In the chapter, “Plant Diseases,” we discussed the disease pyramid. The concept of the disease pyramid is an important one because it helps us make decisions regarding disease management. If we expect any one or more of the points on the pyramid to be lacking (e.g., a site is planted to a non-host), then we could assume that a given disease will not occur and act accordingly. If, on the other hand, we expect disease to occur, all of our available management options are directed at one or more of the three items at the base of the pyramid. That is, we can control the pathogen directly (e.g., with a fungicide), reduce host susceptibility (e.g., plant a resistant variety), or manipulate the environment (e.g., provide drainage and avoid over-watering so as to prevent wet soils that favor root-rot pathogens).

Factors That Influence Disease Development

As we discuss the need for and methods of disease control in this chapter, keep the concept of the disease pyramid in mind.

Disease development is a dynamic process. It depends on:

• The size of the pathogen population,
• The dissemination of the pathogen to a susceptible host,
• The life cycle of the pathogen,
• The reaction of the host, and
• The environmental conditions prevailing at a given time.

Pathogen Populations

The size of the pathogen population is important for several reasons. Although a single fungal spore can start an infection, plants can usually withstand low levels of attack. Also, there is no guarantee that any one spore will actually infect a plant. For example, a spore may germinate in a drop of dew, but if the drop dries before the fungus infects the plant, the germinating spore may dry out and die. The greater the pathogen population, the more likely that enough individuals will infect plants and cause disease.

Pathogen populations are influenced by:

• Their ability to survive during the absence of a susceptible host,
• The relative susceptibility of commonly planted varieties,
• The pathogen host range,
• Plant management practices,
• Climatic conditions,
• How the pathogen is disseminated, and
• The association of vectors.

Dissemination of the Pathogen

Fungal and bacterial pathogens live and survive mostly in living plants, plant debris, or the soil. Fungi are spread from diseased to healthy plants and from one area to another by wind, water, soil, insect vectors, root grafts, and equipment. Wind is a particularly effective means of disseminating foliar pathogens. Surface water and the movement of contaminated soil (e.g., on equipment) are important in the spread of soil-borne pathogens. Splashing rain and irrigation water are frequently of prime importance in the spread of bacterial pathogens. Some are also spread by insect vectors. On a larger scale, people have spread pathogens around the globe through international trade of seed and planting stock.

Phytoplasmas, viruses, and viroids do not survive well outside living tissue. They are usually transmitted from plant to plant by insects and vegetative propagation.

Life Cycle of the Pathogen

It is important to know and understand the life cycle of the pathogens you are dealing with. This will help you determine a way to control or fight it.

Infection. Once a pathogen reaches the host, it must then enter and infect the host. Insect vectors, for example, acquire pathogens from plant tissue and introduce them directly into healthy plant tissue while feeding. Most fungal and bacterial pathogens, on the other hand, are simply deposited on or near the surface of a plant. Bacteria can enter plants only through wounds or natural openings. Many fungi, however, can penetrate directly through intact plant surfaces. Wounding plants during cultural operations makes it easier for many pathogens to invade plants and so may significantly increase disease incidence.

Growth and Reproduction. Once inside a susceptible plant, the pathogen will grow, spread, and reproduce within the plant itself. As the pathogen feeds, it removes nutrients from the plant. Although this can weaken the plant, it does not usually account for most of the damage. Rather, pathogens cause more damage by secreting toxins, enzymes, and other chemicals that impair the plant’s physiological processes.

Reaction of Host

While a plant’s reaction to a pathogen (i.e., susceptible, resistant, or tolerant) is under genetic control, it is also influenced by:

• The plant’s growth stage: Seedlings are often more susceptible to pathogens than older plants. Likewise, new growth may be more susceptible to some diseases.
• Plant nutrition: In most cases, plants that are under nutrient stress are less able to resist pathogens. On the other hand, excessive nitrogen produces succulent tissues that are often highly susceptible to infection.
• Plant injury: Plants that have been injured (e.g., by mowing cuts, wind, pruning) are more prone to disease because pathogens can bypass normal plant defenses and enter the plant directly through wounds.
• Environmental conditions: Extremes in temperature and soil moisture can reduce a plant’s ability to resist pathogens.

Environmental Factors

Rainfall, irrigation, duration and frequency of dew, air and soil temperatures, and soil moisture greatly influence pathogen activity and disease development. For example, many fungi require moisture for their spores to germinate and infect a plant. This is why plant diseases caused by fungi are more common in wet years than in dry years. Plants that are stressed by adverse environmental conditions also tend to be more susceptible to disease.

