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Chapter 8: diseases.


bacteria bactericide dew point fungal pathogen fungicide genetically modified organism necrotic pathogen pesticide plant pathologist sign symptom vector virus

Plants are attacked by numerous organisms from the tiniest viruses to mammals such as rabbits, raccoons, and deer. Plants have developed ways to avoid or defend against such attacks, and pests and disease organisms have developed ways to overcome the plants' defenses. And so the battle continues, with humans intervening on behalf of plants with defenses of their own, such as pesticides, which are chemical controls developed by humans to eliminate or reduce plant disease.

The plant pathologist studies and diagnoses plant disease problems. To identify the problem when a plant fails to thrive, the plant pathologist works somewhat like a detective, looking for signs and symptoms. Signs are actual visible evidence of a disease pathogen, such as spores, hyphae, or mycelium. Symptoms are evidence of infection, such as brown spots in turf, yellow or brown leaves, and so on. In determining the exact nature of the problem, a process of elimination may be used. Photographs of signs and symptoms of plant diseases are available on the Internet. The commercial grower is well advised to obtain a photographic manual with common diseases of the crops they grow (Figs. 8-1 through 8-4).


Several environmental conditions must be present for a disease to occur. Necessary conditions include the presence of the pathogen causing the disease, a favorable environment for the pathogen, and a host plant that is susceptible to the pathogen. Some pathogens require access to plant tissue such as through a wound. Others obtain entry into plants by insect vectors or can simply enter through stomates. Still others are able to attack tissue directly and soften it or are otherwise able to break through epidermal tissue.


Organisms that cause plant disease are called pathogens. The major plant pathogens fall into one of three biological categories: viruses, bacteria, or fungi. Viruses are often spread by insect vectors that transmit viruses while they feed on plant tissue. Although there are no specific chemical controls for viruses, important viral pathogens may be managed by preventing or reducing populations of undesirable insects. Bactericides and fungicides are available for a large number of pathogens. In addition to chemicals, there may be alternative methods of control for specific pathogens. These will be discussed in the sections to follow.




Viruses are smaller than plant cells and consist of nothing more than DNA and a protein coat. They may be transmitted to plants by insects, or they may enter through wounds. For example, white flies, aphids, and thrips pick up viruses from an infected plant, and then, while inserting their mouthparts into a noninfected plant to feed, they transmit the virus. Each time they feed on a new plant they infect it.

Some viruses are borne on the surfaces of seeds or seed coats. Others may be introduced by human hands. For example, cigarette smokers may carry the tobacco mosaic virus (TMV) that they picked up from smoking a cigarette on their fingers. While working with tomatoes, they can easily transmit the virus to the plant or seeds, as tomatoes are susceptible to TMV. Pruning tools are a common means of virus contamination. If you use a pruning tool to remove infected stems of a pear with fireblight, for example, you should disinfect the tool with bleach or rubbing alcohol before making another cut. Propagation tools are another common means of virus transmission from one plant to another. Some propagators disinfect their cutting tools with rubbing alcohol between each new cut they make.

The symptoms of virus infection of a plant are mosaic patterns on leaves or mottling or yellowing of leaves, a deformed, puckered, or warty appearance of the leaves, necrotic spots of dead tissue, stunted growth, and, ultimately, death of the plant. Plants are not always killed by viruses, however, and in this way viruses that can be carried in or on seed tissue are ensured passage on to the next generation of plants. A number of viruses are notorious for infecting plants. Common viral diseases include apple, cauliflower, and cucumber mosaic virus, tomato spotted wilt virus, peony ring spot, and impatiens necrotic spot virus.



Some plants are more susceptible to viruses than others. As mentioned earlier, there are no chemical sprays that will eradicate viruses. Heat treatment and meristem propagation are laboratory techniques that are used during the commercial production of some plants to produce virus-free tissue.

