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Extension > Garden > SULIS > Maintenance > Sustainable Herbaceous Plant Maintenance > Disease and Insects

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Disease and Insects

Diseases and insects go hand-in-hand. Many times, the damage of one opens the door for the other. For example, the wounds caused by chewing insects leave openings in foliage and stems for bacteria and fungi to enter the plant. Some insects act as vectors for disease. In other words, they actually carry the live virus in or on their bodies. When they suck the plant's juices through their stylets, the virus is transferred to the plant where it thrives.

Just like people who get run down, plants that are undergoing water stress, a lack of nutrients, or are being subjected to conditions that are problematic, will be more susceptible to invasions by insects or diseases caused by fungi, bacteria or viruses.

The key to managing insects and diseases in a sustainable landscape is to maintain healthy plants that can tolerate low levels of pests and diseases without suffering significant damage. Like weed management, it is important to identify the insect or disease involved and determine if the problem is significant enough for chemical treatment. If a plant population is healthy, it can usually defend itself against significant damage. However, there may be times, even for healthy plants when some pest control is needed.

For both insects and diseases, regular scouting and monitoring of the landscape is important. The landscape manager should check plants regularly for disease symptoms such as wilting, stunting, galls and abnormal growths, foliar spots, lesions, chlorosis, etc. Insect infestations will also show symptoms such as chewing, leaf mining and signs of the pest including residues such as honeydew, webbing and feces, etc. Unfortunately, symptoms of insect damage are sometimes similar to those of disease as well as those associated with poor cultural practices such as over watering, environmental factors such as pollution, and damage by human activity such as damage to tree bark caused by mechanical means (mowers, trimmers). Sometimes humans cause initial damage through carelessness that can invite secondary invaders such as insects and diseases. The first step to solving any problem is proper monitoring and assessment of the problem and cause. Correct and timely identification will not only save the plant material, but also save time, money and the otherwise unnecessary use of pesticides.

Each disease or insect has symptoms that are associated with it, and needs a specific set of circumstances for those symptoms to occur. These circumstances involve a host, the insect or disease itself, and favorable environmental conditions for the insect or disease to develop. This relationship is illustrated by the Disease Triangle:

Disease Triangle


Diseases are defined as abnormal alterations of the internal (physiological) and/or external (morphological) development of the plant. In many cases, infectious microorganisms, called pathogens, cause disease by entering the plant by penetrating the plant epidermis, through a natural opening such as stomates or, more often, through a wound in the plant. The pathogens reproduce within the plant and cause areas of infection called "infection centers". While chemicals are a last resort for management of a pathogen, most can be managed by employing sound cultural practices, monitoring plants for early diagnosis and management, and by planting resistant species.


Fungi lack chlorophyll and thus cannot produce their own food via photosynthesis, so they must kill the host cells by secreting toxins that break down the cell contents and enable the fungus to absorb nutrients. Fungi produce spores asexually and sexually, on spore-bearing structures called hyphae. Masses of hyphae are visible to the naked eye as white or gray webbing. Spore masses can also be seen with the naked eye. Think of an orange as it rots. The dusty blue-green area is a spore mass while the white webbing around it is the hyphae. Fungi infect through spore dissemination, and they can also infect by penetrating plant surfaces with their hyphae. Once penetration has occurred, the fungus will absorb nutrients from the host cells by breaking down the cell contents.