Disease Symptoms

We categorize disease symptoms based on the nature of the changes occurring in the plant:

• Necrosis is the actual death of cells or entire portions of the plant. Necrotic tissue is usually discolored, frequently appearing brown or black. The necrosis may be general, involving extensive decay or rotting (e.g., dry rots, soft rots, brown rots, white rots) or it may affect only small areas, sometimes only a few cells (e.g., leaf spots, fruit spots, blotches, scabs, stripes, streaks).
• Overdevelopment of tissue results from either increased cell division or abnormal enlargement of cells. Examples are galls, clubroot, leaf curls, and warts.
• Underdevelopment of tissue results from either decreased cell division or below-normal growth of cells. Examples are stunting, dwarfing, and some malformations.
• Discoloration of tissue, other than from necrosis, usually occurs because chlorophyll does not develop properly or breaks down. Yellowing (chlorosis) of green tissue is most common, although reddening or bronzing often occurs. Chlorosis is a symptom of many diseases, particularly in early stages, and may be followed by necrosis. Chlorosis is often a secondary symptom, resulting from damage to another part of the plant (e.g., roots, vascular system.)
• Wilt is characterized by drooping plant parts. It occurs when the plant does not get enough water to its aerial parts (i.e., leaves, stems). Obviously, a plant will wilt if soil moisture is very low. However, pathogens that damage roots or the stem, or that damage or plug the plant’s conductive tissue, also cause wilt. When pathogens are the cause, you may see plants wilting even when you know there is adequate soil moisture.

Predict the Need for Disease Control

Predicting whether or not a disease will occur, and to what extent, can be as difficult as it is important. Because most control measures rely on stopping a pathogen before or very soon after disease develops, you will benefit greatly by being able to determine when disease is likely to occur and when control will be needed.

Monitor the Pathogen

The first clue that a disease might occur is whether or not it has occurred in the same site before. This is especially true for soilborne pathogens, which usually remain in soil once they infest it. Pathogens that overwinter in plant debris (e.g., fallen leaves) may also be a problem unless the debris is removed before spring.

You can check for the pathogen itself by submitting soil or plant samples for analysis. Another important step is to scout frequently so as to detect any disease symptoms as soon as possible. Quite often, a disease may be present but not at a high enough level to warrant control. Other factors, such as the growth stage of the plant, must be considered to make this determination.

Consider the Host Plants

Although planting a resistant variety is in itself a control measure, it can also help reduce the need for other control measures. The growth stage of the plant is also an important consideration in predicting both disease occurrence and the need for control. For example, seedlings tend to be more susceptible than older plants to a number of diseases. Seed treatments and soil drenches applied at planting are specifically designed to protect the plants during this stage of increased susceptibility.

Look for Favorable Conditions

Understand which conditions favor the development of a particular disease. Such conditions can include site characteristics as well as weather.

Site Characteristics. The site is an important factor in disease development. For example, leaf and stem diseases may occur frequently where dew and long periods of high relative humidity occur (e.g., in lowlands adjacent to waterways). Sites that are prone to having waterlogged soils tend to have problems with root rots and many seedling diseases.

The site management plan can also influence disease. For example, proper irrigation can help prevent water-logged soils that favor disease. It follows, then, that you can manipulate conditions at a site to help reduce the likelihood of disease.

Weather. Weather during winter months may be a good predictor of whether certain disease will be a problem. For example, a very cold winter may reduce the survivorship of insects that carry plant diseases. On the other hand, heavy snow can insulate pathogens that overwinter on plant debris and help them survive until spring.

Weather during the growing season is critical to disease development and can help you predict if control will be needed. Important weather effects are air and soil temperatures, soil moisture, rainfall, relative humidity, and leaf wetness. Severe weather can also damage a plant and thus increase its susceptibility to infection.

Duration of Favorable Conditions

The conditions that favor disease development must exist long enough for a problem to actually occur. The duration of leaf wetness (or high relative humidity) and adequate temperature are often among the best predictors of disease. At higher temperatures, for example, infection by fungi often requires fewer hours of leaf wetness than at lower temperatures.

Action Threshold

We previously discussed the action threshold (In Appendix F), which is the level of pest infestation or damage at which you need to apply some control measure to prevent aesthetic damage or serious harm to plant health. However, you cannot detect pathogens until disease symptoms appear. Once symptoms appear, treatments applied in response to these symptoms will protect subsequent plant growth but will not cure plant parts already affected. Therefore, you may choose to act on the basis of suitable conditions for disease rather than on the basis of visible evidence of disease. This will certainly be the case if plants must be blemish free or if the disease can only be controlled preventively (e.g., snow mold, tree decay and canker disease).

Principles of Disease Managements

There are very few plant diseases that can be totally controlled or eradicated. The goal of disease management, then, is to prevent disease or reduce its severity. While efforts are directed at protecting the site as a whole, you must keep in mind that a single plant may serve as a source of infection for an entire planting.

