DISEASES OF INDOOR PLANTS
The occurrence of diseases on indoor landscape plants is typically a result of adverse environmental conditions or the presence of infectious agents. Diseases solely caused by microorganisms (fungi, bacteria, viruses or nematodes) are not common in the indoor landscape setting if the landscape was established using disease-free plant material and conditions conducive to disease development are avoided. It is critical to identify the causal agent or the type of disease that arises in the interior landscape in order to take appropriate corrective measures. Plants are subject to two types of disorders:
To manage plant diseases and disorders successfully, plant maintenance technicians must be able to:
Select the best method(s) for correcting conditions that contributed to the disease development.
Abiotic disorders produce a wide range of symptoms such as reduced vigor, yellowing leaves, leaf drop, or rapid death of plants. The abiotic conditions causing severe damage are easiest to recognize and correct. Less obvious symptoms may go unnoticed and therefore the unfavorable condition or cultural practices may persist for a long period of time. During this time plants are stressed and become more susceptible to pathogenic diseases. For example, if powdery mildew develops on a plant that is usually resistant to this disease, an investigation may reveal that the plant was consistently overwatered, resulting in high moisture and humidity (abiotic conditions) in the planting area. In this case, correcting the cultural practice that allowed the pathogen to become established may be the only corrective measure necessary for managing the powdery mildew.
Many cultural and environmental conditions that lead to poor plant health are mentioned throughout this manual. This section will focus on some of the most likely abiotic disorders found in an interior landscape.
Environmental Conditions Contributing to Abotic Disorders
There are certain ranges of temperature, light and humidity that provide for optimal plant health. The importance of these factors is considered here in relation to the development of abiotic disorders.
Typically interior spaces utilized for living or work environments provide temperatures adequate for growing foliage plants. A temperature range between 50 and 90° Fahrenheit can be tolerated by most foliage plants. Attention must be given to temperatures during times when interior spaces are not occupied by people. With a more energy-conscious society, the thermostat may be set to temperatures that may injure plants when they are exposed over a period of time. Foliage plants can suffer cold damage without freezing. This is referred to as chilling injury. A few plants that are injured by exposure to chilling temperatures between 35 and 50° F for short periods of time include Aglaonema X 'Silver Queen', Dieffenbachia maculata, Dracaena spp. and Polyscias fruiticosa. Symptoms of chilling injury include yellowing or brown water-soaked areas on leaves, loss of foliage, poor growth, and wilting.
Plants exposed to hot temperatures for prolonged periods of time can also suffer injury. Wilting, marginal burn on foliage, and leaf drop may occur. Since most foliage plants can tolerate temperatures as high as 95°F, provided they receive adequate water, the problem is not related entirely to maximum temperature but to utilization of stored food reserves due to elevated respiration levels. When a plant depletes its stored carbohydrates, it may become weak and predisposed to other stresses including invasion by pathogens.
Light affects numerous physiological conditions and processes in plants. When plants are subjected to inadequate light levels, disruptions of these functions may cause stress. Plants may exhibit poor growth and color. These symptoms are easily mistaken for something other than poor environmental conditions.
If plants haven't been adequately acclimatized to the conditions of an interiorscape (lower light levels, lower temperatures and humidity), they may experience yellowing and leaf drop and possible death following installation.
Many growers are producing acclimatized plants which require more time to grow, but are more tolerant of environmental stress at the job site. Be familiar with the growing practices of your supplier and symptoms associated with an inadequately acclimatized plant. If the grower doesn't acclimatize the plants, establish a method and facility to do so before installing the plants directly into an interior landscape.
Cultural Practices Contributing to Plant Disorders
Most plant replacements result from a combination of poor environmental conditions and poor cultural practices. Abiotic disorders symptoms, resulting from poor environmental conditions and cultural practices, are extremely variable and are often misdiagnosed. Pesticide applications will not correct an abiotic disorder. It is vital that plant technicians understand the impact of their cultural practices on the plants they maintain.
The misapplication of water is the leading source of problems related to cultural practices. Do not wet foliage or splash water when performing routine care. Wetting foliage may promote the growth of leaf-spot organisms. Moisture and warm temperatures are required for most diseases to develop on healthy plants.
Water drops on leaf surfaces magnify the intensity of light as it passes through the drop. Plants in direct sunlight can develop burned spots beneath water drops.
