Residential, urban, and agricultural land uses contribute to nonpoint source runoff. Excess nitrogen and phosphorus in runoff can contribute to increased rates of eutrophication. Eutrophication is a stable, naturally occurring process in surface waters. However, water bodies with communities of organisms adapted to low nutrient concentrations (< 0.05 mg/L PO4-P) are more susceptible to overfertilization, whereas naturally eutrophic water bodies are more resilient to additional nutrients because their plant and animal communities are adapted to the more productive environment. Nutrient enrichment and algal blooms occur when the natural balance of an aquatic system is upset.
Pesticide release from both agricultural and residential sources is well documented. Many commonly used pesticides have detrimental effects on organisms from microbes to larger fish species living in creeks, streams, and rivers. For example the pyrethroid pesticide bifenthrin, affects Hyallela azteca a benthic amphipod, at concentrations as low as 1-3 ppt (pg/L). Similarly, the degradation products of fipronil, a fairly new phenylpyrazole pesticide, affect bluegill sunfish (Lepomis macrochirus) at concentrations between 20 and 25 ppb. Local, state, and federal environmental agencies are under pressure to limit pollutant discharges from identifiable nonpoint source contributors to further protect and improve water quality (1, 3). Greenhouse and nursery operations that do not capture and recycle runoff can be significant contributors to agricultural nonpoint source contamination. Currently, California, Florida, Maryland, Oregon, and Texas have adopted regulations mandating runoff capture or control from irrigated agricultural operations. Several nurseries in Southern California have been required to acquire Waste Discharge Permits to control runoff that contains pesticides, nutrients, sediments, and other pollutants. Similar regulation in other states is likely as efforts continue to protect and maintain the quality of surface and ground water resources. In fact, in January of 2009, the US Environmental Protection Agency in concert with the Florida Department of Environmental Protection proposed an expedited schedule for establishing numeric nutrient criteria limiting N and P pollution in Florida lakes, rivers, streams, springs, and canals (2). This precedent could set the stage for regulations in other states in the next decade.
Waterborne phytopathogens are perennial problems across all facets of agriculture and are responsible for billions of dollars of crop losses. A number of chemically-based treatment methods have proved effective at controlling or reducing disease incidence. Drawbacks to chemically-based treatment systems include high initial investment costs, continuous operational expenses, worker safety issues, and a potential for environmental harm if not properly managed.
Figure 1. Phytophthora nicotianae zoospore filled with sporangia (a), releasing sporangia (b), and empty (c) with all sporangia released to potentially infest plant material.