Recommendations for avoiding viruses are as follows:

1. Keep pests that act as vectors at a minimum.

2. Minimize damage to plants during cultivation.

3. Be aware of ways that viruses can be spread by human handling and avoid these.

4. Purchase virus-tested and virus-free plant material.

5. Maintain sanitary growing conditions.


Bacteria are single-celled organisms. They reproduce through cell division. There are approximately 170 bacterial pathogens that infect horticultural plants. See Table 8-1 for common diseases caused by bacterial organisms. The common name of the disease is usually descriptive of the symptoms, such as rot, black spot, or leaf spot. However, there may be several different organisms that actually cause the disease. The organism may vary by plant species or geographical location, or different organisms may coexist and contribute to similar diseases.

Bacteria must enter through wounds or natural openings such as stomata in leaves because they cannot penetrate directly into plant tissue on their own. Notorious bacterial problems include fireblight that affects apples, crabapples, pears, and other members of the rose family and bacterial leaf spots that affect many plant species. Agrobacterium tumefaciens is known for causing galls on woody plants. This feature is exploited in current biotechnological research, where it is used as a method of introducing genes into plant tissue, resulting in genetically modified organisms (GMOs). Pseudomonas, Phytophthora, Cytospora, Xanthomonas, and Erwinia are other bacteria that infect plants. Bacteria cause symptoms such as blight, black spots on leaves, leaves that turn black and die, rapid yellowing and mushiness of leaf tissue, wilting, and death of the plant.

Conditions that favor bacterial growth on plants include the following:

* High humidity

* Crowding and poor air circulation around plants

* Moisture on leaves

* Too much, too little, or irregular watering

* Low light intensity

* Fluctuating temperatures

* Poor soil drainage

* Wrong size container

* Nutrient deficiency or excess

Bacterial pathogens may be treated with streptomycin that is available at nurseries and garden centers, but prevention is the preferred method of control. Do not allow leaves to remain wet for long periods of time and avoid overwatering. Water and fertilize as recommended. Use well-draining soil to prevent plant root systems from remaining wet for too long. Healthy plants are less susceptible to disease than weakened ones, so grow the plants at optimal temperatures, lighting, and levels of fertilization. Avoid droughty conditions and cultural damage and monitor for pests.

Bacterial infections can travel through the conductive tissue in plants. Therefore, it may be necessary to entirely remove affected stems or branches to prevent further spread of the disease. Such infected tissue must be disposed of carefully to avoid spreading the infection to other plants. In some cases, eradication of the entire plant may be necessary and is preferred over the risk of spreading the disease. Culture indexing is used in meristem propagation to ensure that bacterium-infested plants are not being mass-propagated.


Fungal pathogens are perhaps the most common form of pathogen to affect plants (Fig. 8-5). Fungi spread by spores, their reproductive organs, or by vegetative organs called hyphae. Hyphae are long, threadlike structures that are sometimes visible to the naked eye. Hyphae form into masses of mycelium. Fruiting bodies bear the sexually reproduced spores. These are most evident in the form of mushrooms but also exist in less obvious structures. Water, wind, and infected tools can spread spores. Soils are sometimes infested with fungus that is difficult to eradicate. Fungi enter a plant through wounds, stomates, or tissue softened by the fungus itself.

There are numerous fungal pathogens that infect plants. Some of the better known are powdery mildew, corn smut, various rusts, and leaf, root, and stem rots (Figs. 8-6 and 8-7). See Table 8-2 for common plant diseases caused by fungal pathogens.





Hundreds of plant diseases are cause by phytoplasmas. These microorganisms have no rigid cell wall nor a defined shape, and they can only be seen using an electron microscope. Well-known diseases caused by phytoplasmas include aster yellows and grapevine yellows, transmitted by phloem-sucking insects, and bushy plants in soybeans and alfalfa. A phytoplasma has been demonstrated to be the cause of the free-branching trait that is present in some poinsettia cultivars. A stem deformity known as witches' brooms can be caused by phytoplasmas.