Plants with hairy (pubescent) leaf surfaces (i.e. African violets, gloxinia) are easily damaged by cold water on their leaves. Damage from water that is 10 degrees cooler than the leaf surface will appear as distinct "fairy rings."
Misting plants is not recommended because this added moisture on leaf surfaces can spread disease organisms or create conditions favorable for disease development. Foliage should only be wetted when dislodging insects or when removing dust.
Damage from the application of pesticides (phytotoxicity) occurs for several reasons. Products used for shining leaf surfaces should be avoided since they leave heavy residues and clog plant pores.
The misapplication of fertilizers is also a common cultural practice that causes abiotic disorders. Underfertilization can lead to nutrient-starved plants which appear sickly and may be more susceptible to biotic diseases. Overfertilization can led to nutrient imbalances and high levels of salt in the soil. High levels of salts in the growing media can cause damage to plant roots which makes them more susceptible to attack by soil-borne pathogens. The use of water high in minerals may also lead to soil- salt problems.
Soil pH may change over time. Maintaining a proper pH will minimize nutritional problems. When diagnosing plant problems, always check the soil pH to be sure it is in a range adequate for foliage plant growth.
Plant Problems Due to Miscellaneous Activities
Environmental factors and cultural practices can be monitored and adjusted to provide the best possible conditions for plants. Yet there may be harmful activities that occur in the interior landscape that are beyond the control of the plant technician. When symptoms seem to have no other explanation, the following possibilities should be considered:
It is helpful to understand which conditions influence biotic disease (also known as infectious disease) development when managing the health of plants. The disease triangle illustrates the primary factors that must be present and favorable for a biotic disease to occur. These are:
The goal of a plant pest manager is to recognize and manipulate these primary factors to promote stress-free plants and eliminate conditions favorable to disease development. Remember, problems on plants generally do not arise from one isolated cause.
The disease triangle reminds us of the factors necessary for a pathogen to infect a plant. A susceptible host plant and a pathogen must be present along with the proper environmental conditions in order for a disease to occur. Conditions that favor the development of pathogenic diseases can be avoided in the interior landscape setting. Overhead irrigation, splashing water, prolonged leaf wetness, high humidity, crowded blocks of similar plants, and propagation of infected stock are conditions more likely to occur in a greenhouse production setting.
Typically, diseases in the interior landscape result as a secondary infection after a plant has been stressed by adverse cultural or environmental conditions. Correcting the stress often prevents the development of diseases.
Pathogenic organisms can be widespread. A plant technician should be aware of the sources of infectious plant pathogens which include:
When a disease appears in the interior landscape, it is possible that it is a continuation of a situation that began in the greenhouse. The goal of plant growers is to produce high-quality plants as free of pests as possible. Greenhouse growers commonly use pesticides to suppress the development of diseases. The suppressed diseases may become prominent in an interiorscape where fungicide applications are not routine.
Interior landscapers are responsible for maintaining plants under favorable conditions before, during, and after installation to avoid disease development. Prevention is always the preferred method of disease management. Otherwise, once diseases develop, more costly or hazardous methods must be used. Attention to maintenance practices reduces the opportunity for pathogens to become established. Best management practices include:
The use of fungicides is not always possible in an interior landscape setting due to the need for repeated treatments and the limited number of fungicides registered for use in interior landscapes. The only fungicides currently labeled for use in the interior landscape are those that are applied to the soil, typically as a soil drench. If a fungicide is labeled for use on the plant(s) and for the pathogen you need to treat, but is not labeled for use in an interior landscape setting, you may temporarily relocate the plant to a location that is on the label (such as outdoors or in a greenhouse) and then make the treatment. By removing a plant from its interior landscape, the options for fungicide applications are expanded, although temporarily relocating plants may not always be practical. Prevention is the most practical and effective way to manage disease.
Most pathogenic diseases of foliage plants are caused by fungi, bacteria and viruses. Observing symptoms and signs provides clues to the type of pathogen causing a disease but may not indicate the specific agent. Accurately diagnosing the specific pathogen causing the problem is critical for selecting appropriate corrective measures.
Any fungicide will only be effective in controlling those specific pathogens listed on its label. Knowing the exact pathogen causing your problem is important. Accurate diagnosis of plant pathogens may require laboratory analysis. A fresh, representative sample produces the most accurate diagnosis. It is best to collect and submit samples early in the week so that the sample does not arrive during a weekend or holiday when there will be time for it to deteriorate. Large samples will generally enable a quick, correct diagnoses. Send a whole plant if feasible. Otherwise, collect leaves, stems, and roots from plants showing the symptoms. Include the full range of symptoms. Remember that leaf symptoms may be the result of a stem or root problem. Also include a small sample of soil from the roots. Diagnostic assistance can be obtained from your local Cooperative Extension Service.
