Fruit and vegetable growers have historically relied on copper-based fungicides for disease prevention and suppression. Elevated disease pressure on tomato (Solanum lycopersicum) from late blight (Phytophthora infestans) has increased the need for preventive fungicides for organic growers. There are limited options for organic growers that have proven efficacy. During visits to New Jersey farms, organic growers expressed concerns that their use of copper fungicides might be increasing soil copper levels, thus reducing the beneficial microbial populations in the soil and potentially limiting crop growth. In New Jersey, the sampled farms during this study most commonly used the following copper fungicides for tomato: cuprous oxide (Nordox 75 WP; Nordox, Oslo, Norway); copper hydroxide (Champ WG; Newfarm, Alsip, IL); and copper hydroxide (Nu-Cop 50; Albaugh, Ankeny, IA). Usage of cuprous oxide and copper hydroxide involves various maximum annual application rates, resulting in farms applying varying amounts of total copper to their fields based on the product they are using (Table 1). Farm copper inputs are not limited to fungicide applications. Copper is a common component of animal feeds because it is a required dietary mineral. Animal manures can serve as a source of copper where grazing takes place and when animal manures are applied to production areas. The National Research Council (1994) currently recommends that the concentration of copper as a trace mineral in turkey feed should be 4 to 8 mg·kg−1. A study of poultry manure application on fields producing cotton (Gossypium hirsutum) and corn (Zea mays) was conducted in the southeastern United States over a 10-year period. Researchers found that the manure application rate was a critical factor in the levels of soil copper in depths of 0 to 15 cm (He et al., 2009). Another study performed in the southeastern United States evaluated poultry litter application to well-drained soils over a 14-year period and found that the total copper concentrations were significantly greater than those of untreated soils at soil depths of 0 to 2.5 cm and 2.5 to 7.5 cm (Ashjaei et al., 2011). Many farms use rotational grazing of livestock and use animal manures as soil amendments to improve their soil quality. The U.S. Department of Agriculture (2000) suggests a soil pH of 6.5 or higher to reduce the availability of copper to plants consumed by humans and animals. Additionally, soils with organic matter content more than 10% will sequester copper ions, thereby reducing the availability to plants (Evans et al., 2007). This sequestration of copper ions increases the total soil copper levels and can serve as a copper sink with the potential to become available due to changing soil variables. The New York Department of Environmental Conservation and New York State Department of Health (2006) has set the clean-up threshold for brownfield sites as 270 mg·kg−1 for soil copper levels.
Copper fungicide application rates for tomato crops according to three commonly used product label instructions.
Understanding the use and accumulation of copper in agricultural soils is a necessary component of land management for agricultural producers. A research summary by the New Jersey Department of Environmental Protection (1993) found that rural New Jersey soils had a geometric mean of 4.8 mg·kg−1 of total copper. Crop uptake of copper can vary by species and cultivar, is impacted by soil factors, and is typically absorbed into the roots of crops but less so into the shoots (Marschner, 1995). Copper can be translocated into the shoots and leaves in high numbers when soil copper levels are excessive, and the uptake of copper by the plants can be impacted by soil phosphorus deficiencies (Lange et al., 2017; Verdejo et al., 2016). When the copper level in plant tissue is ≥19 mg·kg−1 dry weight, it is considered toxic (Davis and Beckett, 1978).
Growers have expressed concerns about soil copper levels and their impact on soil bacteria and human health. A 2016 study evaluated grassland soils that had been subjected to century-long exposure to normal (≈15 mg·kg−1), high (≈450 mg·kg−1), and extremely high (≈4500 mg·kg−1) copper levels (Nunes et al., 2016). The study results showed a decline in bacterial biomass and fungal biomass with increasing concentrations of copper compounds over time (Nunes et al., 2016). Because more urban and industrial sites are being used for the production of food and outdoor spaces in communities, research involving the impact of heavy metals on soil health will continue to be a priority.
Copper uptake from the soil by plants can vary depending on the crop and can impact the total and available copper in the soil. An evaluation of the accumulation of copper in six commonly grown vegetables found considerable variations according to the cultivar of carrot (Daucus carota), spinach (Spinacia oleracea), and pea (Pisum sativum) (Alexander et al., 2006). A University of Idaho study evaluated copper uptake by potato (Solanum tuberosum) to determine soil copper thresholds for potato production (Moore et al., 2013). Silt loam and sandy soils were amended with a copper rate of 100 mg·kg−1 of copper and produced potato shoots and roots that had removed 2 lb/acre of copper in a silt loam soil and potato shoots that had removed 6.6 lb/acre copper in a sand soil (Moore et al., 2013). A study performed in New Jersey showed that sweet corn (Z. mays) fresh market ears removed 0.014 lb/acre of copper from a silt loam soil (Heckman, 2007). Calculations of soil additions and removal can be complicated for diversified farming situations when an abundance of crop species and cultivars are grown on relatively small acreages with complex crop rotations.
The objectives of this study were to evaluate the use of three commonly used copper fungicides on 15 farms and the resulting soil copper levels and to assess the impacts of these copper levels on lettuce (Lactuca sativa) grown in the sampled soils in a greenhouse study. The gathered information was used to determine if copper levels had changed on individual farms over time and if these levels are excessive enough to cause phytotoxicity in lettuce. This information has been used to recommend best practices for farms using copper-based fungicides. Collectively, the information gathered and current best management practices can assist growers in making more informed decisions about copper fungicide use on their farms.
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