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Canola pod abortion investigated

Pod abortion is not a new story, but it can be puzzling. In 2006, the overwhelming reason for poor podding and seed set was the hot and dry conditions throughout most of the prairies in July and August. No surprise there. But in 2005 in central Alberta, many growers and agronomists puzzled over canola plants with poor pod development when environmental conditions were very favourable.

Agronomists, producers and researchers set out to find out why. The most common symptom seemed to be plants with a few normal pods on the bottom of the main raceme (flower
cluster along the main stem), then a
significant number of undeveloped pods in the middle of the main raceme and then normal appearing pods on top. The suggested causes included variety/ herbicide tolerance issues, herbicide tank residues, herbicide soil residues, disease, fertility and poor root structure.

“In investigating the problems on different fields, we didn’t find an answer that would cover all the fields, but we did eliminate a lot of possibilities,” says Doon Pauly, a crop specialist with the Alberta Ag-Info Centre in Stettler.

Bloated and stubby pods can be an indication of heat stress, moisture stress, or sulphur deficiency, says Murray Hartman, provincial oilseed specialist with Alberta Agriculture, Food and Rural Development (AAFRD). Flower blast, on the other hand, can indicate heat/moisture stress, sulphur or boron deficiency and lygus bug damage.

With sulphur or boron deficiency, the abnormal pods and flower blast tend to get worse up the main raceme, unless a subsoil layer of sulphate is exploited by roots later in the season.

So, Hartman says a section or branches with poor pods followed by normal pods would probably indicate periods of heat or moisture stress, as was common in 2006.

Hartman says that sections of a raceme with flower blast (just the stubs left) could be caused by periods of heat or drought, or by lygus bug feeding on a flower cluster. With lygus bug, the flower blast can be irregular on a few plants with sections of flower blast and other plants with less or more damage. With heat or drought, the flower blast is more uniform. However, if the plant emergence was uneven, then the flower blast due to heat or drought may be uneven as well.

What about herbicide injury in herbicide resistant canola?
With the widespread adoption of herbicide resistant canola (HRC), some growers and agronomists wondered if poor herbicide tolerance was a cause of pod abortion in 2005, especially as it relates to glyphosate. Neil Harker, a research scientist with Agriculture and Agri-Food Canada at Lacombe, conducted a comprehensive literature search to see if any scientific papers had been published on pod abortion due to poor herbicide tolerance in HR canola.

“In the literature, we didn’t find a problem,” says Harker, “but it did raise some questions.”
Mark Kidnie, a researcher with Monsanto Canada at Winnipeg, says the event that confers glyphosate tolerance in canola is either there or it is not. If the event is present, Roundup Ready canola will be tolerant to glyphosate at labelled rates with no adverse effect on yield.

Kidnie says that all commercial Roundup Ready varieties are tested for tolerance at twice the label rate of glyphosate. He says that application beyond the recommended leaf stage or at a higher rate is not recommended, since this may lead to additional plant stress. Kidnie also says that Monsanto has never seen any differences in Roundup Ready tolerance when applied with different formulations of Roundup herbicide.

Greenhouse research by Brian Schilling, who collaborated with Harker and George Clayton of AAFC Lethbridge, indicated that glyphosate could reduce canola shoot dry weight even at recommended leaf stages and application rates. However, when these same trials were conducted in the field, Harker says that they did not see any effects on canola shoot development.

“None of the field trials included application after the recommended six leaf stage of canola, and that could be a factor,” says Harker.

In 2005, anecdotal evidence from the field indicated that some growers might have applied glyphosate to Roundup Ready canola at later than recommended stages when flowers were visible. The late applications occurred because growers had a hard time getting into the field due to wet weather that delayed spraying.

Harker says it is possible that late spraying could have compromised the tolerance. In some field trials, Harker and Clayton observed glyphosate injury to Roundup Ready canola under field conditions when applied late. The recommended window for spraying glyphosate is cotyledon to six leaf stage. However, since 1996, the Roundup label has cautioned that applications at the four to six leaf stage may cause temporary yellowing of the crop.

Pauly says it was evident that there were a number of fields with problems in pod development by the end of 2005. Most were likely associated with some sort of stress. Some fields were successfully diagnosed to fertility problems or sprayer tank contamination and some to root maggot damage. The jury is still out on herbicide stress.

Harker says that pod abortion due to late glyphosate application is still a question mark and that more research into the issue is required. In the meantime, researchers and agronomists alike recommend staying within label guidelines when it comes to application timing and rates. And if pod abortion looks suspicious next season, Pauly recommends working with local agronomists to try to diagnose the stress that might have caused the problem.