Top Ten 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 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 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 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.