Potted Lisianthus:

Secrets of Success – Disease Control

by Robert J. McGovern, Brent K. Harbaugh, and Jim P. Price
University of Florida, Gulf Coast Research and Education Center – Bradenton, FL
Published in Greenhouse Grower, February 1998

Part 1 - Plug Production

Part 2 - Bedding and Potted Plant Production

Part 3 - Protecting Lisianthus from Fungus Gnats and other Arthropod Pests

Part 4 - Control Diseases affecting this Crop
 

Although it may not be the easiest to produce, lisianthus has been growing in popularity as a potted plant each year.  Our final installment in this four-part series focuses on controlling diseases affecting this crop.

The first three articles in this series covered general horticultural methods of plug and potted lisianthus production and insect management.  This final article addresses the control of plant pathogens and should be considered in combination with the information we previously presented when trouble-shooting lisianthus problems.

1.)                Determine that the problem is in fact disease related.

2.)                Correctly identify the disease.

3.)                Take a holistic or integrated approach to disease management.

Rule out other Causes

            The first action taken to control a suspected disease problem shouldn’t be reaching for the nearest fungicide container without being sure about the cause of the symptoms that are observed.  A wider focus than merely narrowing in on symptoms is needed, one that considers each aspect of your production program.  Plants may exhibit symptoms that at first appear to be the result of infection by pathogens, such as fungi, bacteria, or viruses.  However, they may be responses to different types of stress, including suboptimal growing conditions (water and nutrient imbalances), insect feeding, misapplication or incompatible mixing of pesticides, etc.

            For example, water stress may cause yellowing, leaf spots, and necrosis of leaf margins that may be misinterpreted as disease symptoms.  Similarly, the root damage and wilting resulting from fungus gnat feeding mimic infection by root-rotting fungi.

            Therefore, the first step to effective disease control is an accurate general diagnosis:  Is the problem the result of disease or not?  Once the noninfectious factors are ruled out, disease recognition becomes paramount.  Below are descriptions of diseases of lisianthus we commonly have observed.

Make an Accurate Diagnosis

1.)        Gray mold (Botrytis blight).  As its name suggests, gray mold, also known as Botrytis blight, is characterized by the prolific production of fuzzy, gray fungal growth, including masses of spores (Figure 1).  Spores of the casual fungus, Botrytis cinerea, are easily spread by air currents and water splash, making gray mold one of the most commonly encountered and difficult-to-control diseases in lisianthus.  All above-ground plant parts (stems, leaves, and flowers) are susceptible to B. cinerea, which is favored by cool, moist conditions.

            Symptoms on petals begin as small, bleached spots that rapidly enlarge to blight the entire flower.  Senescing flowers and damaged tissue are particularly susceptible to

B. cinerea, and it is common to find the fungus using such an infection site as a spring board to healthy tissue.

            Early symptoms on stems consist of brown lesions that may remain localized or enlarge and kill the entire plant.  Localized lesions that may be somewhat concentric, often girdle the stem, and lead to wilting and death of tissue above the infection site.

2.)        Curvularia leaf blotch.  A leaf blotch of lisianthus appears to be caused by the fungus Curvularia sp. and begins as minute tan specks that slowly enlarge into tan to beige blotches (Figure 2).  These blotches resemble symptoms of micronutrient toxicity caused by low pH, especially on lisianthus seedlings.  (This can easily be ruled out by determining soil pH; a soil pH below 6.5 can lead to micronutrient toxicity in lisianthus.)

            Curvularia has rarely been reported as a pathogen but it is a common saprophyte or secondary invader of damaged plant tissue.  Its invasion of lisianthus is favored by warm temperatures around 75ºF.

3.)        Impatiens Necrotic Spot Virus.  Symptoms caused by the Impatiens Necrotic Spot Virus (INSV) in lisianthus are variable and may include stunting, circular to irregularly shaped necrotic leaf spots, and distortion of leaves and shoots (Figure 3).

            Infection by severe INSV strains may result in systemic necrosis, wilting, and/or plant death.  The Western Flower Thrips (Frankliniella occidentalis) is currently the only known vector of INSV.  Immature thrips acquire INST and can transmit the virus as adults for their lifespan.  The virus has an extremely broad host range, including other flowering potted crops such as gloxinia, impatiens, and chrysanthemum.  Older crops, volunteers, and many weeds may serve as reservoirs for INSV.

4.)        Fusarium crown and stem rot.  The causal fungus of this disease, Fusarium avenaceum, primarily attacks the crown and stems of lisianthus, but may also cause rot in the tap

root and a few larger feeder roots near the soil line.  The first above-ground symptom is a gradual loss of green coloration in leaves, followed by tan leaf flecks, browning of leaf veins, and tanning of entire leaves (Figure 4).

Wilting and a brown stem rot occur as the disease progresses, and infected plants rapidly die.  Orange spore masses form at the bases of these rotted stems and these, along with veinal necrosis, are diagnostically very important.

            Fusarium crown and stem rot may resemble gray mold, but the two diseases can be easily differentiated based on their spore coloration.  Fusarium crown and stem rot is favored by cool temperatures.

             Fusarium avenaceum may be spread in contaminated transplant trays and on pruning tools.  The fungus has been reported to infect many cultivated crops, and we are currently investigating its potential to attack other potted ornamentals besides lisianthus.

