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Plant Disease Epidemiology: Temporal Aspects
Plant Disease Management Strategies

Since the beginning of agriculture, generations of farmers have been evolving practices for combating the various plagues suffered by our crops. Following our discovery of the causes of plant diseases in the early nineteenth century, our growing understanding of the interactions of pathogen and host has enabled us to develop a wide array of measures for the control of specific plant diseases.

From this accumulated knowledge base, we can distill some general principles of plant disease control that can help us address the management of new problems on whatever crop in any environment. One such set of principles, first articulated by H. H. Whetzel in 1929 and modified somewhat by various authors over the years, has been widely adopted and taught to generations of plant pathology students around the world. These "traditional principles", as they have come to be known, were outlined by a committee of the US National Academy of Sciences, 1968.

Traditional Principles of Plant Disease Control

  1. Avoidance—prevent disease by selecting a time of the year or a site where there is no inoculum or where the environment is not favorable for infection.

  2. Exclusion—prevent the introduction of inoculum.

  3. Eradication—eliminate, destroy, or inactivate the inoculum.

  4. Protection—prevent infection by means of a toxicant or some other barrier to infection.
  5. Resistance—utilize cultivars that are resistant to or tolerant of infection.

  6. Therapy—cure plants that are already infected.

While these principles are as valid today as they were in 1929, in the context of modern concepts of plant disease management, they have some critical shortcomings. First of all, these principles are stated in absolute terms (e.g., "exclude", "prevent", and "eliminate") that imply a goal of zero disease. Plant disease "control" in this sense is not practical, and in most cases is not even possible. Indeed, we need not eliminate a disease; we merely need to reduce its progress and keep disease development below an acceptable level. Instead of plant disease control, we need to think in terms of plant disease management.

A second shortcoming is that the traditional principles of plant disease control do not take into consideration the dynamics of plant disease, that is, the changes in the incidence and severity of disease in time and space. (See: Disease Progress.) Furthermore, considering that different diseases differ in their dynamics, they do not indicate the relative effectiveness of the various tactics for the control of a particular disease. They also fail to show how the different disease control measures interact in their effects on disease dynamics. We need some means of assessing quantitatively the effects of various control measures, singly and in combination, on the progress of disease.

Finally, the traditional principles of plant disease control tend to emphasize tactics without fitting them into an adequate overall strategy.

Does this mean that we should abandon the traditional principles? Of course not! We merely have to fit them into an appropriate overall strategy based on epidemiological principles.

Strategies versusTactics

Ask a handful of pest management experts to name the major plant disease control strategies, and you are sure to find disagreement. The problem is generally one of semantics rather than of fundamental disagreement over the important means of disease control. The dictionary definitions for the two terms are similar, but generally speaking, an overall plan for reaching a particular objective is called a strategy, while the specific means for implementing a given strategy are called tactics. Like the goals and objectives that they are intended to achieve, strategies and tactics tend to occur in hierarchies. What is a "strategy" at one level of focus could be called a "tactic" at another level of focus.

The important point to remember is that countless human undertakings, be they military operations, political campaigns, football games, or any other kind of organized effort, have failed, despite flawless tactics, for lack of a sound strategy. Any endeavor that requires a series of connected tasks for its completion also requires some kind of overall plan. Each individual task, no matter how skillfully executed or how successful its outcome, will not advance progress toward the final objective unless it has a coherent relationship with all of the other necessary tasks.

A Hierarchy of Objectives

Objectives tend to occur in hierarchies. The most general objective, in order to be realized, will have several sub-objectives that first must be successfully accomplished. Each of those sub-objectives can also have sub-objectives, and so on, in a hierarchy that can consist of several layers of objectives.

For example, suppose that our general objective (goal) is to reduce the losses caused by potato late blight in a particular field. We could construct a hierarchy of objectives as follows:

First-level objective (Goal): Reduce the losses caused by potato late blight

Second-level objective: Reduce the level of foliage infection

  • Third-level objective: Reduce the level of initial inoculum
    • Fourth-level objective: Rotate potatoes with other crops
    • Fourth-level objective: Remove volunteer potatoes
    • Fourth-level objective: Plant certified seed
    • Fourth-level objective: Remove piles of discarded potatoes
  • Third-level objective: Reduce the rate of disease development
    • Fourth-level objective: Plant partially resistant varieties
    • Fourth-level objective: Plant a living barrier to isolate adjacent potato fields
    • Fourth-level objective: Apply fungicides as necessary
  • Third-level objective: Reduce the duration of the epidemic
    • Fourth-level objective: Plant an early-maturing variety