Mycoplasmas are minute parasitic organisms that lack a cell wall. They live in the phloem of plants. Diseases, such as yellows of many plants were once thought to be caused by viruses but are now known to be caused by mycoplasmas. They are spread by insect vectors. For example, the glassy-winged sharpshooter spreads Pierce's disease of grape and leafhoppers spread X-disease of peach.


Many plant pathogens, particularly bacteria and fungi, survive and spread in humid, moist, or wet conditions. When humidity is high, disease is more prevalent. After rain, bacterial and fungal infections on developing fruit are greater. When the dew point is high (dew forms at a high temperature), turfgrass tends to stay wet for long periods, especially at night and through the morning. During such times, turfgrass diseases are difficult to combat. Golf course managers typically will spray fungicides daily to weekly, if they have the budget for it, to minimize such diseases. Regular irrigation during periods of high dew points should be avoided as much as possible. Irrigation during the night is a common practice on golf courses, but problems with fungi are only alleviated if the turf actually dries out long enough to prevent fungal spores present on the leaf surface from germinating and penetrating leaf tissue. Morning irrigation would allow the grass to dry out later in the day and remain dry through the night. However, this is usually not feasible because it interferes with regularly scheduled activities at the golf course.

Temperature plays a role in the success of plant disease pathogens. In general, the warmer temperatures are, the more rapidly pathogens grow. However, some pathogens grow better at cooler temperatures. Downy mildew is favored by cool temperatures and shady conditions in the landscape. Gray snow mold can grow at temperatures slightly below freezing and stop growing at temperatures above 45[degrees]F. Pink snow mold grows between temperatures of 32[degrees]F and 60[degrees]F.

Many soils are infected with root rot fungi. Soils that remain wet for too long are more problematic. One solution is to construct raised beds to improve drainage. Commercial growers may choose to disinfect the soil using fumigation. Home gardeners may choose to use soil solarization (see chapter 10) to kill pathogens present in the soil.


Sometimes a plant has symptoms of a problem, yet no pathogen is present. For example, overwatering of houseplants is the number one cause of plant death. In a pot that lacks drainage holes, the plant sits in water, which cannot drain out. This results in a lack of oxygen at the root surface. Leaves turn yellow and wilt, eventually falling off.

Another noninfectious problem is air pollution. Ozone and acid rain are the primary air pollutants that affect plants. Ozone results indirectly from motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents. Clover, a particularly sensitive species, exhibits ozone damage in yellow-green mottled leaf spots. Rain becomes acidified as it picks up air pollutants, primarily from electrical generation plants. The major offenders found in acid rain are oxides of sulfur and nitrate. Whereas normal rain has a pH of about 5.5 due to carbon dioxide dissolved in it, the most acidic rain in the United States has a pH of 4.3.


Problems associated with pH are most likely exhibited as nutrient deficiencies or toxicities. These are discussed in detail in chapter 6. Although there are some symptoms of general nutrient deficiency and toxicity, each species will exhibit symptoms specific to it (Fig. 8-8). Refer to a photographic manual, extension agent, or other source to identify the exact problem. Always conduct a soil test before gardening or planting in a new area, and repeat the test every 3 to 4 years to check for problems. High pH is not usually as much of a problem as low pH, which is a good thing, because low pH is more easily corrected. To raise soil pH, add lime or another calcium-containing compound. To correct for nutrient deficiencies when pH is in the acceptable range, you will need to apply fertilizer. Consult a soil laboratory or your local extension agent for guidance. Nutrient toxicity problems are very difficult, if not impossible, to remedy, although there are some plants that are able to remove certain toxic elements from the soil, and they may be able to help in soil remediation efforts.


Many disease organisms are prevalent in our environment. Even the air over the North Pole has been found to contain fungal spores! So, we should not assume that we can ever eradicate a pathogen from our environment. We should rather seek to reduce disease incidence by using a multipronged strategy. Such a strategy should include as many of the following as possible (Fig. 8-9):

* Grow healthy, nonstressed plants.