Disease symptoms on foliage or stems are the most likely to be noticed. A quick response can stop the spread of a pathogen on the host and to other plants. If plant damage is not severe and has been checked by your management procedure the plant may still be attractive enough to remain on site. Root diseases are less likely to be noticed until damage is severe. When an infection becomes severe, the best alternative is removal of the plant and infested growing media.
Fungi are the most prevalent plant pathogen of indoor ornamental plants. They cause a number of leaf spot, stem, and root rot diseases. Most foliage diseases caused by fungi develop circular, dead spots or lesions on leaves, stems and flowers. The gray-to- brown center is dry and papery, with a darker edge. As surrounding tissue is invaded, lesions may exhibit concentric rings which gives the affected tissue a "target-like" appearance with a faded green halo forming the outermost ring. These circular lesions can overlap, forming larger lesions, producing a blotchy appearance. Also, look for black, pinpoint-like pustules within the lesion. These black specks are the reproductive structures of the fungus which develop on or in diseased tissue. (Not all fungi produce these structures.) Inside these structures are millions of spores that are easily carried by air currents, water or soil movement, and human activity. After reaching a host plant, spores can remain inactive for long periods until conditions are favorable for growth and infection of plants. Temperatures from 60 to 80°F and high moisture levels encourage fungus growth.
There are many different fungal pathogens found on foliage plants. They produce different sizes, shapes, and colors of lesions. Refer to Table 1 for an overview of common fungal leaf-spot diseases of foliage plants.
Although plant diseases caused by bacteria are less common than fungal diseases, bacterial pathogens can cause some of the most economically damaging diseases. There are no chemical controls available for managing bacterial infections in indoor landscapes. Prevention is directed at reducing the number of bacterial sources and avoiding conditions that are favorable for growth of bacteria. Cultural control and sanitation are important aspects of prevention.
Moist conditions and high humidity from splashing or misting contribute to the spread of bacterial disease. Bacteria are single-celled, microscopic organisms. There are two types of bacterial plant diseases:
Systemic infections which occur in the plant's vascular tissues
(water- and nutrient-conducting tissue), and
Localized infections which appear as leaf and/or stem lesions.
Systemic infections are limited to the vascular tissues of stems, crowns, and sometimes roots. They cause wilting and general yellowing of plants. Occasionally systemic bacterial diseases may cause rotting or cankering of the stem tissue. These cankers or rots will be soft and mushy in appearance and may have an unpleasant odor. Early stages are almost impossible to diagnose. Later stages are easier to diagnose, however, the disease is so extensive by this point that treatment is impossible. If bacterial diseases are diagnosed, the best action to take is to remove the infected plant from the site and destroy it.
Localized bacterial disease symptoms can be in the form of leaf spots and/or stem rots. Bacterial leaf spots may be distinctly different from fungal leaf spots. They are dark green with a greasy, water-soaked appearance when viewed from the underside of the leaf. Eventually these areas turn tan, dark brown, or black depending on the plant species and the bacterium involved. A distinct yellow band (referred to as a halo) often surrounds the periphery of infected tissues. The yellow region can also be described as chlorotic, meaning the tissue becomes yellow because the chlorophyll has been destroyed. A small slice of the lesion placed in a drop of water may emit a stream of bacteria from the tissue. The water will appear cloudy. Early detection will allow you to prune out infected plant parts and possibly save the aesthetic value of the plant. Clean hands with soap and water and disinfect pruning tools in 70 percent alcohol after such removal actions.
The spots or lesions of localized infections can enlarge rapidly and consume the entire leaf within a short time. Such fast-moving infections often spread into leaf petioles and stems. Plant tissue becomes soft and mushy, often with a foul odor. In advanced stages, brown ooze is found in veins throughout the plant. Control of these diseases generally involves prompt removal of infested plant parts. Refer to Table 2 for examples of bacterial leaf spot and stem rots.
