Top Ten Problems of Vegetative Annuals

June 12, 2003 - 12:15

Industry experts have gotten together to share some of the most common problems of vegetative annuals.

Vegetative annuals are excellent niche plants for spring,
summer and fall production. The wide spectrum of available colors and plant
forms has contributed greatly to their success. While these plants are
"hot" with consumers, sometimes growers may have a lukewarm attitude
because of common production problems.

Below are the top ten problems of vegetative annuals we have
observed during extension visits to growers and through plant samples submitted
to the North Carolina State University Plant Disease and Insect Clinic.
Recognizing potential problems and how to prevent or manage them will,
hopefully, keep these problems off your top ten list.

Diseases

Pythium root rot.
Symptoms associated with Pythium root rot may include stunting, nutritional
deficiencies (yellow or purple lower leaves) or wilt. Root tips are typically
infected first and will turn brown in color or will be missing when the plant
is uprooted. Eventually, the entire root system becomes brown and soft.

Under conditions that favor disease development, the fungus
can move up the root to the crown and stem. At this stage, the stem will be
soft and appear black in color. Pythium irregulare, P. aphanidermatum and P.
ultimum are the most common species found in greenhouses sampled in New York
and Pennsylvania. On New Guinea impatiens, Pythium irregulare causes symptoms style="mso-spacerun: yes"> that are more similar to a vascular
wilt disease than a root rot. In this case, thin, black streaks running up the
stem often go unnoticed until the plants begin to wilt.

Cool, wet substrates with poor drainage generally favor
Pythium root rot; however, there are Pythium species that thrive under warm
temperatures. Sanitation, good cultural practices to avoid crop stress and
fungicide applications are used to manage this disease. Fungicide rotation is
important to help prevent the development of fungicide resistant strains of
Pythium. Systemic fungicides such as Banol and Subdue MAXX, used in rotation
with Truban, Terrazole or Banrot, will help manage this disease.

Rhizoctonia leaf spot.
Leaves may become infected with Rhizoctonia leaf spot when they contact the
root substrate or the fungus is splashed onto the leaves resulting in leaf
lesions. Leaf spots are typically brown and dry and have a discrete margin.
This disease favors warm, moist/humid conditions.

Rhizoctonia crown rot.
This disease starts at the soil line or just below. The stem becomes soft and
mushy, and the plant wilts. Plants may appear stunted and have yellow leaves
that may later become "water-soaked." Roots are sometimes affected,
but rot is primarily found at the crown. Web-like strands of the fungus on the
soil surface may be visible with a hand lens. Once a rooting strip is infected,
the fungus can move through the entire strip, infecting other cuttings. The
entire strip should be discarded if any diseased cuttings are found. Fungicides
may interfere with rooting, so it is important to test the product on different
cultivars prior to treating the entire crop. Heritage, Medallion, Chipco 26GT
or Sextant are among a number of fungicides that are effective for managing
Rhizoctonia.

Botrytis. Wounded or
senescent plant parts are usually the first to be colonized by Botrytis
cinerea, the causal agent of gray mold. Symptoms can vary depending on the host
and tissue invaded, but a proliferation of fluffy brown/olive gray fungal
spores on the infected tissue is typical. Healthy tissue will often become
infected if it comes in contact with diseased tissue (such as an infected petal
that drops and falls onto a leaf). The basal end of cuttings can be infected,
and stem cankers are not uncommon.

High humidity and cool temperatures favor the development of
this disease. An integrated approach that includes lowering the humidity,
removing infected plant material and applying protective fungicides is
necessary for management. Rotate fungicide classes to reduce the development of
fungicide resistance. Rotating either Decree, Medallion or Daconil with either
Pathguard or Chipco 26GT and Sextant have been demonstrated to provide
effective disease management for Botrytis blight when used as part of an
integrated pest management strategy.

Virus diseases.
Symptoms vary by host and include yellow or necrotic spots on stems or leaves,
leaf distortion, leaf mosaic/mottling, black leaf spots, black ringspots,
overall yellowing or stunting. Commonly encountered viruses infecting
vegetative annuals are Impatiens Necrotic Spot Virus (INSV), Tobacco Mosaic
Virus (TMV), Cucumber Mosaic Virus (CMV) and Tomato Spotted Wilt Virus (TSWV).
Discoloration and stem dieback are frequently observed for INSV-infected
torenia and nemesia. Transmission of INSV and TSWV is primarily by western
flower thrips. TMV and CMV can be transmitted by tools, worker's hands and, in
the case of CMV, aphids.

Because there is no cure for an infected plant, it is
important to be able to recognize disease symptoms so infected plants can be
identified and removed from the greenhouse before the virus spreads throughout
the entire facility. Infected plants should be destroyed or disposed of well
away from the crop. Inspection and isolation of new plant material being
brought into the greenhouse, and the careful management of insect populations
and weeds are important in managing viral diseases. Be sure that the
propagation stock or cuttings you are purchasing are top quality and certified
to be virus free. Plants should not be carried over to the next season if a
virus has been identified in the greenhouse. Diagnostic test kits are available
to growers for the rapid identification of viruses in a greenhouse or field
setting. Alternatively, samples can be sent to a diagnostic laboratory for
confirmation.