Fungi thrive in a variety of conditions - cool or warm, damp or dry. Most favor damp, acidic conditions. Symptoms of fungi are many and include necrosis, lesions, leaf blotches, spots, cankers, water-soaking, chlorosis, wilting, rots, and others. Unfortunately, many of these symptoms are also common for bacteria, viruses, and even some abiotic (environmental) causes. Before starting a treatment, take note of the plant's environment. Good cultural practices will usually prevent fungi from invading plant materials and keep the plants from becoming stressed and thus less susceptible to fungi and other pathogens. Some of these techniques include:

  • Let the soil dry out between watering. This will prevent soil from being perpetually damp and thus encouraging the growth of fungi that attack roots;
  • Water plants at the base of the plant instead of from above. This prevents soil-borne fungal spores from splashing up onto leaves and keeps their foliage dry, preventing infection by some fungi;
  • Mulch plants to hold in moisture (not too much) and to prevent soil-borne fungi from splashing onto leaves when it rains;
  • Water in the early morning so excess moisture can evaporate. Watering in the evening will cause the excess water to remain on plant foliage, creating a positive environment for fungi and other pathogens to thrive.
  • Monitor plant materials closely, especially at times of high humidity and warm temperatures. If symptoms are noted, contact the Plant Pathology Clinic at the University of Minnesota where for a nominal fee, a plant pathologist will identify the pest and make recommendations for treatment.

If treating with chemicals is the only means of control, be sure to select a fungicide that is manufactured for the specific fungus that is being managed. Note that fungicides are preventative, not corrective, and thus must be used prior to infection. Always follow the directions on the label for safety precautions, protective clothing, temperature and conditions during applications, etc.


Like fungi, bacteria are widespread and easily transmissible between plants. They can be spread via rain, wind, plant debris, insects, or seeds, and they have an extremely high reproductive rate. While fungi can infect via natural openings, wounds or penetration, a majority of bacteria infect via a wound in the plant epidermis. They can also enter a plant via natural openings such as stomates, but the primary course of entry is via wounds. Hail storms, mechanical damage, animals and humans are a few of the ways plants are wounded, creating openings for bacteria to infect.

Symptoms of bacterial infection are similar to those of fungi - leaf spots, blight, wilting, scabbing; however, where spores are evidence of a fungus, bacterial infections may ooze or stream. Like fungi, bacteria infections can also affect the vascular system of a plant, "plugging" it up and causing bacterial wilt.

Bacterial infections require different methods of control. The best control focuses on proper cultural practices:

  • Keep plants stress-free by maintaining good plant health overall as good health is the best defense against infection;
  • Control insects - insect feeding creates wounds, allowing bacteria to enter and infect plant material;
  • Avoid injuring plants; lawnmowers, weed trimmers, vehicles, etc. can damage trees and shrubs, by creating cuts and wounds in the bark, which creates openings for bacteria to enter and infect;
  • Remove weeds from plant beds; weeds are often hosts for bacteria;
  • Remove plant debris and compost and do not use infected plant debris in a compost pile. Dispose of this in a separate compost area designated for weeds and infected material where it can decompose. It can also be burned. Do not put it in the trash.


Viruses are extremely small and they are extremely dependent on a living host for energy and for its own replication. Viruses are also immobile and depend on a host for dissemination, the most critical of these being the aphid that is a vector for the virus. Viruses enter plants via wounds made by insects, such as the wound made by an aphid as it feeds. They can also enter via infected pollen grains and by wounds made mechanically by humans or animals. The most common symptoms of viruses are stunting, localized lesions, and systemic lesions such as mosaics, mottling, abnormal leaves and leaf or stem thickening. Symptoms may be similar to herbicide damage, excessive fertilizer damage or nutrient deficiency. Because of their immobility, viruses will not appear in specific infection centers - randomly affected areas of the planting with one or more of these symptoms. Rather, if a plant here and there is displaying symptoms of abnormal leaves, chlorosis, and stunting, this is a good indication that a virus is the problem.

It is important to prevent infection when managing viruses as well as bacteria and fungi. This includes limiting vectors (e.g. insect control), and by following a good Integrated Pest Management (IPM) program. For more information on IPM, visit the Center for Urban Ecology and Sustainability.

Virus management techniques include:

  • Sanitation - the number one management technique recommended for virus management. Viruses are easily transmitted by gardening tools, human hands, equipment, etc. Cleaning tools, washing hands, sterilizing pots, and being conscious about avoiding the spreading viruses are all good cultural practices.
  • Control insects - This is especially true of sucking insects that vector viruses such as aphids, and thrips. Beneficial insects and insecticides can be used for management.
  • Eliminate / limit the inoculum source (the infected plants).
  • Chemicals will not control viruses.