Viruses are much smaller than bacteria. They enter the cells of a plant and are multiplied by the host. Viruses live and multiply only within living cells. Viral diseases are usually introduced into interior landscapes by the use of infected plants or by insects. They can be spread to healthy plants by the feeding activity of sucking insects such as aphids and leafhoppers, or on the hands and tools of maintenance workers. Currently there is no chemical control for a virus once it infects a plant. Although most viruses are specific to only a few types of plants, prompt and complete removal to prevent its potential spread is recommended when a virus is discovered.
Symptoms of viral diseases are very diverse. Some viral diseases have symptoms similar to fungal diseases. Symptoms of viral infection include:
Refer to Table 3 for examples of viruses on indoor plants.
Diagnosing a viral disease should not be based only on symptoms. If you suspect a virus, isolate the diseased plant and obtain assistance from your local Cooperative Extension office.
Root rots are the most common disease problem with indoor ornamental plants. Most root rot diseases are caused by fungi. See Table 4 for examples of fungal root rots.
Infectious root rots can be diagnosed to some extent by direct observation of the root system. Healthy roots of herbaceous plants should appear white and firm. Offcolor or brownish- to-blackish, limp roots often indicate that root rot is present. Being able to pull off outer root tissue with your fingers (leaving the string-like center of the root behind) is a good sign that root rot is present. In order to determine the health of a root system, you should know what a healthy root system looks like. Conditions of the above-ground portions of the plant that indicate a root problem include smaller and less vigorous growth, new leaves that may be smaller and fewer than normal, and old leaves that turn yellow and fall beginning at the base of the stem. Also, the plant may droop and new shoots may emerge from the lower stem.
Two of the most common root rot fungi involved in root rot disease are the water molds, Pythium spp. and Phytophthora spp. These organisms are called water molds because of a spore stage that is adapted to spread by swimming in water. These organisms, even if present, are not a problem unless plants have been subjected to poor environmental conditions or cultural practices. High moisture levels in potting soil due to overwatering, poorly drained media, or water standing in the bottom of the containers induce infection of roots by these fungi. Root damage caused by overfertilization, build up of salts in the soil, chilling or freezing temperatures, or phytotoxicity caused by improper soil drenches can also lead to root rots.
Drainage of the media and establishing proper irrigation, fertilization and good sanitation practices help avoid and/or correct root rot diseases in general. Be aware of the changes in the physical structure of the rooting media. As media ages it may settle and pack in the bottom of containers and hinder drainage.
Rhizoctonia solani is a soil-borne organism which is especially damaging to roots and lower stems. Under high humidity and cool weather conditions this organism, as well as the water molds, can cause serious aerial blights (diseases of above-ground plant parts). Only occasionally is this a problem for indoor plants. R. solani has a wide host range and needs only minimal environmental requirements for disease development. Therefore, this organism poses a serious threat where poor horticultural practices exist or when this organism has contaminated the potting media.
Other pathogenic organisms which can be equally detrimental under the right conditions are Fusarium spp., Cylindrocladium spp., Sclerotinia spp., and Thielaviopsis basicola. These organisms are soil borne and persist in soil or artificial media for quite a while. Therefore, the best management of these pathogens, as all the others, is prevention: Use only disease-free plants, avoid stressed plants or those with discolored roots or stems, monitor watering practices closely, and practice good sanitation measures.
Several types of tiny roundworms cause plant diseases on plants used indoors. Lesion nematodes (Pratylenchus spp.) and spiral nematodes (Helicotylenchus spp.) cause plant stunting and poor growth because their feeding weakens the root system. The root knot nematode (Meloidogyne spp.) causes nodules to form on roots, thus impairing root function. This causes stunting. The foliar or spring-crimp nematode (Aphelenchoides spp.) lives within the leaf tissues of many indoor plants. It causes death of the leaf tissue, resulting in brown lesions on older leaves. Nematode diseases tend to be rare on indoor plants.
Good sanitation is the primary means of controlling these pathogens. Soil sterilization prior to planting will kill adults as well as eggs of root nematodes. Fumigants are as effective as steam for this purpose. Fumigants can be used as a nematode management tool in a production setting but are not registered for use in interior landscapes.
Material adapted from Michigan State University Extension Bulletin E-2308 Interiorscape Pest Management. A Training Manual for Commercial Pesticide Applicators. Julie Stahecki, Editor.