Insects

Aphids. Usually
green and 1/8 inch long, although red or pink color forms may also be seen,
aphids feed on new growth, which becomes stunted and distorted. They excrete
honeydew in which sooty mold can grow. Aphids also vector viruses such as
cucumber mosaic and alfalfa mosaic. Inspect plants for aphids or their cast
skins. Look at leaf undersides, stems and buds. Only winged aphids will come to
yellow sticky traps. Control measures include Azatin, BotaniGard, Endeavor,
insecticidal soap and Marathon.

Two-spotted spider mites. Look for very fine yellow stippling on the upper surface of the leaf.
Heavily damaged leaves can turn dry and defoliate. Stippling will not be seen
on ivy geraniums, which may develop edema in response to mite feeding. High
populations of mites produce webbing. When stippling is noted, inspect the
undersides of older leaves for mites or webbing, or sharply tap the leaves over
a sheet of white paper to check for mites. They are about 1/16 inch long, green
to red in color, with two distinctive black spots. These mites prefer areas
with low relative humidity and high temperatures. Control measures include
Akari, Avid, Floramite, Hexygon, horticultural oil or Pylon.

Broad mites. Another
type of mite that can occur is broad mites. Look for stunting or twisting of
new growth and flowers, blackening and death of young growth, and leaves that
are smaller and harder than normal. This mite will be difficult to detect
during routine monitoring and is most likely to be found on growing tips. Check
plants under a microscope for these yellowish-white mites. They do not produce
webbing. Control measures include Avid or Pylon.

Western flower thrips.
Thrips damage usually appears as scarred, stunted or distorted foliage or
flowers, or as white areas on leaves or petals. Thrips are also a concern as
vectors of INSV and TSWV. Blow gently in to flowers or buds to draw out the
hiding thrips. Tap sturdier plants over a white board or sheet of paper to
check for an infestation. Use yellow sticky traps to monitor adult activity.
Adults are thin and yellow to light brown, about 1/16-1/8 inch long. A hand
lens may be needed to distinguish them from pieces of soil. Adult thrips are
attracted to open flowers and may be seen in much higher numbers than on sticky
traps. Thrips move through greenhouses on air currents, so traps should be
placed in areas of air movement. Place them near openings, including ceiling
vents. Traps can also be placed among plants suspected of harboring thrips.
Control measures include Azatin, Avid, BotaniGard, Conserve or Pedestal.

Nutritional Disorders

Low pH. One factor
that can induce nutrient problems in vegetative annuals is low substrate pH.
The general pH range for vegetative annuals in a soilless substrate is 5.4-6.8,
but maintaining the pH between 5.8 and 6.2 is recommended. Certain
macronutrients such as calcium and magnesium can become less available at pH
values below 5.4. Toxicities (manganese and iron) can also occur if pH values
drop too low. Iron toxicity symptoms occur on lower, mature leaves, but
symptoms may vary by crop. On geraniums, symptoms begin as numerous pinpoint
spots that quickly turn to yellow and ultimately become necrotic. On other
crops, such as dahlia, fuchsia and strawflower, the older leaves develop
numerous pinpoint black necrotic spots across the blade. The entire leaf may
die as the spots enlarge.

High pH. High
substrate pH can induce nutrient problems in vegetative annuals. Low uptake of
nutrients, particularly boron, copper, iron, manganese and zinc, can occur if
the substrate pH is above 6.5. Iron deficiency is the most common problem that
occurs with high substrate pH. Symptoms occur on newly developing leaves and
appear as either an interveinal chlorosis (yellowing) on crops such as petunia
and snapdragon, or as a complete yellowing of the top growth. In late stages,
the leaf blade may lose nearly all pigmentation, appearing white. Determining
if the substrate pH is above 6.5 will help diagnose the problem. (Sulfur
deficiency also appears as a complete yellowing of the upper foliage, but the
substrate pH may not be above 6.5.) Deficiencies can also result from root
death, over-irrigation, poor substrate drainage or insect damage. Inspecting
the roots will help determine the cause of the problem.

Physiological Disorder

Edema. Sometimes
called oedema or intumescence in sweet potatoes, edema appears as small
pimple-like swellings commonly found on the underside of the leaf. These
swellings can enlarge, coalesce, turn brown and Á style="mso-spacerun: yes">
become corky in appearance. They most
commonly occur in ivy geraniums but can be found in zonal geraniums and
sweet potatoes. Symptoms are
typically observed during cool, cloudy weather when the root medium is warm and
moist.

Edema is a water balance problem. Water uptake is greater
than what the plant can transpire, resulting in the formation of the style="mso-spacerun: yes"> swellings. The swellings ultimately
burst, and the brown, corky scars develop. Avoiding excessive irrigation during
cloudy periods and increasing air circulation will help prevent the problem.

About The Author

Colleen Warfield is an assistant professor in ornamentals pathology, Brian Whipker is associate professor, James Gibson and Brian Krug are graduate research assistants in floriculture, Christine Casey is an assistant professor in ornamentals entomology at North Carolina State University, Raleigh, N.C., and Raymond Cloyd is an assistant professor in ornamentals entomology at the University of Illinois.

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