Just as the last clouds of Tropical Storm Cindy were rolling out, we received a call from a grower about a "rust" spreading across the nursery. The grower described it as coming out of nowhere and affecting most of his crops. Typically when a grower describes a problem occurring suddenly and across many plant species it is rarely a disease or biotic problem. The grower indicated that he recently sprayed CuPro 5000 (by SePro) just before the rain set in for the next 4 days. Being familiar with this facility, we knew that the well water pH was often below pH 5.5. With those variables in mind we immediately suspected a copper burn. Most copper products formulated with the active ingredient copper hydroxide warn applicators about using tank water with a pH below pH 6.5. It also warns about using this product during conditions of extended wet weather (Figure 1). The precautions are in one of those boxes at the front of a label that we are all guilty of skimming over.
Figure 1. Excerpt from CuPro 5000 label listing special precautions with acidic spray solution. https://www.sepro.com/documents/CuPRO-5000_Label.pdf
CuPro 5000 and several other copper fungicide/bactericides are formulated as copper hydroxide. Copper hydroxide is a more stable form of copper than copper sulfate. This formulation allows for long residuals and safe application of high concentrations. Copper hydroxide has the chemical formula Cu(OH)2. If you break down this formula Cu is copper and (OH)2 is hydroxide. Think of the hydroxide as a base (opposite of an acid) and think of acid as the concentration of hydrogen in a solution. The greater the concentration of hydrogen the lower the pH of a solution. When copper hydroxide mixes with acidic water, hydrogen replaces copper and combines with hydroxide to form water (H2O). In this scenario, the copper is now free and potentially at a toxic concentration. If the spray solution pH remains above 6.5, then the copper will become available slowly and safely. When the pH is too low there is too much hydrogen available and copper becomes available at too fast a rate and can cause problems.
H + Cu(OH)2 = H2O + Cu
After visiting the nursery we found that our suspensions were correct. All fingers pointed to a copper burn. Symptoms included necrotic tissue and leaf drop. Peaches are very sensitive to copper when in leaf. Often copper damage to peaches is seen as a yellowing of the foliage and then defoliation. In this case it showed up as red to brown necrotic areas on foliage and some leaf drop (figure 2), possibly because the damage occurred so rapidly. A dead giveaway was that damage matched spray patterns. Silhouettes were observed where leaves overlapped and shielded lower leaves from the copper spray (Figure 3).
Figure 2. Example of copper phytotoxicity on peach
Figure 3. Spray pattern on fig.
The grower has used copper at this stage and season in the past without any issues. What was the different this time? Here are two possible reasons:
We are all guilty of skimming labels, and this scenario is a good reminder for us all to read the entire label. It may be worth the time writing notes on pesticides to remind applicators of special precautions and restrictions. If you do not have a pH meter to check your irrigation or tank water, for little expense you can use the pool testing litmus paper kits. Copper can be a safe pesticide if used correctly but even the safe products have precautions and labels restrictions. Another important item on many copper labels is a long restrictive reentry interval (REI). Cupro 5000 has a REI of 48 hours. The label also requires eye flushing equipment available to employees entering the copper treated area during the 48 hour REI. If you have a questions about pesticide labels contact your Commercial Horticulture Extension Agent or the product manufacturer.
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