Plant disease management involves the application of one or more of the following principles:

• Resistance involves planting varieties that are able to resist or withstand infection.
• Eradication technically means the complete removal of a pathogen from an area. In practice, reducing the size of a pathogen population at its source is a more realistic goal.
• Protection involves establishing a chemical barrier between a host and a pathogen to prevent infection.
• Exclusion keeps a pest out of an un-infested area using such methods as planting disease-free seed.
• Avoidance is accomplished by planting at times when, or in areas where, a pathogen is rare or ineffective. Avoidance can also be achieved by making conditions unfavorable to pathogens.
• Therapy involves reducing disease severity in an already-infected plant and is used quite often in the maintenance of woody ornamentals (e.g., pruning to remove infected plant parts).

Each control method we discuss will fit into one of these management strategies.

Methods of Disease Control

A single procedure may control a plant disease. Most management strategies, however, use several methods and usually involve an integrated program of cultural, mechanical, and chemical control measures. Note that many of the control measures are implemented by the customer, not a custom applicator.

Prevention

A disease management program begins with steps to keep the area as free of pathogens as possible. Screen incoming plant material and destroy or isolate for treatment any diseased plants. When such is available, only certified disease-free seed or plant material should be planted. This is especially important in preventing disease cause by bacterial or viruses because chemical control (other than vector control) is not really an option.

Resistant Varieties

Planting disease-resistant varieties is usually the most effective and economical way to control plant diseases, assuming acceptable varieties are available. Remember that there are variable levels of resistance and no one variety is resistant to all pathogens. Resistance to wilt, bacterial, and virus diseases is especially useful because of the lack of chemical options in controlling such diseases.

Plant Management Practices

In general, cultural and management practices that promote the health and vigor of desirable plants will diminish the plants’ susceptibility to infection. Consider the role of the following practices in promoting plant health and controlling diseases.

Site Selection. As stated earlier, some sites may be more favorable for disease development, as with sites in lowlands that have long periods of continuous dew or high relative humidity. Sites that tend to have water-logged soils may have problems with root rot. Microenvironmental conditions also influence disease development. For example, ornamentals and turf susceptible to powdery mildew are more vulnerable to infection when planted in the shade rather than the sun.

Planting Practices. Avoid establishing a monoculture of ornamental plants, as was the case with the American elm, which was used almost exclusively to adorn city streets. Because of the lack of diversity in these plantings, Dutch elm disease spread rapidly, devastating many an urban forest.

Proper Plant Nutrition. Many plant diseases are less severe when there is a proper balance of soil nutrients. Excessive nitrogen fertilization, which promotes rapid, vigorous growth, can open the door to disease problems in some plants (e.g., powdery mildew on roses). Less nitrogen and more potassium will reduce the chances for powdery mildew infections on these plants. On other plants (e.g., turfgrass), however, powdery mildew is favored by slower growth and added nitrogen can promote growth and help counteract powdery mildew. To successfully manage disease, then, you need a thorough knowledge of a plant’s nutritional responses to disease situations.

Proper Irrigation. Manage irrigation carefully. Overly wet soil will favor root rot and overhead irrigation increases the risk of leaf diseases. If irrigation is needed, timing can be critical. For example, you should irrigate early in the day to allow adequate drying time to reduce the risk of infection by foliar pathogens.

Sanitation. Sanitation is important in keeping pathogens that are present in soil or on tools from contacting plants. Disinfect pruning tools (e.g., in bleach or alcohol) between uses to avoid spreading pathogens from diseased to healthy plants. Sterilize or pasteurize growing media for interiorscape plants and remove dying plant material before it falls on other plant parts or growing media.

Raking up fallen leaves or cultivating them into the soil in the fall can help reduce the overwintering pathogen populations. For example, leaves of crabapple trees infested with scab disease should be raked and disposed of in the fall. This will restrict the size of the initial pathogen population, but remember that inoculum originating in other areas may supplement that population later in the year.

Mechanical Control

Pruning infected or infested plant tissue will reduce the inoculum present in a site. It’s best to prune during the dormant season. However, if you must prune during the growing season, do so during dry periods to reduce the risk of the wound getting infected.

You should keep equipment and workers out of plantings when the foliage is wet so that they do not inadvertently drag pathogens from diseased to healthy plants.

Control of Other Pests

Disease management is part of an overall pest management program. Other plant pests can increase the incidence and severity of plant diseases in the following ways:

• Insects can transmit plant pathogens from infected plants to healthy plants or create wounds that facilitate infection by pathogens,
• Weeds can serve as alternative hosts for plant pathogens or insects that serve as vectors, and
• Stresses caused by plant pests reduce the plant’s natural abilities to fight off disease.

Controlling insect vectors can be the key to preventing the spread of plant viruses. After all, we have yet to develop a chemical that can kill a virus and yet leave the host plant unharmed.

Remember that plant disease control must fit into the overall pest management program. Reduced spacing between plants may be an effective way to control weeds by increasing plant competitiveness. However, the denser canopy will produce a higher relative humidity and reduce the drying effect of wind, thus possibly increasing the likelihood of plant disease.