* Maintain sanitary growing conditions.

* Use pesticides when they will help reduce pathogens.

* Reduce or eliminate pests that act as vectors of diseases.

* Grow genetically tolerant or resistant crops.


Growing Healthy, Nonstressed Plants

Stressed plants may emit chemical signals that identify them as an attractive host to invading pests. For example, borers often infect trees that are already drought stressed or are growing poorly because of insufficient fertilizing. A tree that is incorrectly pruned is left without its natural defense mechanism for compartmentalizing damage and avoiding infestation by pathogens or pests. Grass that is mowed with a dull blade has a larger cut surface and is more susceptible to fungal or bacterial infection.

Growth of healthy plants requires adequate water and fertilizer and appropriate pH. But it also means using the proper cultural practices for growing and maintaining healthy plants. Growing garden plants in the correct setting in a garden or field is also important in growing healthy plants. To do this, one must understand the growing requirements of the plants with respect to soil, sun exposure, and space.

Garden Sanitation

Some practices in the garden lend themselves to transmission of plant disease, regardless of the cause. Diseased plant material must be removed promptly and disposed of in a way to minimize contact with other garden plants. Do not compost diseased plants! And do not use them for mulch. If in doubt, play it safe. If you follow the rules for pasteurization, by heating moistened media to 160[degrees]F for 30 minutes, it may be possible to eradicate pathogens from composted material. Clean shovels, pruning clippers and loppers, and other tools between uses. If you are a smoker, you may be transmitting TMV to solanaceous plants, including tomato, potato, eggplant, and peppers. You can get it on your fingers while smoking. Because it is a virus, it can persist for very long periods of time in a cigarette. It may be necessary to handle your tomato plants with gloves on, even if you have washed your hands.

Do not introduce diseased plants from other gardens. All gardeners enjoy exchanging plants with their friends, but this can be a costly practice if the plants you receive (or give) are infected with a disease pathogen. Seeds may also be infested. Seeds that are hot-water treated are less likely to have pathogens on their seed coat.


Rotate crops to minimize buildup of pathogenic organisms. Grow plants from different families in each location when you rotate them. For example, follow a solanaceous crop with a legume or cole crop, and so on. You may be able to grow one crop in the same location for 2 or 3 years, but it is best to rotate after that. Potting soil, or soilless media, should not be reused. If you must reuse soilless media, pasteurize it. This will kill most disease-causing organisms.

Use of Pesticides

Pesticides designed to treat pathogens are mainly limited to fungicides, although there are a few bactericides available. Reducing or eliminating pests that act as vectors of viruses, mycoplasmas, and phytoplasmas will offer better control of these diseases than any treatment that is available. However, heat treatment of meristems can rid some plants of these pathogens. This laboratory procedure is particularly useful for vegetatively propagated plants and is used extensively for crops such as geraniums that are widely propagated and susceptible to the devastating bacterial blight, Xanthomonas. In this manner, thousands of disease-free plants can be produced.

Fungicides are used primarily to prevent fungal infection (Fig. 8-10). They are not very effective in treating existing fungal infections and cannot repair damage that has already occurred to the plant. Vascular diseases, such as Fusarium and Verticillium wilt, cannot be adequately controlled by fungicides. There is a higher risk of a pathogen's developing resistance to a fungicide that is used after it has already infected a plant because of the specific mode of action of the fungicide. Repeat applications of fungicides with the same or similar modes of action should be avoided to minimize the risk of pathogen resistance to the fungicide.

Disease forecasting systems for timing of fungicide applications are available for many crops. These are based on environmental factors such as temperature and precipitation, humidity, or dew point or even leaf wetness. Other systems have been developed on the basis of visible symptoms in the crop. This method requires regular scouting of the crop and good record-keeping. When a predetermined threshold has been reached, fungicides are applied. These systems require adequate knowledge of the disease cycle and favorable environmental conditions required for successful infection by a pathogen. Such a system can be economical and environmentally sound when used properly.


