By Brian E. Whipker, James L. Gibson, Dharmalingam S. Pitchay, Paul V. Nelson, James R. Baker, James E. Faust, Paul A. Thomas, Mike Benson, Todd J. Cavins and Jean L. Williams-Woodward
Many problems can occur during pansy production. The Fred C. Gloeckner Foundation supported a cooperative project involving North Carolina State University, Clemson University and the University of Georgia that examined the most common problems observed by growers. Following is a list of these problems, including descriptions of the symptoms, and techniques for the prevention of or remedy for each problem. For a more complete discussion of pansy production, a packet of pocket-sized diagnostic cards and a production manual also are available. Contact information is at the end of this article.
Numerous fungi cause foliar diseases on pansies, but growers often have difficulty in making the correct diagnosis of the pathogen; the symptomatic spots often look very much alike. Fortunately, the control measures generally are the same.
Foliar diseases are favored by extended periods of leaf wetness. Watering late in the day so that plants go through the evening with wet leaves can increase disease. This is because most leaf-spotting fungi require a minimum of eight to 12 hours of constant leaf wetness to infect the plant. Infection cannot occur unless water is present on the leaf for the required amount of time.
Promptly remove infected leaves and severely infected plants to reduce disease spread. One infected leaf or plant, if left in the tray or greenhouse, can spread the disease to all of the other plants. Fungicides can reduce infection of new leaves, but they will not cure already infected leaves. Remember, the test for the effectiveness of a fungicide spray is not whether the existing leaf spots go away (because they never will), but whether new growth remains free of infection. Repeat applications will be necessary to control these diseases.
Cercospora Leaf Spot:This is the most common leaf spot disease on pansies in the South. Large, spreading purple spots start on the lower leaves; the spots have a feathered margin. As the disease progresses, tiny purple spots and flecks appear on the upper leaves. Over time, the leaf spots develop tan centers with purple borders. Spots are often irregular in size and shape. Lower leaves often turn yellow and fall off, reducing plant vigor and appearance.
Disease Cycle:Caused by Cercospora violae (a fungus specific to pansy), this disease favors warm temperatures and wet conditions, and is most often seen in the fall and late spring. The fungus infects the lower leaves first. Infected leaves fall off the plant and it is presumed that the fungus survives within the leaf debris. Numerous spores that usually are spread by splashing water to adjacent leaves or plants are produced within the spots.
Management: Remove severely infected plants to reduce disease spread. All plant debris should be discarded or destroyed at the end of the production cycle to reduce pathogen survival. Applying fungicides, including mancozeb, myclobutanil, chlorothalonil, fludioxonil, or thiophanate methyl + mancozeb, at seven to 14 day intervals can reduce disease development and spread.
Anthracnose:This is another very common leaf spot on pansies. The leaf spots are typically pale or grey with thick, black margins that sometimes appear to have a concentric ring pattern. The spots may range in size from 1/8 to 1/4 inch. Sunken, elongated brown to tan lesions often develop on the petioles, peduncles, or stems of infected plants. The lesions eventually girdle the stem, killing the plant. Anthracnose also may cause brown spotting on infected flowers.
Disease Cycle:Anthracnose is caused by the fungus Colletotrichum violae-tricoloris. Infection may be associated with stressed or injured tissue. The disease is most severe during periods of warm, wet conditions in the fall and late spring. The fungus produces abundant spores within the leaf spots or other affected plant parts that are spread to adjacent plants by water splashing. The fungus survives from year-to year in infected plant residue.
Management:Removing and destroying infected plants, as well as plant residue, at the end of the production season can reduce disease survival and spread. Applying fungicides such as mancozeb or thiophanate methyl + mancozeb at the first sign of disease can reduce disease spread. Avoid stressing plants and prolonged periods of plant wetness which can favor anthracnose infection.
Pansies can be infected by the usual root-rot pathogens such as Pythium, Rhizoctonia, and Fusarium, but they also are susceptible to Thielaviopsis, the cause of "black root rot." Black root rot is the most serious root disease of pansies primarily because it is very difficult to control.
Root rot diseases typically occur in wet substrates due to over-watering, poor drainage, or containers standing in water. All root rot pathogens are soil-borne, meaning that they survive in substrates and debris. They usually develop in the production area due to poor sanitation practices such as re-using potting substrate and unclean containers, as well as from plant and soil debris on benches and floors.
Anything that moves or contains substrate (dirt and debris) has the potential to introduce or spread root rot pathogens, including dirt on shoes, the outer sides of potting mix bags, tools, hoses, and hands.
Foliage symptoms of root diseases are very similar. Plants often are chlorotic or purplish, wilted, stunted and weak. A root rot disease cannot be diagnosed by examining only the foliage. The best way to diagnose the root rot disease is to look at the roots.