5.)        Pythium root rot.  Lisianthus that turn yellow and wilt suddenly even when the soil is adequately most may be suffering from Pythium root rot.  This diagnosis may be confirmed by looking for the presence of mushy, darkly discolored roots (Figure 5).  The outer rotted and discolored layer of Pythium infected roots is often easily pulled off leaving behind a central, stringy fiber.

Pythium root rot is common in water-logged soils with poor aeration.  Cycles of under- and overwatering that lead to root damage are very conducive to outbreaks of Pythium.  Fungus gnats create infection sites through their root feeding, and may disseminate the fungus.  Pythium, depending on the species, has a broad host range.  It is spread by water splash and may contaminate potting soils and irrigation systems.

6.)        Rhizoctonia crown and stem rot.  The fungus Rhizoctonia solani can infect the roots, stem, and foliage of lisianthus.  The fungus most typically produces a rot and dark discoloration in stems at the soil, leading to wilting and rapid collapse of the entire plant (Figure 6).  Under humid conditions, growth of R. solani may rapidly envelop and blight leaves and shoots; the disease is then known as Rhizoctoniia web or aerial blight.  Leaf lesions may have a concentric appearance.

Rhizoctonia solani, similar to the majority of the pathogens already mentioned, has a broad host range that encompasses most economically important ornamentals.  The fungus is very active at warm temperatures, effectively colonized and survives in growing media, and is spread by water splash and airborne particular matter.

Use an Integrated Approach

            Taking an integrated approach to disease management that considers all aspects of lisianthus production is as essential as it was for initially diagnosing the problem.  Listed below are components of an effective strategy for disease management in lisisanthus:

1.)        Transplants.  The use of vigorous, pathogen-free transplants is necessary for reducing plant loss, as well as costly and futile pesticide application.  In addition, this practice can help to avoid the introduction of plant pathogens into your production facility that are often difficult to eliminate once established.

2.)        Sanitation.  Sanitation should be rigorous and multifaceted.  Structures, that either come in contact with plants or collect soil and debris, should be routinely washed and disinfested.  Production house walkways should be made of concrete so that they may be easily cleaned and disinfested.

            Personnel should routinely disinfect their hands and shoes before entering production houses.  Disinfestant dispensers and foot baths should be set up at production house entrances.  Infected plants should be immediately rogued.  (Avoid handling wet plants to reduce fungal and bacterial spread.)

            Plant debris must be promptly removed from production areas.  Do not establish cull piles and trash areas up wind from, or near to, production facilities.  Following removal of infected plants, personnel should disinfest their hands, shoes, and tools before resuming regular duties.

3.)        Growing medium.  The growing medium should provide adequate aeration for roots and should be free of pathogens and weed seed.  In addition, growing media should be stored in original bags or in covered containers until used to prevent contamination.

4.)        Cultural practices.  Proper cultural practices, such as providing adequate water and fertilizer and avoiding excesses of these factors, will reduce root system injury and disease. 
            Fertilizer and water imbalances lead, respectively, to nutrient deficiencies or soluble salt excesses, and drought stress or water-logged soils, and commonly predispose plants to soilborne disease.  If possible, irrigate early in the morning to avoid prolonged leaf wetness.  Increasing plant spacing promotes better air circulation, thereby reducing leaf wetness and disease.

5.)        Other pests.  Control of insects (especially aphids, fungus gnats, and thrips) and other arthropods will decrease feeding damage and the spread of fungi, bacteria, and viruses.  Removal of volunteers and weeds reduces sources of plant pathogens and the insects that spread them.

6.)        Scouting.  Thorough, routine scouting should be practiced in each production house for early detection of disease and insect problems.  Severely infected plants should be promptly rogued to prevent pathogen spread.  Workers should not resume duties before personal disinfestations and decontamination of tools.

7.)        Biological control.  Although researchers have actively investigated the use of beneficial microorganisms (biological control) for suppression of plant disease over the last 3 decades, this area of disease management is still in its developmental stage.  Research data indicate that bacteria such as Bacillus, Pseudomonas, and Streptomyces, and fungi such as Gliocladium and Trichoderma may suppress a variety of fungal pathogens when introduced into transplant media, or when used as seed treatments.

            Commercial formulations of biocontrols and disease-suppressive transplant mixes are becoming increasingly available but should be thoroughly tested for effectiveness on a limited scale before routine use in your lisianthus program.

8.)        Pesticide application.  Application of pesticides to control diseases of lisianthus should be done preventively and therapeutically.  Spray schedules should be based on scouting reports and the presence of environmental conditions conducive to disease development.  Spray delivery systems should be routinely calibrated and in good working order.  Insufficient coverage is one of the most common reasons for pest management failure.

            Avoid untested tank mixes and applying pesticides to water-stressed plants.  Follow-up inspections will help evaluate pesticide effectiveness.  Rotate different classes of fungicides to prevent the buildup of pathogen resistance.  Listed in Table 1 are fungicides with broad ornamental clearances that permit use on lisianthus.  (Since lisianthus is a new crop it is not specifically mentioned on fungicide labels.)

Table 1.  Fungicides for the management of diseases of lisianthus.*