Fungicides may be applied as liquids, granules, dust, or gas. They are applied to seeds, bulbs, seedlings, flats of plants, and leaves and roots of mature flowers and trees. They may be applied as a drench to container plants in a greenhouse or sprayed from above by crop dusters. Gas-applied fungicides are called fumigants, and they may be applied inside a greenhouse or injected into the ground to control soil-borne pathogens. Even harvested produce may be treated with fungicide. This treatment is usually accomplished by dipping the produce into a liquid solution. Tree trunks are sometimes injected with fungicide. Repeat applications are usually required during the growing season. The need for repeat applications is determined by several factors, such as moisture levels and temperature, as well as the prevalence of a given pathogen during the growing season. Certain diseases are worse in some years than in others.

Pesticide Handling

Fungicides and all pesticides, for that matter, should be handled and treated as poisons. Several agencies are involved in the registration, use, handling, and disposal of pesticides. These include the Environmental Protection Agency (EPA), U.S. Department of Agriculture (USDA), and other national and state agencies. Pesticide handling procedures and licensing for professional applicators have been developed and are administered by each state. Although restricted use pesticides cannot be purchased without a license, home gardeners do not generally require special licensing for application of general use pesticides on their own property. Nevertheless, everyone should follow labeled uses and instructions. Pesticide labels are legal documents that indicate the proper clothing and protective gear to use as well as safe handling procedures and proper disposal. As with all poisons, keep them out of the reach of children and pets.

Biological Controls

Biological controls, also known as biopesticides or biofungicides, have been identified as a natural approach to pest control. Naturally occurring antibiotics are sometimes found in high enough levels in the native soil that they effectively reduce the level of pathogen or the extent of damage caused by such pathogens. Other naturally occurring antibiotics have been isolated and identified. An example is Trichoderma harzianum, a fungus that is available commercially as a seed treatment product. It is a mycoparasite of several damping-off pathogens including Pythium, Rhizoctonia, and Fusarium. Another example is the biocontrol agent Trichoderma hamatum T382 that suppresses Botrytis cinerea.


In many ways genetic control methods are superior to other methods because they reduce or eliminate the need for chemicals. There are two types of genetic resistance, traditional breeding methods and genetic modification. Traditional breeding requires identification of resistant plants or closely related species from which resistance genes can be introduced. Genetic modification requires the identification of one or more resistance genes and the ability to isolate and reproduce that gene and introduce it into the plant of interest using biotechnological methods. In the latter case, genes from the offending pathogen may be useful in controlling the pathogen. For example, insertion of a gene from a disarmed virus may serve as a sort of vaccine against the virus. Animals and insects may also provide the genetic material for antipathogen activity. For example, genes that code for enzymes that break down fungal cell walls have been isolated from animals, amphibians, and insects and are being tested in some plants for fungicidal activity.


Pest control and crop management have undergone changes in many areas in the past 50 years. The changes have been dramatic, sophisticated, and wide-ranging. During that time, we have made great strides in areas of insect physiology and life cycle, effectiveness of a variety of biological controls, and genetic basis for pest infection and for plant resistance to attack. The results of research in these areas has led to the development of more sophisticated chemicals, less toxic chemicals, improved timing of chemical application, alternating the use of chemicals with different modes of action, and breeding for genetic resistance within plants. Genetically modified crops are the direct outcome of research in the latter arena. Scientists continue to seek to isolate genes that confer resistance to disease, with the intent to replicate them and insert them into species of plants that lack such resistance genes.

