LETHBRIDGE – Twisted pea plants and purple-tinged canola can be signs of herbicide damage.
Sandi Scott, a research technician at the Alberta Research Council, works like a detective when diagnosing the injured plants that arrive at her laboratory every year.
Like any good investigator, she needs to know the history and the whereabouts of those sad specimens, she told farmers at the Southern Alberta Research Association’s crop diagnostic school held in Lethbridge July 7.
Canola is especially susceptible to damage from herbicide residue from previous crops, but discolouration or misshapen leaves could also indicate insect damage, frost or nutrient deficiency.
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Cold stress generally causes more uniform damage across a field, while herbicide damage is more spotty.
Purpling and cupping of canola leaves could indicate low sulfur levels or herbicide damage.
“We can confirm injury you have seen in the field or we can predict it,” Scott said, but to do so she needs information such as soil pH, rainfall and organic matter in the soil.
“This test by itself isn’t foolproof,” she said. “We want it used with a lot of other parameters for the best outcome. Because of the variability in a field, we can’t say 100 percent.”
She also recommends checking fields at early leaf stages and recording the damage, symptoms and recovery. Photographs are also a good idea.
Canola can be injured if exposed to low soil concentrations of some Group 2 herbicides. Damage ranges from minimal plant injury to total crop loss.
Group 2 herbicides block an enzyme that produces amino acid and starves the plant to death. Group 4 chemicals may cause swelling in the stems.
Producers should read labels to determine how long to wait before planting canola in fields where certain herbicides were applied. With herbicides such as Glean or Pursuit, they may have to wait up to six years.
Weed control companies working for the oil and gas industry often use industrial strength herbicides around well sites, which could also affect surrounding crops for years to come.
Most plant injury occurs when the normal microbial breakdown of herbicide chemicals is affected by drought, low organic matter and soil pH.
“Acid soils encourage the carryover of these products,” Scott said.
Her lab does not see many samples from the Edmonton to Red Deer corridor because of richer soil and good moisture, but damage was reported in drought years such as 2002.
Soil is more alkaline around Lethbridge and acidic around Vegre-ville, which means damage is seen more often in those areas.
Scott expects to see problems this year because of dryness.
Damage can be caused by spray drift from other fields, usually along the edges, or from spray tank contamination in which tanks, booms, filters or sump pumps were not cleaned properly between applications.
Herbicides such as 2,4-D could drift farther into a field. In young plants, drift injury can be distinguished from soil residue injury because drift causes damage on a larger leaf area.
Tordon, which many municipalities use for road and ditch weed control, can create runoff problems that can persist for 10 years.
It can cause swollen stems and twisted plants in peas and damage tomatoes if it seeps into irrigation water used in greenhouses.
“Once you get it in well water, you can’t get it out,” she said.
Producers who submit samples for bioassays or chemical analysis should mention what kind of chemical damage is suspected so technicians know where to start their search.
Bioassays detect herbicides at a lower level but do not name the actual chemical.
“If you were going through some legal decision, you would have to have the actual chemical from a chemical analysis, ” she said.
It is also important to check plants early.
Damage to cotyledons could be confused with frost damage. Severe injury to the cotyledons are unlikely to form true leaves, mature or flower.
