Problem Statement

Rationale.  Relationships between plants and their pests and pathogens are delicately balanced. Plants possess intricate defense mechanisms, whereas pests and pathogens possess elaborate strategies to cope with plant defenses. Any factor that upsets normal plant metabolism can affect plant defenses and threaten overall plant health. For the first time in human history, anthropogenic emissions have resulted in tropospheric ozone concentrations high enough to disrupt plant metabolism and cause the ozone-induced injuries noted earlier, which result in suppressed growth and yield. It is not surprising that this relatively sudden change in earth=s air quality also has affected interactions between plants, pests, and pathogens. Research to unravel effects of tropospheric ozone on pests and pathogens of agricultural crops has been fragmented and sporadic. Plant stress caused by ozone can increase, decrease, or have no effect on pests and pathogens, but mechanisms to explain these responses are unknown. Ozone-induced increases in pest and pathogen populations would further suppress crop yield and increase pesticide use. Increased use of pesticides would threaten environmental quality. Data are required to estimate the environmental and economic impact of such changes.

What is known.  Reports of stimulatory responses of insects feeding on plants exposed to air pollutants predominate over reports of inhibitory responses. Early investigations in Europe showed that growth rate and degree of infestation of various aphid species often were greater on plants in nonfiltered urban air than in charcoal-filtered air although two aphid species were inhibited by nonfiltered air. Specific atmospheric components responsible for effects attributed to nonfiltered air in European reports were not identified, but evidence from later experiments with specific pollutants indicates that ozone is a prime candidate. Various stimulatory responses (increased feeding, faster development, better survival) have been reported for larvae of several leaf-chewing insects when host plants were exposed to ozone. Populations of the two-spotted spider mite on white clover and peanut increased faster on plants exposed to ambient concentrations of ozone than on plants exposed to lower concentrations of ozone. Increased fecundity and shorter development time caused this increase in mite populations.

Our present knowledge of pollutant effects on plant disease stems largely from short-term experiments performed mostly in the greenhouse or laboratory, dealing with only one stage in the parasite cycle. Studies with diseases caused by fungi have predominated over diseases caused by other organisms. Foliar diseases have been studied more than diseases of other plant organs.

It is generally agreed that effects of ozone on pests and pathogens are mediated mostly through effects on host plant physiology. Effects on plant concentrations of carbohydrates, nitrogen compounds, or metabolites that may be directly involved in plant defense often have been cited as possible indirect mechanisms.

Gaps.  Effects of tropospheric ozone have been studied for only a small percentage of important pests and diseases. Most research has involved short-term exposure to one or two relatively high pollutant concentrations to measure effects on individual life stages. Studies employing long-term exposure to a wide range of pollutant concentrations allowing measures of changes in multiple life stages and population dynamics are rare. Most research has been performed in greenhouse or environmentally controlled exposure systems. Few studies have been performed in systems under near-ambient environmental conditions. Changes in host plants that may account for observed effects on pests or disease include changes in host suitability as a food source and increases or decreases in metabolites that may be involved in defense mechanisms or host attractiveness. However, pollutant-induced changes in specific host nutrients or specific metabolites have not been proven as cause for pest or disease response. Recent reports show that carbon dioxide enrichment can prevent ozone stress in many crops. Other soil-related and climatic factors also can alter plant response to ozone. Effects of carbon dioxide enrichment and other environmental factors on pest or pathogen response to ozone have not been adequately addressed.

Goals

• Determine effects of chronic ozone exposure of plants on multiple life stages and population dynamics for representative agricultural pests and pathogens;
• Identify ozone-induced changes in plant chemistry that control pest infestation; and
• Estimate effects of observed responses on crop yield and pesticide use.

Approach

Experimental ozone exposures will be performed in controlled environments, greenhouses, or open-top field chambers. Exposures will be chronic, mimic real-world exposure dynamics, and span the range of concentrations that occur in the troposphere at various locations throughout the world. Exposures will be performed before, during, and after pests or disease organisms are introduced to host plants. Temperature, rainfall, humidity, and solar radiation will be routinely recorded throughout the experiments to examine possible influence of these factors on measured responses.

Economically important pests and parasites will be selected for study, and host plants with variable degrees of tolerance to ozone will be used when possible. Measurements of individual and multiple life stages will be made to allow estimates of long-term effects on population dynamics. Measures of ozone effects on plant biochemistry will be made to identify mechanisms of pest and parasite responses. Effects of carbon dioxide enrichment and other factors such as temperature or soil nutrition on pest or pathogen response to ozone will be included as experimental variables when appropriate to increase extrapolation of results to a wider range of environments

Outcomes

• Estimates will be developed for risks to crop production from ozone-induced effects on pests and diseases.
• Ozone-induced changes in pesticide use caused by ozone impact on pests and disease will be assessed.

Impact

Minimal adverse effects on crops caused by ozone influence on plant pests and diseases and

improved regulatory and policy decisions to minimize ozone effects on agriculture

Linkages to Other ARS National Programs

• Global Change
• Integrated Agricultural Systems
• Plant Biological and Molecular Processes
• Plant Diseases
• Plant Microbial and Insect Genetic Resources, Genomics, and Genetic Improvement