The increased sophistication of pest control and crop health management has been identified by a number of different monikers in an attempt to express a more accurate attitude toward plant health. Integrated pest management (IPM) is a widely used term that captures this intention. Other terms include integrated crop management and total crop management. Part of the reason for this name change is to imply that rather than controlling pests, one should aim to manage them using comprehensive methods and understanding. To take that thinking a step further is the idea of managing the comprehensive health of a crop, not just killing pests or eradicating diseases as they appear. The concept is congruent with the concept of ecology of the natural world. There is a certain ecology that exists in the production of crop species. The soil environment, daily temperature fluctuations, geographical location, and history of plants in that location all interact to determine the pests and diseases that will be present in a crop production environment. Because plants are more susceptible to pests or disease when they are weak or are grown under suboptimal conditions, improving those conditions can minimize risk. Good horticultural practices (a "good offense") are the best defense against diseases and pests:

* Fertilize as recommended, provide organic matter as much as possible to build soil structure, provide slow-release nutrients, and encourage earthworm populations that aid in aeration of the soil.

* Trap and monitor pests.

* Rotate crops.

* Plant in the right location, use proper spacing, and keep weeds under control.

* Avoid overly wet conditions, provide good drainage, and use raised beds where necessary.

* Keep plants adequately watered.

* Provide adequate light/avoid too much light.

* Grow at optimal temperatures.

* Monitor environmental conditions to time pesticide applications.

* Use sanitary practices in field, nursery, garden, and greenhouse.

* Take advantage of biological controls; often these occur naturally.

* Practice safe handling, storage, and disposal of pesticides.


Plants are attacked by pathogens that, in turn, cause disease. Signs of disease are those visible entities that are part of the pathogenic organism. Symptoms of a disease are exhibited by plants, indicating that there is a problem. Fungus, bacteria, and viruses are the major pathogens that cause diseases in plants. Environmental conditions that favor infestations vary, but for many pathogens moist, cool conditions are favor able. Some common symptoms of a pathogen infestation are yellow or brown spots, mosaic patterns, and stunted growth. A fuzzy appearance is a sign of fungal growth. Animals, including humans, and insects can transfer pathogenic organisms, and insects can act as vectors to transmit viruses directly into plants.

Plant disease can be prevented to some extent by good cultural conditions and by maintaining healthy plants. Optimal plant health is obtained by correct watering and fertilizing and avoiding an excess or deficiency of either. Optimal temperatures, lighting, and humidity and proper pH also contribute to plant health. Preventing conditions that lead to stress is also important in maintaining good plant health. When infested plants do appear, proper sanitation is beneficial.

Genetically improved plants are available for many species and for the major pathogens to which they are susceptible. Crop rotation can reduce pathogens that build up in the soil from one season to another. Controls for pathogens may be manufactured chemicals or biological controls. Integrated pest management and integrated crop management are crop management programs that incorporate several integrated strategies to prevent, reduce and, if possible, eliminate disease infestations while at the same time minimizing adverse effects on human health and the environment.


* Select a plant pathogen and learn about its life cycle and host plant(s), including conditions that favor its growth.

* Find a plant with symptoms of disease. Identify the plant and the disease. Write a short paper on where the plant was located and the prevailing conditions. Is there anything that could be changed to avoid or prevent the disease in the future?

* Select a crop plant and research the disease(s) that commonly affect it. Write a short paper describing the pathogen and symptoms, discussing ways to manage or avoid the disease, and including prevention and controls that may be implemented.

* Find a disease forecast system for a crop and write a report on how it works, conditions that favor the disease pathogen, thresholds, and timing of fungicide application. Include details such as recommended fungicide(s), application rate, crops covered, and so on.


1. What is the difference between a sign and a symptom?

2. Disease organisms that cause plant disease are called--.

3. What are the three major categories of plant pathogens?

4. Which type of pathogen that is commonly transmitted by insect vectors is nothing more than DNA and a protein coat?

5. The vegetative structure of fungus is called--or--.

6. Describe and discuss two nonpathogen causes of plant problems.

7. What is biological control and how does it differ from chemical control?

8. Name and discuss two garden sanitation activities that can prevent or minimize disease.

9. To avoid or minimize fungal infections, the best time of day to water is--.

10. IPM stands for--.


Agrios, G. N. (1978). Plant pathology. New York: Academic Press.