Black Root Rot (Thielaviopsis):Pansies infected with black root rot produce above-ground symptoms typical of other root-rotting diseases. Infected plants may be stunted and/or wilted, but the most diagnosable symptom is leaf yellowing (chlorosis).
Roots infected with black root rot develop dark spots or bands that are easily seen against the normally white pansy roots. Early infection is most often seen at the tips of secondary feeder roots, but as the disease progresses, the entire root system becomes black and water-soaked. The black discoloration is due to fungal spores within the root. The spores can be seen with the aid of a 15-20x magnifying glass, hand lens, or microscope. All stages of pansies can be infected and killed. The disease can be especially destructive during late summer months when temperatures are high and when the substrate pH is higher than 6.0.
Disease Cycle:Black root rot, caused by the fungus Thielaviopsis basicola (also known as Chalara elegans), is a widespread and destructive root disease of pansy. Thielaviopsis survives in soil and plant debris. Healthy roots become infected when they come in contact with infected roots or fungus spores within infested substrate and/or debris. The fungus then grows and multiplies rapidly, producing spores within the infected plants. Spores also can spread from pot to pot by splashing water.
The disease can be introduced into greenhouses from infected plugs. Always examine plugs carefully for symptoms of infection such as uneven growth and poor foliage color before introducing them into production areas. If you suspect a problem with the plugs, immediately submit a sample plug tray to a diagnostic clinic. Spores of Thielaviopsis can survive on greenhouse benches and plug trays for up to two months (and within plant debris under benches for longer periods of time).
Management:Good sanitation, proper plant care, and fungicides can help reduce black root rot disease development. Use only new soilless, pathogen-free, potting mix. Never re-use potting mixes! Use new containers for each planting and keep all equipment and the planting area clean of old potting substrate and debris. Avoid water and heat stress, especially in late summer.
Do not over-fertilize plants.Apply the recommended rate of nitrogen for pansy production, but avoid fertilizers high in ammonium-nitrogen. Maintain a root substrate pH between 5.4 and 5.8. Always check your plants regularly for signs of disease and discard infected plants immediately. If needed, fungicides such as thiophanate methyl, triflumizole, or fludioxonil can reduce black root rot infection, but they will not "cure" already infected plants.
Pythium Root Rot:Infected roots appear tan to brown and water-soaked or slimy. The outer root cortex is easily stripped from the thread-like inner root (stele) with your fingers. Pythium typically attacks the root tips first and then progresses upward within the root system. Lower leaves of infected plants often yellow. Pythium also causes damping off.
Disease cycle:Pythium is very common and it cannot be eliminated from greenhouse production. Pythium survives in soil and on anything that contains soil, including hands, shoes, pots and equipment, as well as in contaminated water sources. Over-watering and excessive overhead misting during propagation will favor disease development by providing ideal conditions for Pythium spore germination and movement. The disease is easily spread pot to pot by splashing water or soil movement. High soluble salt concentration in the potting substrate also favors Pythium infection by stressing the root system.
Management:Fungicides typically used for control of Pythium such as etridiazole, fosetyl-Al, mefenoxam, and propamocarb can stunt young pansies, so apply fungicides with caution during the first week after transplant.
Pansyworms (variegated fritillary) are small, spiny caterpillars that are appropriately named because they feed on pansies and other plants in the violet family. It is the immature stage of one of a group of four-footed butterflies called fritillaries. The variegated fritillary is a brownish-orange, spotted and speckled, medium-sized butterfly that is sometimes noticed on pansies in mid to late summer and early fall.
Damage:Very small caterpillars rasp away at leaves, displaying much the same evidence as do slugs. Older caterpillars chew holes in leaves or consume leaves altogether. The caterpillars also leave unsightly dark droppings on the leaves. Often the droppings are more conspicuous than are the young caterpillars.
Life History:Pansyworms overwinter as caterpillars feeding on pansy, violet, alyssum, Johnny-jump-up and other plants in the violet family. In spring, the caterpillars molt into a chrysalis (pupa) and from that stage emerges a generation of adult butterflies. These butterflies are not particularly rare; they range throughout the U.S. except for the Pacific Northwest, so it is common to find pansyworms feeding on pansies throughout the Southeast.
Management:Pansyworms are not resistant to pesticides, so chemical control is not difficult; however, the worms are small enough to do damage before they are discovered. Variegated fritillaries are large enough to be easily excluded from greenhouses and shade houses with screening.
Boron (B) Deficiency symptoms are initially expressed on the new leaves and stems, with growth being distorted or clubby. With advanced conditions, death of the growing point can occur, thus resulting in axillary shoot growth. It is important to correct B deficiency when symptoms first appear because death of the meristematic tissue or distorted leaves can not be reversed. Excessive levels of K or Ca can have an antagonistic effect on B availability. B is less soluble in soilless substrates and when the pH is above 6.2.
