Best Management Practices for Copper Fungicide Use

Fruit, vegetable, and field crop growers use copper-based fungicides as a protectant for many foliar diseases. The key with copper is that it must be applied before the presence of disease in order to be effective. Organic growers frequently rely on copper fungicides–the only currently approved organic option shown to have worthwhile efficacy against diseases such as late blight and downy mildew. These diseases have the ability to wipe out crops, therefore the use of copper and the disease control it provides are critical to both organic and conventional farming operations. Copper is also an essential nutrient for crops. Its importance in plant growth is outlined in Rutgers Cooperative Extension’s Fact Sheet FS720, Copper: Evaluating Micronutrient Needs of Soils and Crops in New Jersey.

Due to the often and frequent use of copper, many growers express concern with how copper interacts with the biology of the soil, with the ability of crops to take up excessive levels of copper, and elevated soil copper levels impacting overall crop growth. The impact of copper applications on farm soils and how to best use copper is the focus of this fact sheet.

Soil Interactions with Copper

Copper applied as a foliar disease preventative is sprayed on the plant leaves. However, copper washes off the leaf tissue during rainfall or with overhead irrigation and enters the soil. Therefore, frequent applications are necessary to maintain copper on the leaf surfaces. Soil texture and pH play a role in the activity of copper in the soil, including its ability to bind to surfaces or leach out of the soil. Copper binds more readily to well-decomposed organic matter and clay in the soil, so high organic matter content soils and clay textured soils tend to have higher total copper levels. Soil pH levels also affect the availability of copper; the higher the soil pH, the more likely copper is to bind to clay and soil organic matter. Available copper can be leached out of soils during rain events and heavy irrigation. Plant uptake of copper from the soil can vary greatly, with some plants, such as potatoes, acting as bio accumulators.

Soil Testing to Evaluate Soil Copper Levels

Growers should soil test for pH and fertility levels regularly when producing food crops. The soluble levels of copper can be measured through a standard nutrient soil test. The Rutgers Soil Testing Laboratory offers testing services which include macro-nutrients, micro-nutrients, and soil pH. For an additional fee, the lab will report on the percentage of soil organic matter. This information is valuable in understanding existing soil copper levels and the potential for leaching or elevated total copper levels in the soil. Lab results should be compared from year to year to track changes over the long term. Research sampling results show great variation in soil copper levels from farm soils, which were not necessarily dependent on their history of copper use. Regular soil testing is the best way to understand soil copper levels changes over time on your farm.

Tissue analysis can be used to determine crop uptake of copper. Tissue samples can be sent overnight to labs specializing in plant tissue analysis. Rutgers researchers found lettuce, which is more sensitive to high levels of soil copper, did not take up excessive levels of copper, nor did it express any toxicity symptoms when grown in soils with a history of frequent copper sprays.

Proper Use of Copper Fungicides

Copper fungicides are pesticides and should be used according to state law and the product label. When copper fungicides are used in a farming operation, New Jersey state law requires that the applicator have a pesticide applicator license for both organic and conventional applications. For information on how to obtain a pesticide applicator's license see the New Jersey Department of Environmental Protection’s Pesticide Licenses and Product Registration webpage, or you can obtain more information from your local Rutgers NJAES Cooperative Extension office. Applicators in other states should check on their state’s specific application and licensing requirements.

Growers who use copper should be mindful of their copper applications and monitor their soil nutrient levels through an annual soil test. Regular soil testing of farm soils will give an idea of how one’s farming practices are affecting soil copper levels. Copper use will increase levels that can be measured by soil testing. When looking at your soil test report keep in mind that soil copper levels below 5ppm are considered deficient and that copper levels above 20ppm should be monitored.

The following best management practices are advised when using copper fungicides:

1. Read and follow the product label.
2. Certified organic producers should verify that the product is listed on their organic system plan and that it is approved by their certifier.
3. Adhere to the listed re-entry period as stated on the product label.
4. Crop rotations should be used to reduce multiple-year applications to fields. For example: rotate crops that you are likely to apply copper on with crops that are less likely to benefit from a copper application.
5. Copper-based fungicides are preventative and should be used prior to disease infection.
6. Track rainfall amounts to help make decisions on spray schedules for copper applications. Less than one inch of rainfall may not warrant re-application. Between 1 and 2 inches of rain may not require reapplication; however you may want to cut the interval in half (example: if you are on a 10-day application schedule and it rains, you could spray within 5 days of the last application due to rainfall. Greater than 2 inches of rainfall may warrant a full reapplication to protect the plant, even if the last application was within a few days.
7. Care should be taken to spray only the foliage of the target crop and try to minimize ground spraying of copper.
8. Soil copper levels should be monitored annually through a reputable soil testing laboratory.
9. Growers should be mindful of animal manures and feeds that may contribute to further copper buildup.

Copper-based fungicides are an important disease management tool for both organic and conventional growers. When using copper for disease control the applicator should always follow the pesticide label instructions. Additionally, applicators should consider proper application techniques, track rainfall conditions, schedule overhead irrigation, soil test, and use other management strategies to ensure proper and efficient use of copper for disease management.

January 2020