Part I: Biotic and Abiotic Plant Problems

DISEASE MANAGEMENT

Part I: Biotic and Abiotic Plant Problems

Cynthia L. Ash and Bert T. Swanson

Insects, animals and infectious organisms are called biotic agents because they are living organisms. Noninfectious factors are called abiotic because they are non-living agents. Both agents can cause serious visible plant damage. Healthy plants are less susceptible to most insects and diseases and less severely damaged by biotic or abiotic problems than are plants under stress.

Plant Health Care (PHC) is a way of managing plants and their problems by focusing efforts on maintaining the health of the plant. PHC maintains that if the plant is kept healthy it will thrive simply by minimizing stress factors such as not planting in the proper location, not providing the right amount of sunlight, moisture and nutrients in the correct soil type, plus poor maintenance.

Integrated Pest Management (IPM) provides a variety of treatment strategies to combat the presence of insects and diseases, but may not be the only focus of a plant or landscape management program. PHC is a proactive approach which attempts to prevent, remove or limit situations favoring pest problems. This is accomplished through the use of environmentally sensitive treatment strategies designed to keep disease and insect populations below harmful levels.

Diseases vary in severity from year to year and from one locality or geographical area to another depending on environmental factors such as the amount and frequency of rain or heavy dews, relative humidity, air and soil temperature, and plant nutrition, plus the relative resistance or susceptibility of the host plant, and the virility of the causal or disease-producing organism. All factors must be present and "in balance" for a disease to develop. This can be formulated into as a simple equation:

Susceptible Plant + Disease-Producing Organism

+

+

= Disease

Proper Environment + Method of Distribution

Categories of Plant Diseases Biotic Plant Diseases ? A Biotic plant disease is the product of the plant, the pathogen, and the environment interacting over time (Figure 1). It is important to recognize that all three factors interact to produce diseases and that diseases can be prevented or controlled by manipulating any one of the three components. It is also important to recognize the time factor. Some diseases develop quickly within a plant, while others develop slowly. There is also a time factor related to the spread and increase or method of distribution of the pathogen population within the host population. Weather is a major contributor to this equation as it directly affects the biotic agent. It is also important to recognize and understand any plant to insect interactions that may exist.

Abiotic Plant Diseases ? Diseases can also be caused by abiotic agents such as high or low temperature, phytotoxic gases, nutritional imbalance, soil-oxygen deficiency, moisture stress, and other abiotic factors as listed in Table 1.

Decline Plant Diseases ? Major emphasis in plant pathology has been directed toward single biotic or abiotic primary-causal-agent diseases. There is a third category of diseases, called declines, which result not from a single causal agent, but from an interacting set of factors as shown in Figure 2. Terms that denote the symptom syndrome such as dieback and blight, are commonly used to identify these diseases. One must be cautious, though, because these are not terms used exclusively for declines.

Disease Recognition It is necessary to study the diseases of individual plants, and the diseases of plants in a population to recognize differences among the three categories of diseases. The biotic, abiotic, and decline diseases can be compared using four criteria: symptoms, signs, host specificity of disease, and spatial distribution.

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Figure 1. A Biotic plant disease is the product of three interacting factors over time.

Table 1. Common Abiotic factors causing plant problems.

Lawn mower Injury Frost or Freezing Injury Drought Nutrient Excess Improper Site Improper Soil Type Cultivating to Close Weed Whip Injury Shipping Damage

Construction Damage Hail Damage Excessive Soil Moisture Wind Damage Improper Maintenance Improper Pruning Improper Fertilization Improper Soil pH Sun Scald

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Soil Oxygen Deficiency Air Pollution/Phytotoxic Gases Nutrient Deficiency High or Low Temperatures Root Girdling and Constriction Planting Equipment Injury Harvest Equipment Injury Improper Pesticide Application Improper Plant Spacing

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Figure 2. Factors influencing the Decline category of plant diseases.

Symptoms ? Biotic agents of plant diseases produce symptoms on specific plant parts. The affected parts are usually not randomly distributed on the plant as only a portion of a plant may be affected. Environmental or inoculum dispersal factors may account for the unevenness of disease symptoms. A progressive invasion of tissues is also a good symptom of a biotic-induced disease. However, a range of symptom expression may occur with biotic diseases as a result of genetic variation and the spread patterns of the pathogen.

Abiotic diseases may or may not be plant-part-specific, but they are usually uniform in symptom expression on the plant. Exceptions to a general distribution throughout the plant could be caused by portions of the plant not being exposed. For example, the lower branches of a tree may be covered with snow and therefore not exposed to salt spray or the desiccating effects of winter winds. An abiotic disease does not generally occur as a progressive invasion like a biotic infection, so that evidence of callous ridges on a stem infection, or necrosis of leaves surrounded by chlorotic or other colored tissue, is usually not seen. The most characteristic symptom of a disease is the progression of symptom expression on individual plants and between plants. Another characteristic symptom is a reduction in growth. Some plants show very slight

symptoms, others are dead, and others are intermediate in condition.