Strategy for Boron:Irrigation water may contain adequate or even excessive levels of B. Test the irrigation water to determine available levels. Supplement with additional B in your fertilization program if B is <0.3 ppm. No additional B may be needed if B levels are between 0.3 to 1.5 ppm. Select fertilizers that do not contain B if irrigation water levels are >1.5 ppm.
Continual Fertilization:Most commercial fertilizers that provide micronutrients include B. Remember to add supplemental B applications if using calcium nitrate (Ca(NO3)2) + potassium nitrate (KNO3) and your irrigation water lacks B.
Corrective Fertilization:Corrective drench applications of Borax or Solubor at 6.25 ppm of B (see mixing rates below) can be applied to soilless substrates. (Visible improvements will be very slow.) It is important to correct B deficiency when symptoms first appear because death of the meristematic tissue or distorted leaves cannot be reversed. (Measure carefully, overdoses can be lethal to pansies.)
For a corrective application, add 0.75 oz. (21.3 g) of Borax (11 percent B) per 100 gallons of water, or 0.43 oz. (12.2 g) of Solubor (20 per cent B) per 100 gallons of water.
Nitrogen Deficiency:Plants deficient in nitrogen exhibit slow growth, stunting, lack of lateral shoot growth, or (with advanced conditions) lower leaves that initially turn greenish-purple to yellow (chlorosis). Leaf abscission occurs after prolonged deficiency conditions. N is a mobile element within the plant; therefore, deficiency symptoms will appear first on the lower, older leaves.
Excess levels of N will result in dark-green coloration, reduced plant growth, and delayed flowering. Excessive N can reduce uptake of potassium (K).
Strategy for Nitrogen (N):Pansies are susceptible to ammoniacal-nitrogen (NH4-N) toxicity. Symptoms appear as an interveinal V-shaped chlorosis with green veins, and downward cupping of the younger leaves. Ammoniacal-nitrogen toxicity can be avoided by supplying >75 percent of N in the nitrate (NO3-) form. (Categorize urea-nitrogen with ammonical-nitrogen when calculating the ratio.)
Options:Use a fertilizer that provides N at 125 ppm constant liquid fertilization or 175 to 200 ppm constant liquid fertilization with excessive leaching (outdoor production). Examples include calcium nitrate (Ca(NO3)2) with potassium nitrate (KNO3), 20-10-20, 15-5-25, Excel 15-5-15 Cal-Mag, 15-2-20, and others.
If using calcium nitrate (Ca(NO3)2) with potassium nitrate (KNO3), remember to supply P, Mg, and micro-nutrients to the plants.
If using 20-10-20 or 20-20-20, remember to supply Ca and Mg to the plants.
Corrective Fertilization:Applications include calcium nitrate (Ca(NO3)2), potassium nitrate (KNO3), or Excel 15-5-15 Cal-Mag at the rate of 300 to 400 ppm N. A single corrective N fertilization will return the lower leaves to the normal green color within one to two weeks. Do not overapply. It is important to correct N deficiency when symptoms first appear because lower leaf drop may occur under severe conditions.
Mottle Pansy Syndrome
During fall production in the mid 1980s, growers in warm regions began noticing Mottle Pansy Syndrome (MPS). Pansies that express this malady have variegated leaves that first are seen at the plug stage.
If night temperatures are cool and daytime highs are near 80° F, the plant loses the variegation and appears normal until heat stressed. However, under daytime heat and warm nights, the variegation resumes on new growth.
As this syndrome progresses, leaves become contorted, develop finger-like projections, or become feathery. This bizarre growth will continue until temperatures are lowered or fertility is increased. Note that increasing fertility or reducing temperature is not a cure. Once the consumer places the plant outdoors, mottled and contorted leaves will return with warmer weather.
Source of Problem:Although extensive tests have been performed for virus, mycoplasma-like organisms, bacteria, and nutritional deficiencies, MPS remains a mystery attributed to a genetic defect in pansies and violas. Unlike the occasional variegated leaf in plugs, this defect may cause significant crop loss. It has been expressed in several lines of pansies distributed by most major suppliers.
Corrective Action:Discard the plant. Plugs express the variegated leaves almost immediately, which enables a grower to hold a plug tray up into sunlight and to remove the affected seedlings. By scouting plug trays, growers can eliminate these plants before they become a problem. The cost of scouting is only a fraction of the potential production loss.
Avoidance Review:1) Scout plug trays as they arrive. Remove all variegated-leafed plugs. Monitor new growth on transplanted plugs for signs of lanceolate, twisted, or elongated leaves. Maintain a cool production environment for pansy plugs. Scout the entire crop for signs of variegated leaves during warm temperature conditions.
2) Purchase quality seed and report significant amounts of variegation to the seed supplier. Do not try to grow affected plugs. Applications of boron, iron, and magnesium only mask the genetic problem under ideal conditions and may cause loss of business reputation and income once the plant is sold and re-expresses the symptoms.