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30 POTATO GROWER | APRIL 2018 Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In HERBICIDES | By Andy Robinson, North Dakota State University/ University of Minnesota What dicamba damage does to potatoes, and how to fi ght it Injury Prone Plant injury from dicamba has been in the news for the past two years. All this talk about dicamba injury is often associated with the release of dicamba-tolerant soybean, or the RoundupReady 2 Xtend soybean. However, dicamba is not new and has been in use for several decades in cereal crops, on pastureland, and as a weed burndown. In the potato world, there are always concerns of potato plants' unintended exposure to herbicides. Low rates of herbicides in potato can cause yield reduction, reduce marketable potatoes, or cause complete crop loss. I could go on and on about all the different and crazy herbicide injury stories, but let's talk here about dicamba, how it affects potatoes, and how to protect your potato crop. We have been gathering data to determine what effect dicamba has on seed tubers and potato plants. Before we dive into this, let's focus on dicamba for a moment. Dicamba is a Group 4 herbicide, commonly called a synthetic auxin or plant growth regulator. Dicamba is a translocating herbicide, meaning once it enters the plant it will travel to that plant's growing points—the youngest leaves, roots and tubers (if present). This is why when a potato plant is exposed to a low amount of dicamba, the injury is seen in the youngest leaves a week or two later. Injury manifests as wavy leaf margins, crinkled leaves, cupping or curled leaves, twisted stem and leaves, or callused stems. Similar symptoms can occur from other plant growth regulator herbicides; thus, it is important to take a sample of the injured leaves and have them tested at a laboratory for herbicide residues to verify the specific chemical causing the injury. Dicamba injury on potato tubers is expressed as elephant hide, smaller tuber, and/or malformed/cracked tubers. We typically see greater tuber malformations as a result of herbicide injury when plants are exposed during tuber initiation through early tuber bulking. At high enough concentrations, dicamba residues can carry over in tubers. This is problematic for commercial production, because dicamba residues are not allowed by the EPA Malformations of daughter tubers from dicamba injury that occurred at tuber initiation include elephant hide, malformed tubers and/or cracked tubers.