June 2003

Top Ten Problems of Vegetative Annuals By Colleen Warfield, Brian Whipker, Christine Casey, Raymond Cloyd, James Gibson and Brian Krug

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

Below are the top ten problems of vegetative annuals we haveobserved during extension visits to growers and through plant samples submittedto 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, nutritionaldeficiencies (yellow or purple lower leaves) or wilt. Root tips are typicallyinfected first and will turn brown in color or will be missing when the plantis uprooted. Eventually, the entire root system becomes brown and soft.

Under conditions that favor disease development, the funguscan move up the root to the crown and stem. At this stage, the stem will besoft and appear black in color. Pythium irregulare, P. aphanidermatum and P.ultimum are the most common species found in greenhouses sampled in New Yorkand Pennsylvania. On New Guinea impatiens, Pythium irregulare causes symptomsthat are more similar to a vascularwilt disease than a root rot. In this case, thin, black streaks running up thestem often go unnoticed until the plants begin to wilt.

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

Rhizoctonia leaf spot.Leaves may become infected with Rhizoctonia leaf spot when they contact theroot substrate or the fungus is splashed onto the leaves resulting in leaflesions. 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 andmushy, and the plant wilts. Plants may appear stunted and have yellow leavesthat may later become “water-soaked.” Roots are sometimes affected,but rot is primarily found at the crown. Web-like strands of the fungus on thesoil 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. Theentire strip should be discarded if any diseased cuttings are found. Fungicidesmay interfere with rooting, so it is important to test the product on differentcultivars prior to treating the entire crop. Heritage, Medallion, Chipco 26GTor Sextant are among a number of fungicides that are effective for managingRhizoctonia.

Botrytis. Wounded orsenescent plant parts are usually the first to be colonized by Botrytiscinerea, the causal agent of gray mold. Symptoms can vary depending on the hostand tissue invaded, but a proliferation of fluffy brown/olive gray fungalspores on the infected tissue is typical. Healthy tissue will often becomeinfected if it comes in contact with diseased tissue (such as an infected petalthat 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 ofthis disease. An integrated approach that includes lowering the humidity,removing infected plant material and applying protective fungicides isnecessary for management. Rotate fungicide classes to reduce the development offungicide resistance. Rotating either Decree, Medallion or Daconil with eitherPathguard or Chipco 26GT and Sextant have been demonstrated to provideeffective disease management for Botrytis blight when used as part of anintegrated 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 infectingvegetative annuals are Impatiens Necrotic Spot Virus (INSV), Tobacco MosaicVirus (TMV), Cucumber Mosaic Virus (CMV) and Tomato Spotted Wilt Virus (TSWV).Discoloration and stem dieback are frequently observed for INSV-infectedtorenia and nemesia. Transmission of INSV and TSWV is primarily by westernflower thrips. TMV and CMV can be transmitted by tools, worker’s hands and, inthe case of CMV, aphids.

Because there is no cure for an infected plant, it isimportant to be able to recognize disease symptoms so infected plants can beidentified and removed from the greenhouse before the virus spreads throughoutthe entire facility. Infected plants should be destroyed or disposed of wellaway from the crop. Inspection and isolation of new plant material beingbrought into the greenhouse, and the careful management of insect populationsand weeds are important in managing viral diseases. Be sure that thepropagation stock or cuttings you are purchasing are top quality and certifiedto be virus free. Plants should not be carried over to the next season if avirus has been identified in the greenhouse. Diagnostic test kits are availableto growers for the rapid identification of viruses in a greenhouse or fieldsetting. Alternatively, samples can be sent to a diagnostic laboratory forconfirmation.

Insects

Aphids. Usuallygreen 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 excretehoneydew in which sooty mold can grow. Aphids also vector viruses such ascucumber mosaic and alfalfa mosaic. Inspect plants for aphids or their castskins. Look at leaf undersides, stems and buds. Only winged aphids will come toyellow 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 seenon ivy geraniums, which may develop edema in response to mite feeding. Highpopulations of mites produce webbing. When stippling is noted, inspect theundersides of older leaves for mites or webbing, or sharply tap the leaves overa sheet of white paper to check for mites. They are about 1/16 inch long, greento red in color, with two distinctive black spots. These mites prefer areaswith low relative humidity and high temperatures. Control measures includeAkari, Avid, Floramite, Hexygon, horticultural oil or Pylon.

Broad mites. Anothertype of mite that can occur is broad mites. Look for stunting or twisting ofnew growth and flowers, blackening and death of young growth, and leaves thatare smaller and harder than normal. This mite will be difficult to detectduring routine monitoring and is most likely to be found on growing tips. Checkplants under a microscope for these yellowish-white mites. They do not producewebbing. Control measures include Avid or Pylon.

Western flower thrips.Thrips damage usually appears as scarred, stunted or distorted foliage orflowers, or as white areas on leaves or petals. Thrips are also a concern asvectors of INSV and TSWV. Blow gently in to flowers or buds to draw out thehiding thrips. Tap sturdier plants over a white board or sheet of paper tocheck 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 handlens may be needed to distinguish them from pieces of soil. Adult thrips areattracted to open flowers and may be seen in much higher numbers than on stickytraps. Thrips move through greenhouses on air currents, so traps should beplaced in areas of air movement. Place them near openings, including ceilingvents. 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 factorthat 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. Certainmacronutrients such as calcium and magnesium can become less available at pHvalues below 5.4. Toxicities (manganese and iron) can also occur if pH valuesdrop too low. Iron toxicity symptoms occur on lower, mature leaves, butsymptoms may vary by crop. On geraniums, symptoms begin as numerous pinpointspots that quickly turn to yellow and ultimately become necrotic. On othercrops, such as dahlia, fuchsia and strawflower, the older leaves developnumerous pinpoint black necrotic spots across the blade. The entire leaf maydie as the spots enlarge.

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

Physiological Disorder

Edema. Sometimescalled oedema or intumescence in sweet potatoes, edema appears as smallpimple-like swellings commonly found on the underside of the leaf. Theseswellings can enlarge, coalesce, turn brown and çbecome corky in appearance. They mostcommonly occur in ivy geraniums but can be found in zonal geraniums andsweetpotatoes. Symptoms aretypically observed during cool, cloudy weather when the root medium is warm andmoist.

Edema is a water balance problem. Water uptake is greaterthan what the plant can transpire, resulting in the formation of theswellings. The swellings ultimatelyburst, and the brown, corky scars develop. Avoiding excessive irrigation duringcloudy periods and increasing air circulation will help prevent the problem.