Common diseases of herbaceous landscape plants


Remove infected flowers and plants. Clean up infected foliage. Rotate species if possible. Gray Mold of Bedding Plants.

Damping off

Attacks seedlings. Prevent by sanitation of tools, pots, etc. Avoid over watering. Use fresh seed-starting mix. Mix pulverized sphagnum peat moss in seed starter - it contains a natural fungicide. Damping-Off of Seedlings.

Powdery mildew

Prevent by choosing resistant varieties. Increase air circulation by spacing plants appropriately and pruning / thinning plants. Avoid overhead watering. Avoid planting sun-loving plants where they get too much shade. Remove infected plant parts and destroy. If necessary, you could begin a fungicide spray program using sulfur or chlorothalonil. (sold as Daconil 2787) at the earliest signs of infection. For additional information see Powdery Mildew of Ornamentals.


Prevent with the appropriate fungicide. Remove infected plants and destroy.


For details, see the Oedema Fact Sheet


Insects are the most abundant group of animals on earth and they affect our daily lives as human beings in many ways including their effects on plants. The evolution of insects over millions of years is nothing short of incredible and they deserve the utmost respect.

The management of insects in the sustainable landscape focuses on an integrated pest management program that employs proper cultural practices, plant selection, and monitoring insect levels. Chemicals are an important means of management, but the last resort. Insecticides may kill detrimental insects and harm beneficials as well such as bees and other pollinating insects, earthworms and other soil insects. Select a product that will have the least toxic effect on other organisms and the environment. Investigate organic, natural and biological controls as well. They may offer an alternative, safer means of pest management in the landscape and greenhouse environment, especially where people are in contact with the plant material on a regular basis.

For an in-depth look at the insect world, IPM, and managing various insects, visit the Center for Urban Ecology and Sustainability.

Common pests and diseases of herbaceous plants


Chemical treatments include horticultural oil, insecticidal soap, and neem spray. Beneficial insects include ladybugs, preying mantis, parasitic wasps and hoverflies. Graham Rice in Discovering Annuals, recommends planting buckwheat (Limnanthes douglasii) to attract hoverflies whose larvae prey on aphids.


Treatment includes Bacillus thuringensis (Bt); hand-picking; Sevin.


Treat with insecticide at dusk when they come out to feed. Earwigs

Leaf miner

Treat with a systemic insecticide; hand-pick leaves with larvae inside.

Red spider mite

Treat biologically with Phytoseiulus persimilis. Spider mites are generally worse in hot, dry summers than when the weather is cool and moist. Vigorously growing plants are better able to tolerate mite feeding, so make sure phlox are watered and fertilized adequately to insure their vigor. An insecticidal soap spray can be moderately effective, particularly if you catch the mite infestation in its early stages. If the mite problem is severe, check your local garden center for Kelthane, a miticide labeled specifically for use on ornamentals or flowering perennials. Apply any spray very early in the day when temperatures will remain cool for several hours. Spraying during the heat of the day is more likely to damage foliage.


Treat with molluscicide; hand-pick; trap. For details on slugs and slug control, visit:


Nematodes are non-segmented round worms that belong to the phylum Nematoda. There are about 625,000 species with approximately 0.5% of all nematodes being plant parasitic. They can be easily seen with a dissecting microscope. Not all nematodes are pathogenic; some are being studied as biological control for insects and damaging soil-borne organisms including pathogenic nematodes.

