and James L. Gibson and Todd J. Cavins are graduate research assistants in floriculture at North Carolina State University, Raleigh, N.C. Colleen Warfield is assistant professor in ornamentals pathology at NCSU and Raymond A. Cloyd is assistant professor in ornamental entomology/integrated pest management at the University of Illinois, Urbana, Ill. They may be reached via phone at (919) 515-5374 or E-mail at brian-whipker@ncsu.edu.

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