Signs ? Fruiting structures or other fungal structures are signs of biotic agents of diseases. Signs are most useful with fungal-induced diseases. The fungus can be identified and recognized as a casual agent of disease by the presence of specific fruiting structures. With abiotic diseases, a confusing array of fungal structures may be seen. These can be identified and recognized as saprophytic organisms and are, therefore, not signs of plant pathogens. Signs associated with decline diseases are also not uncommon. Some of these may be confusing saprophytes such as those often found on plants suffering from abiotic diseases, and others are facultative or weak parasites that contribute to the decline. Identification of a specific organism as a known contributor to the decline of a plant is a good indication of a decline disease.

Host Specificity ? Biotic diseases are usually hostspecific or occur on a limited numbers of related or unrelated hosts. This concept is most applicable to fungal and bacterial pathogens. Other pathogens can be more general in their hosts. The most characteristic feature of abiotic diseases is the occurrence of similar

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symptoms on two or more totally unrelated hosts. In contrary, decline diseases are host-specific, but more than one species in a region may have its own specific decline syndrome.

Spatial Distribution ? Because biotic diseases are caused by infectious agents, they usually show a clumping distribution pattern of diseased individuals. Inoculum produced by diseased plants is most concentrated around the diseased individuals, thereby contributing to a higher incidence of disease in localized areas. Only with initial infection caused by inoculum dispersed from a distance, does the distribution of disease approach randomness. Topographic features that produce moisture or temperature conditions favorable for inoculum production, dispersal, and infection, may contribute to clumped disease distribution patterns typical of biotic diseases.

An abiotic disease is usually random in a plant population except when the agent is distributed in a concentrated or specific pattern. For example, a point source of pollution will produce a progressive intensification of symptoms as it nears the source. Decline diseases can have a random symptom distribution pattern within a given location.

All three types of diseases occur randomly. Thus, differences from one stand to another can be caused by many factors such as site, environment, genetic factors of the host, and many others.

Diagnosis of Plant Problems Correct identification of the causal agent and the plant affected are the first and most important steps in solving a plant problem. An inaccurate diagnosis will result in the implementation of the wrong management strategy and little success in solving the problem. Be aware of the fact that plants may have more than one problem, and the obvious one may not necessarily be the most important one. If the location where the plant is growing is the problem, there may not be a good solution.

Many diagnostic resources are available including books, extension publications, websites, university clinics, and personal experience. These should be used to develop an appropriate problem diagnosis routine. A Color Diagnostic Guide for Diseases of Trees and Shrubs is enclosed as Part II of this Chapter.

Diagnosing problems can be simple or difficult. Start with a complete plant and site history and inventory,

and obtain as much detail as possible. In commercial production or landscape situations, the time spent maintaining good records can return great rewards in problem solving and future problem prevention. When constructing the history of a problem, it is best to have a set of standard questions. As a minimum, the University of Minnesota Plant Disease Clinic Information Request Form should be used or adapted for this purpose. This form can be copied from Figure 3 of Part I of this Disease Management Chapter, or it can be obtained from the Plant Disease Clinic web site at plpa.agri.umn.edu under Extension and Outreach, Plant Disease Clinic. In completing the report, provide all plant symptoms or any responses of the plant to the problem. Include drawings, with measurements, and any other additional information. Send a completed information form to a Lab with every plant sample. Photographs and samples are helpful, especially when discussing the problem with others who have not visited the site. Digital images can also be sent to clinics and other knowledgeable individuals to receive good advice almost instantly. The more information obtained, coupled with a thorough on-site investigation, the better the diagnosis will be.

After determining the cause of a plant problem, evaluate the severity and extent of the problem. For the cause and the cure, explore all management possibilities, keeping in mind past, current and future situations and any potential economic factors. Pesticides may be used, but they may not be the most desirable or effective management method. However, they may serve as a temporary solution until other measures, or the plants own defense system, can correct the situation.

Abiotic Plant Problems Many Abiotic factors exist which can cause plant problems as shown in Table 1. These factors are called "Non-infectious" because they are not alive and they do not multiply like fungi, bacteria and viruses. Abiotic or non-infectious plant problems may be very visually apparent, such as mechanical damage to a plant. It may also be very obscure such as "mouse ear" of River Birch which is due to the minor nutrient element Nickel deficiency in the plant. Sometimes the only way to identify a problem as abiotic is to rule out all possible infectious disease and insect problems.

Proper knowledge and implementation of cultural and maintenance practices for plants in greenhouses, nurseries and landscapes can prevent most abiotic plant

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Figure 3. University of Minnesota Plant Disease Clinic Information Request form (Page 1). Also available at plpa.agri.umn.edu.

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