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.

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.

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.

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.

• 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)

• 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

• 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

• 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