Pathogenic nematodes are part of a "complex" - a combination of different factors that, together, create an infection site within a plant. Some nematodes attack the roots of plants; others crawl up the stem of plants onto the leaves and enter the plant via the stomates such as foliar nematodes. Some secrete chemicals from their dorsal esophageal gland that cause cells to enlarge within the plant root and cause necrosis of the epidermal and cortical cells of the root. Damage to the root tip creates an infection site and invites secondary invaders such as bacteria and fungi to further degrade the plant tissue. Some species of nematodes create galls and tumors on plant roots or cockles on foliage that weaken the plant, making it vulnerable to other pathogens.

In a sustainable landscape, cultural practices are the best form of nematode management. There are no nematocides available today that are effective and safe to use. Most pathogenic nematodes are attracted to plants weakened by drought or nutrient deficiency or soils that are low in organic matter. Maintaining plant health or using resistant varieties of plants is the best form of management for these organisms. For more information, see Nematodes Fact Sheet from the University of Nebraska-Lincoln.

Integrated Pest Management (FUTURE)


  • Cornell University Plant Disease Diagnostic Clinic, "Oedema Fact Sheet", updated October 1999.
  • Cornell University Plant Disease Diagnostic Clinic, "Powdery Mildew Fact Sheet", revised by Diane Karasevicz, April 1996.
  • Ohio State University Extension, online publication, "Disease Control in the Landscape", bulletin 614.
  • Ohio State University Extension, "Insect and Mite Control on Woody Ornamentals and Herbaceous Perennials", Ohioline bulletin 504.
  • Purdue University Cooperative Extension, "Winter Injury of Ornamentals", BP-2-W, Paul C. Pecknold Reviewed 5/01.
  • Purdue University Cooperative Extension, "Powdery Mildew of Ornamentals", BP-5-W, Paul C. Pecknold. Revised May 2001.
  • Purdue University Cooperative Extension, "Pesticides and Personal Safety", PPP-20, Fred Whitford, C. Richard Edwards, Jonathan J. Neal, Andrew G. Martin, John Osmun, and Robert Hollingsworth. May 2001.
  • Purdue University Cooperative Extension, "Pesticides and the Label", PPP-24, Fred Whitford, Daniel T. Barber, David Scott, C. Richard Edwards, John Caravetta.
  • Purdue University Cooperative Extension , "Pesticides and the Home Landscape", PPP-29, Fred Whitford,, Robert M. Corrigan, Gail Ruhl, B. Rosie Lerner, Zachary Reicher, and Timothy J. Gibb. April 2001.
  • Purdue University Cooperative Extension, "Pesticides and Wildlife", PPP-30, Fred Whitford, Brian Miller, Richard Bennett, Margaret Jones, and Larry Bledsoe.
  • Purdue University Cooperative Extension , "Pesticides and Pest Prevention for the Home, Lawn and Garden", PPP-34, Fred Whitford, Gail Ruhl, Allen Boger, Andrew Martin, B. Rosie Lerner, Zachary Reicher, Robert Corrigan, and Cliff Sadof. May 2001.
  • University of California, online information, UC Pest Management Guidelines. 2002.
  • University of Minnesota Extension, "Damping-Off of Seedlings", FS-1167-GO, F. L. Pfleger and S. L. Gould. 1998.
  • University of Minnesota Extension Service, "Earwigs", Yard & Garden Newsletter Vol. 1, No. 6, Jeff Hahn, July 15, 1999.
  • University of Minnesota Extension Service, "Gray Mold of Bedding Plants", Yard & Garden Brief, P147G, Janna Beckerman, July 2001.
  • University of Minnesota Extension Service, "Piercing / Sucking Feeders", FO-06953. 1997.
  • University of Minnesota Extension, "Slugs in the Garden", publication #446, Jeffrey Hahn and Doug Foulk. 2000.
  • University of Minnesota, "Center of Urban Ecology and Sustainability (CUES)", Dr. Vera Krischik.
  • University of Minnesota, "Controlling Slugs: Cultural, Mechanical, Biological and Chemical Techniques", online presentation, Julie E. Weisenhorn. May 2001.
  • University of Nebraska - Lincoln, Department of Nematology
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