The source of potato herbicide injury isn’t always easy to pin down, especially when the usual suspects aren’t the guilty ones.
During a recent Ontario Potato Board webinar, Andy Robinson, extension potato agronomist at North Dakota State University/University of Minnesota (NDSU/UMN,) said pinpointing how herbicides came into contact with a plant is often a challenge but knowing the signs helps.
Why it matters: Herbicide injury can have a significant impact on a potato stand, its ability to transform sunlight into carbohydrates, and can result in reduced yield and marketability.
Herbicide injury generally occurs through seed contamination, soil carryover, drift, spray equipment contamination or misapplication. It results in poor emergence, chlorosis, necrosis and distortion of growth in plants and tubers.
Initiating some simple on-farm management protocols can mitigate instances of drift, spray tank contamination, misapplication and soil carryover – recognizing and addressing seed contamination usually involves lab testing for residue or growing a few seeds out to test the quality.
“Make sure you follow your labels and do whatever your labels allow you to do. That’s the important thing,” said Robinson. “The labels are really there to protect you from causing yourself problems.”
Soil carryover impacts symptoms that can be present in wide swaths or within the whole field and display as emergence issues, pruned, brown or bottle-brushed roots, stunted growth after emergence, shortened stolons and early tuber set including malformed tubers.
“If you can detect (an injury) pattern in a big area of the field it tends to be a soil carryover issue,” he said but cautioned environmental stress, nutritional imbalances, disease, viruses and sometimes genetics also cause cracked tubers and mimic herbicide injury.
Using an image where emergence on half the field was uniform while the other side was barren, Robinson said initially one would assume it was soil crossover.
Upon closer inspection, they found the left side was fumigated against a high population of root-lesion nematodes, but not the right where the tubers, once dug up, showed RLN penetration at the stems.
While soil carryover injury symptoms are usually linear in a field, drift shows the bulk of the injury at the edge of the field and then, depending on wind direction and strength, lowering along a gradient into the field. Misapplication and tank contamination generally have concentrated areas.
“Things can look very similar to herbicide (injury) which is why looking at the patterns in the field is important,” Robinson said. “You can also take samples and send them in for analysis to determine what it might be.”
Robinson said ALS inhibitors (Group 2), growth regulators (Group 4), and EPSP synthase inhibitors (Group 9) are the usual suspects with translocation herbicide injury occurring on new growth points, including leaves, tuber and roots and contact herbicide injury impacting whatever it touches but doesn’t move within the plant. Both styles impact tuber growth.
Herbicide injuries can appear within days or over weeks, he said.
Glyphosate symptoms, young leaves yellowing and tubers cracking, occur within one to three days.
The ALS inhibitors symptoms of yellowing of new growth, leaflets elongating and wrinkling, boat-shaped leaflet tips and cracking tubers, occur much later and late-season injury generally shows no symptomology on the tubers or plant leaves because all the energy is focused on tuber growth.
“It’s important to document (injury) right away because over time, typically, injury symptoms will worsen or get hidden under new growth,” he said. “It makes it more difficult to identify, and it makes it more difficult to get a positive lab test as the plant is breaking down.”