Plant diseases, caused by infectious viruses, bacteria, phytoplasmas, fungi, and nematodes, result in problems in agriculture, landscape, and forest settings. These problems include reduced yields, lower product quality or shelf-life, decreased aesthetic or nutritional value, and, sometimes, food and feed contaminated with toxic compounds. Control of plant diseases is essential to providing an adequate supply of food, feed, and fiber. Growers currently spend large sums to achieve partial control of pathogens that attack crops and other plants. Even then, crop and commodity losses because of diseases cost billions of dollars each year. Reducing such losses has long been a high priority objective for agriculture and so for the Agriculture Research Service (ARS). Besides the obvious benefits for producers and processors, plant health protection is an important mechanism for increasing food supplies without increasing the land under cultivation. Knowledge and management of plant diseases of quarantine significance are vital, not only for protecting our domestic crops from foreign disease, but also for maintaining and expanding export markets for plants and plant products.

Strategies for the control of plant diseases include planting resistant crop varieties, changing crop cultural practices or storage conditions to those less favorable for disease development, employing biological controls, applying chemical pesticides, and using integrated disease management (combining two or more of the above approaches). The ability to develop any of these strategies depends first on identifying the pathogen causing the disease, then learning how to interrupt its disease cycle. The more known about the genetic, biochemical, and physiological processes that operate in the host and pathogen as infection and disease progress, the more likely a control method can be devised. Understanding the ecology of pathogens is also important (that is, how they survive, are dispersed, and otherwise interact with their environment). In addition, an understanding of the epidemiology or outbreak of disease and spread of pathogens is important for deciding which control actions are most effective in various situations.

As with all microorganisms, plant pathogens exhibit a remarkable ability to change and adapt. Newly discovered pathogens and more virulent strains of old pathogens continually arise that overcome resistant crop varieties or are no longer controlled by strategies and chemicals that were once effective. Continuing research to develop new control methods is necessary to increase or even maintain current levels of crop production and commodity preservation. Further complicating the situation, public concern has grown in recent years regarding the use of chemicals to control diseases. This concern stems from the fear that such chemicals may contaminate food or accumulate in the soil and ground water (and so be introduced into the food chain). As a result, pressure has increased to develop nonchemical approaches to plant disease control. Typically, however, nonchemical controls do not exist, are less effective, or too costly. More research is needed to develop or improve the approaches.

Host plant resistance to plant diseases is a desirable control strategy because it can be highly effective, is environmentally benign, and usually entails little or no additional expense to producers. To be most effective, such resistance should be durable, (not readily overcome by mutations in pathogens that increase virulence). This requires a clear idea of the pathogens that are present, their ability to change, and the nature of the resistance processes themselves. Also required are rapid, reliable pathogen detection and identification procedures for rapid, reliable disease diagnoses. Improved detection and identification procedures are also becoming more important as international trade of plant products increases and as trading partners seek to protect themselves from the introduction of unwanted diseases.

For diseases where host plant resistance is unavailable or only somewhat effective, strategies are needed that integrate cultural, biological, and chemical control procedures. For example, different crop rotations or improved cultural practices can sometimes suppress development of pathogenic organisms while maintaining a high level of productivity. Biological control shows great potential for disease control, but has sometimes been unreliable or only marginally effective. Additional research is needed to understand the interactions of microorganisms in the soil, plant parts, and on the mechanisms that biological control agents use to reduce or prevent the onset and development of disease. Finally, the appropriate use of chemicals will continue to be an important tool in the battle against plant diseases, especially where alternative controls are not effective. Additional research is needed to develop safer chemicals and to increase the efficiency of chemical applications.

Recently developed biotechnology tools provide promising new approaches for achieving control of diseases. Genetically engineering plants for resistance to virus, bacterial, fungal, and nematode pathogens has shown some success, but more research is needed. Also, techniques have been developed that allow researchers to trace specific genes and thus facilitate breeding resistant varieties. Biotechnology offers the exciting possibility of developing disease resistance in plants that cannot be accomplished through conventional breeding procedures (for example, by introducing genes for resistance from unrelated species).

The overall goal of the Plant Diseases National Program is to develop and improve ways to reduce crop losses caused by plant diseases. The program focuses on developing effective disease control strategies that are not environmentally harmful, do not threaten the safety of consumers, and are compatible with sustainable crop production. The ARS program will be conducted in cooperation with related research in other public and private institutions.