Greenwood, P., Halstead, A., Chase, A. R., & Gilrein, D. (2000). American Horticultural Society Pests and diseases: the complete guide to preventing, identifying and treating plant problems. New York: DK.

Mikolajski, A. (2004). How to get rid of garden pests and diseases. London: Anness.

Pirone, P. P. (1978). Diseases and pests of ornamental plants. New York: John Wiley.

Sinclair, W. A., Lyon, H. H., & Johnson, W. T. (1987). Diseases of trees and shrubs. Sacramento, CA: Comstock.

Smith, M. D. (1994). The Ortho problem solver. San Ramon, CA: Solaris Group.

Waller, J. M., Lenne, J. M., & Waller, S. J. (2002). Plant pathologist's pocketbook. New York: CABI Publishing.

Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.

Common Plant Diseases Caused by Bacterial Organisms

NAME                    ORGANISM                PLANT(S)

Bacterial               Xanthomonas             Geranium
  blight                  campestris pvr.         (Pelargonium)

Bacterial leaf          Pseudomonas             Philodendron, pothos,
  spot                    cichorii                Chinese evergreen,
                                                  and Monstera

Bacterial soft          Erwinia spp.            Potatoes, other root
  rot                                             crops, some

Black rot of            Physalospora            Apples, crabapples
  apple                   obtusa

Black rot of            Xanthomonas             Radish, mustard,
  crucifers               campestris              cauliflower

Crown gall              Agrobacterium           Hundreds of woody
                          tumefaciens             and herbaceous

Fireblight              Erwinia amylovora       Ornamentals and fruit
                                                  in the Rose family

Leaf blight of          Erwinia carotovora,     African violet, iris
  African                 Erwinia
  violet                  chrysanthemi

NAME                    CONDITION/REMEDY

Bacterial               Water-soaked leaf spots,
  blight                  stem rot, sudden wilt, and
                          collapse of tissue. Avoid
                          spreading the disease
                          organism. Remove and
                          destroy infected tissue,
                          dead leaves, flowers, etc.

Bacterial leaf          Brownish black lesions on
  spot                    leaves. Avoid wetness,
                          poor drainage, crowding,
                          poor air circulation.

Bacterial soft          Pathogen is soil borne and is
  rot                     more troublesome during
                          warm, wet weather. Can
                          affect root crops in
                          storage. Lesions on
                          leaves, roots enlarge and
                          go soft underneath as
                          tissue liquifies. Dig and
                          destroy infected plants
                          and those nearby. Rotate
                          vulnerable crops.

Black rot of            Fruit rot that usually starts at
  apple                   the calyx end, leaf drop,
                          stem cankers, and stem
                          and twig dieback. Grow
                          healthy trees and prevent
                          stress, remove mummified
                          fruit and prune out
                          diseased or dead branches.
                          If not controlled, cankers
                          will develop.

Black rot of            Wilted and pale green leaves
  crucifers               turn chlorotic and
                          become necrotic. Leaf
                          spots or circular lesions.
                          Cauliflower heads turn
                          brown. Maximize
                          drainage and avoid using
                          contaminated equipment
                          in clean crop fields.

Crown gall              Enters through freshly
                          wounded tissue, often at
                          soil line. Gall appears as a
                          rounded growth. Affected
                          trees are weakened or
                          killed. Remove entire
                          affected plant.

Fireblight              Rapid dieback of shoot tips,
                          entire stems and leaves,
                          flowers turn brown and
                          die almost overnight.
                          Remove 6-12 inches
                          below observed damage.

Leaf blight of          Water-soaked spots on
  African                 leaves. Isolate infected
  violet                  plants and remove
                          diseased foliage. Disinfect
                          tools after such use.