Table 1. Examples of Common Fungal Leaf Spots
Colletotrichum spp., Glomerella spp.
|Dracaena spp., Ficus
spp., Brassaia sp., Philodendron spp., Dieffenbachia spp., Sansevieria spp., ivy, palms, and others.
|Lesions form on leaf or stem, limited in size. Spots often have a raised edge.|
|Dieffenbachia spp.||Young leaves develop small red spots with dark edges. Spots may merge before leaf yellows and falls. Fungus enters insect-feeding wounds.|
|Cercospora spp.||Brassaia actinophylla, Cordyline spp., Ficus spp., Peperomia spp., palms, and others.||Pinpoint lesion on underside of leaves that resemble edema. Some hosts have many small red-to-brown lesions with pale edges. Affected leaves yellow and fall.|
|Fusarium Leaf Spot||Fusarium moniliforme||Dracaena spp., Pleomele spp.||Attacks growing point, which may be totally rotted away. Yellow-to-red rounded or oval leaf spots may appear. Cream-colored spores may appear.|
|Gray Mold or Botrytis Blight||Botrytis cinerea||Wide range of hosts, including many flowering plants.||Quick rot of flowers, buds, stems, and leaves. Gray-colored hyphae and spores easily seen in severe cases.|
|Phyllosticta Leaf Spot||Phyllosticta dracaenae||Dracaena spp.,
|Irregular spots with brown centers and yellow halos, merge to destroy large areas of leaf.|
|Powdery Mildew||Erysiphe cichoracearum
and other fungi.
African violets, and many others.
|Usually affects leaves, but all above-ground parts can be affected. Gray-white spots or patches. Leaves may discolor before drying out, turning brown and falling. Infected buds fail.|
Table 2. Examples of Bacterial Leaf Spots and Stem Rots
Philodendron spp.,and others.
|Spots from pinpoint size up to 3/8"develop and merge to cover large areas, except for midrib of leaf. Spots round to oval, yellow or yellow orange with a dull green center.|
|Agleonema spp., Dieffenbachia spp., Philodendron
Syngonium spp., and others.
|Symptoms range from distinct spots on blighted leaves to mushy, foul-smelling rot of main stem. New leaves can yellow and wilt. Also rapid, mushy leaf collapse.|
Table 3. Examples of Viruses of Indoor Plants.
|Cactus Virus X
Cucumber Mosaic Virus
Dasheen Mosaic Virus
Fig Mosaic Virus
Tobacco Mosaic Virus
Tradescantia Mosiac Virus
Aglaonema spp., Caladium spp.,
Dieffenbachia spp., Philodendron spp.
Rhoeo spp. , Columnea spp.
Table 4. Examples of Fungal Root Rots
|Wet Rots||Pythium spp.
|Diffenbachia spp. Maranta spp.,
Philodendron spp., Brassaia spp.,
and many others.
|Begin at root tips, move toward stem. Roots turn brown to black, outer layer is mushy and may slip off, leaving center of root intact.|
|Dry Rots||Rhizoctonia solani
and many others.
|Source of damping off in seedlings. Older plants develop dry, reddish-brown lesions at the soil line. Leaves can be affected if they are near soil level.|
Table 5. Susceptibility of Various Plants Used Indoors To Various Diseases*
|Host Plants||Systemic Bacterial Infection||Bacterial Leaf Spots||Nematode Diseases||Virus Diseases||Powdery Mildew||Water Mold
and Crown Rots
|Root and Stem Rots||Fungal Leaf Spots And Blights|
|Chamaedorea spp. (see Palms)|
|Chrysalidocarpus spp. (see Palms)|
|Dizygotheca spp. (see Aralias)|
|Fatshedera spp. (see Aralias)|
|Fatsia spp. (see Aralias)|
|Howea spp. (see Palms)|
|Nephrolepis spp. (see Ferns)|
|Phoenix spp. (see Palms)|
|Polyscias spp. (see Aralias)|
|Raphis spp. (see Palms)|
* Disease occurences are noted from the articles Nematode Pests of Tropical Foliage Plants and Leatherleaf Fern by A. R. Chase, D. T. Koplan, and L. S. Osborne (Foliage Digest July 1983, pp. 3-6) and Guide to Diseases of Foliage Plants 1983 by A. R. Chase (Foliage Digest, June 1983, pp. 7-9), and as listed in Index of Plant Diseases in Florida, Bulletin 11, Div. of Pit. Ind., Fl. Dept. of Agr. and Consumer Services.
** Includes Erwinia carotovora.
*** Includes anthracnose diseases.
This is not intended to be a comprehensive table