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.
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