Second-level objective: Reduce the level of tuber infection

  • Third-level objective: Reduce the amount of inoculum that can infect the tubers
    • Fourth-level objective: Control the foliar epidemic
    • Fourth-level objective: Hill the rows to keep the tubers deeply buried
    • Fourth-level objective: Kill the vines and let them dry before harvest
  • Third-level objective: Reduce the rate of disease development
    • Fourth-level objective: Dry the the surface moisture on the potatoes before storage
    • Fourth-level objective: Move the potatoes into cold storage as quickly as possible
  • Third-level objective: Reduce the duration of the epidemic on stored tubers
    • Fourth-level objective: Sell any potentially infected potatoes as soon as possible

Note that this is just a slice out of a much larger hierarchy of objectives. What we, as plant pathologists or pest control specialists, might set as our most general objective would simply be an intermediary objective for a grower, who has to manage other pests, manage the crop, and be concerned with the productivity of the whole farm. At the other end of the scale, under some of our fourth-level objectives we could create a fifth level and perhaps even a sixth. For example, "Apply fungicides as necessary" would require some determination of the susceptibility of the variety that we had planted, an assessment of the inoculum available, and an assessment of the environmental conditions, perhaps requiring the use of a predictive model.

Note also that there are many valid ways to structure a hierarchy of objectives to accomplish a given goal, even starting with the same lowest-level objectives. The nature of the plan for organizing the objectives depends on the biases of the planner, and while some plans may be better than others for accomplishing the goal, the others are not necessarily wrong.

The Epidemiological Basis of Disease Management

Plant disease epidemics can be classified into two basic types, monocyclic and polycyclic, depending on the number of infection cycles per crop cycle. (See: The Cyclical Nature of Plant Disease.) The early stages of a monocyclic epidemic can be described quite well by a linear model, while the early stages of a polycyclic epidemic can be described with an exponential model. Since we are concerned with keeping disease levels well below 100%, there is no need to adjust the models for approaching the upper limit, and we can use the simple linear and exponential models to plan strategies:

Examining these models, we can see that in both there are three ways in which we can reduce x at any point in the epidemic:

  1. Reduce the initial inoculum (Q in the monocyclic model and x0 in the polycyclic model). (Actually x0 is the initial incidence of disease, which is proportional to the initial inoculum.)
  2. Reduce the rate of infection (R in the monocyclic model and r in the polycyclic model)
  3. Reduce the duration of the epidemic (the time, t, at the end of the epidemic)

These, then, can be used as three major strategies for managing plant disease epidemics, and we can organize our plant disease control tactics under one or more of these overall strategies. Furthermore, by means of the model we can assess the quantitative impact of each strategy, not only by itself, but in its interaction with others.

The monocyclic model

It is clear from the above model of a monocyclic epidemic that Q, R, and t have equal weight in their effect on x. A reduction in the initial inoculum or the rate of infection will result in a reduction in the level of disease by the same proportion at any time, t, throughout the epidemic. If t can be reduced (for example, by shortening the season), disease will be reduced proportionately.

The polycyclic model

  • If r is very high, the apparent effect of reducing x0 is to delay the epidemic.
  • If r is very high, x0 must be reduced to very low levels to have a significant effect on the epidemic.
  • Reducing r has a relatively greater effect on the epidemic than reducing x0.
  • Reducing x0 makes good strategic sense only if r is low or if r is also being reduced.

It is easier to understand (and remember!) these concepts if we actually select different values for x0 and r, plug them into the model, and graph the outcome. This can be done easily with a calculator that has an exponential function, or with the accompanying simulation.