Common Plant Diseases Caused by Fungal Pathogens

NAME                ORGANISM                     CROP

Anthracnose         Colletotrichum spp.,         Leaves of trees:
                      Colletotrichum               ash, oak,
                      lindemuthianum,              elm, maple,
                      Colletotrichum               dogwood,
                      gloeosporioides var.         etc.
                      minor, Gloeosporium,

Black knot          Apiosporina morbosa          Plums, cherries,

Botrytis blight,    Botrytis cinerea             Many
  gray mold                                        vegetables,
  rot                                              greenhouse

Phytophthora        Phytophthora                 Potato blight,
  blight              infestans                    late blight of
                                                   plants in the

Powdery             Erysiphe, Odium,             Leaves of
  mildew              Podosphaera                  woody and
  (Fig. 8-1)          leucotricha,                 non-woody
                      Sphaerotheca fuliginea,      plants.
                      Sphaerotheca pannosa
                      var. persicae, and
                      Uncinula necator

Rust (Fig. 8-5)     Gymnosporangium spp.,        Many
                      Melamspora medusae,          ornamental
                      Melamspora larici-           plants,
                      populina, Uromyces           roses, corn,
                      spp., Puccinia spp.,         sweet pea,
                      Phragmidium                  brambles,
                      mucronatum,                  etc.

Scab                Venturia inaequalis,         Apple and
                      Pseudomonas                  crabapple
                      marginata,                   gladiolus,
                      Sphaceloma violae,           pyracantha,
                      Streptomyces spp.,           potato
                      Streptomyces scabies

Verticillium        Verticillium dahliae,        Many plant
  wilt                Verticillium                 species

Witches'            Interaction of powdery       Hackberry
  broom               mildew or other
                      fungus and an
                      Eriophyid mite

NAME                SYMPTOMS

Anthracnose         Small tan to black irregular
                      spots, usually on
                      leaves. Leaves die and
                      tree becomes
                      defoliated; shoots and
                      twigs die back.

Black knot          Abnormal growth on twigs,
                      branches, and fruit
                      spurs. Knots encircle
                      branch, become black.
                      Tree eventually declines
                      and dies. Remove
                      infected tissue and
                      destroy it.

Botrytis blight,    Gray to brown
  gray mold           discoloration, water
  rot                 spots, whitish gray to
                      tan mycelium. Cool,
                      humid conditions favor
                      this disease. Pretreat
                      seeds with fungicide.
                      Use pasteurized soil or
                      media. Keep water off
                      the foliage and water
                      early in the day.
                      Remove and destroy
                      diseased plant parts.

Phytophthora        Water-soaked lesions on
  blight              leaves turn into large
                      brown spots; eventually
                      white fungus is
                      apparent. Use resistant
                      cultivars; if cool, wet
                      conditions prevail,
                      apply fungicide.

Powdery             White to gray powdery
  mildew              area usually on leaves.
  (Fig. 8-1)          Leaves become stunted
                      and eventually drop off
                      as mildew spreads.
                      Favors cool, shady

Rust (Fig. 8-5)     Begins as small red lesions
                      on leaves. Become
                      yellow, orange, brown,
                      and black. On junipers,
                      reddish-brown galls
                      form having gelatinous
                      tendrils. Some of the
                      rusts require two hosts
                      to complete their life

Scab                Spots on underside of leaf
                      begin brown, turn
                      green, and then dark
                      brown and black. On
                      fruit, spots are circular
                      green lesions. Older
                      lesions resemble black,
                      cracked scabs. Favors
                      warm moist conditions.

Verticillium        Leaves turn wilt and turn
  wilt                brown. Other
                      symptoms are seen in
                      the wood, but resemble
                      other diseases, as well.

Witches'            Thin, twiggy stems that are
  broom               numerous at shoot tip.
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Author:Loehrlein, Marietta M.
Publication:Home Horticulture: Principles and Practices
Date:Jan 1, 2008
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