Clearly developing a sound disease management strategy requires enough knowledge of the biology of the pathogen and host to select the appropriate epidemiological model. It also requires at least "ball-park" estimates of the model parameters and the magnitude of the impact of each specific tactic on the initial inoculum or the apparent infection rate. Failure to adopt such a quantitative approach can lead to some embarrassing or even very costly errors. (Example)

The Traditional Principles Revisited

To make the conceptual leap from disease control to disease management, the traditional principles can be modified by fitting them as tactics within each of the three major disease management strategies and by slightly changing the wording to reflect the quantitative impact of the action rather than an absolute effect:

Tactics for the Reduction of Initial Inoculum

  • Avoidance—reduce the level of disease by selecting a season or a site where the amount of inoculum is low or where the environment is unfavorable for infection
  • Exclusion—reduce the amount of initial inoculum introduced from outside sources
  • Eradication—reduce the production of initial inoculum by destroying or inactivating the sources of initial inoculum (sanitation, removal of reservoirs of inoculum, removal of alternate hosts, etc.)
  • Protection—reduce the level of initial infection by means of a toxicant or other barrier to infection
  • Resistance—use cultivars that are resistant to infection, particularly the initial infection
  • Therapy—use thermotherapy, chemotherapy and/or meristem culture to produce certified seed or vegetative planting stock

Tactics for the Reduction of the Infection Rate

  • Avoidance—reduce the rate of production of inoculum, the rate of infection, or the rate of development of the pathogen by selecting a season or a site where the environment is not favorable
  • Exclusion—reduce the introduction of inoculum from external sources during the course of the epidemic
  • Eradication—reduce the rate of inoculum production during the course of the epidemic by destroying or inactivating the sources of inoculum (roguing)
  • Protection—reduce the rate of infection by means of a toxicant or some other barrier to infection
  • Resistance—plant cultivars that can reduce the rate of inoculum production, the rate of infection, or the rate of pathogen development
  • Therapy—cure the plants that are already infected or reduce their production of inoculum

Tactics for the Reduction of the Duration of the Epidemic

  • Avoidance—plant early maturing cultivars or plant at a time that favors rapid maturation of the crop
  • Exclusion—delay the introduction of inoculum from external sources by means of plant quarantine

Example: Dry bean seed production

Several important pathogens of dry beans, including Pseudomonas syringae pv. phaseolicola (the causal agent of halo blight), Xanthomonas phaseoli (the common blight pathogen), and Colletotrichum lagenarium (the fungus responsible for anthracnose) are seedborne. Recommendations for the control of these diseases, therefore, always include the reduction of seed infection through some kind of "clean seed" program.

The seed for most of the dry bean production in the United States is grown in the semi-arid areas of the Pacific Northwest, where there is very little development of these important seedborne pathogens. In most years the seed produced in these areas has a vanishingly low incidence of seed infection.

In the dry bean producing areas of the central and northeastern US, however, the weather during most summers is at least moderately favorable for the development of epidemics of these diseases. By planting only western-grown seed, dry bean producers in the rest of the country can escape serious infection. Suppose, however, that for reasons of economics and politics the eastern growers decide to establish their own local certified bean seed production program. They know, of course, that they are likely to get some seed infection, but they can afford to invest a bit more in protecting the seed crop with fungicides and bactericides than they can the rest of their beans, and new technologies permit the detection of very low levels of seed infection in their certification program.

Despite the frequent use of the term "disease-free seed", zero infection is impossible, and so in any seed certification program it is necessary to establish an acceptable level of seed infection. Without getting into sampling error and sensitivity of the seed assay, which, of course, are important considerations, we can calculate the maximum allowable seed infection very roughly using our knowledge of the epidemiology of the disease(s) in question.

We begin by working backward from harvest, where we have to decide what level of disease we can allow at the end of the season. This is usually based on economic criteria and yield-loss models, and let us suppose for the sake of this example that we have determined that in the case of halo blight the final incidence of disease allowable is 25% of the plants infected.

We next have to decide which of the epidemiological models to use, and since halo blight clearly is polycyclic, we select the logistic model. Now we have to estimate the apparent infection rate of halo blight under the conditions to which the beans are likely to be exposed. (Ideally we would make several estimates of r, each under different environmental conditions, to calculate the acceptable level of seed infection under the whole range of conditions that we expect to encounter in the field.) This can be done by conducting a series of field trials or by looking up some published disease progress data. (See Estimating Model Parameters: Some Examples.) The rest is simply a matter of plugging in our estimates for r, the final disease incidence, and the length of the season into the simple exponential model and solving for initial disease incidence. (See Practical Uses of Epidemiological Models.)

What becomes painfully obvious in this case is that the maximum allowable level of initial disease incidence is so low that it is not practically achievable by seed selection alone. Our best tactic is to purchase seed produced in semi-arid environments where the level of seed infection is, in fact, exceedingly low. Many eastern bean producers could have saved themselves large sums of money by making these simple calculations.

 Next: Management Simulation