Lack of disease resistance information hinders control

Australian farmers have carried out an ongoing struggle against blackleg.

Genetic resistance hadn’t been keeping up to the onslaught of virulent pathogen types by the early 1990s, but then a new source of resistance was incorporated into canola from a wild relative, brassica rapa subsp sylvestris.

The sylvestris resistance, which provided what looked like an almost complete immunity to blackleg, was incorporated into a variety and introduced as Surpass 400 in 1999.

Surpass 400 was the dominant variety grown in Australia by 2001, but the resistant gene had totally collapsed two years later and crops were devastated in the Eyre Peninsula region of South Australia.

This tale illustrates what can happen when we rely on single gene resistance to overcome a pathogen.

Sadly, it is not isolated.

France had a similar experience in the 1990s, again with canola and blackleg.

We are also experiencing a similar breakdown in canola in Canada, but this time it is with clubroot.

Articles calling on farmers to rotate clubroot resistant varieties have been appearing in the western Canadian farm press as recently as 2014.

Little did we realize that all of the available varieties carried the same resistance mechanism.

There are 15 hybrids, by my count, that now claim resistance to clubroot, and from what I have been able to determine, 14 of them carry the same resistance mechanism derived from the European variety Mendel.

This leaves us with the following best management practices for controlling the spread of clubroot:

  • canola rotations with more than a two-year break
  • avoiding fields that are known to have high clubroot inoculum
  • avoiding fields that are not scouted for clubroot regularly
  • planting canola hybrids that have the same resistance mechanisms in that rotation
  • control canola volunteers and other clubroot susceptible weeds that may increase the pathogen in the non-canola years
  • avoiding tillage that is more than zero till
  • avoiding operations that limit soil movement between fields and keeping soil in the field where it belongs
  • cleaning soil and mud from equipment before moving to a new field
  • practising proper bio-sanitary procedures when moving between fields

For years, we have promoted the use of rotation when it comes to herbicide groups and weed resistance. It has proven to be effective in delaying the onset of herbicide resistant weeds.

The industry provides us with helpful tools such as listing herbicide groups on the label. These groups are based on modes of action.

Farmers and agronomists are able to use this information to build a plan that uses a mixture of groups. These tools are also available for fungicides and insecticides.

Farmers should strive to rotate pesticide modes of action as well as resistance genes in the crop to improve sustainability and reduce production risk.

Why then, are there not tools like this that would show the basis of resistance for blackleg and clubroot?

Industry experts say simple screening tools are available that can determine what races of blackleg to which varieties and hybrids are resistant.

I have also been told that seed companies are reluctant to publish this information because it is complicated and “proprietary knowledge.”

We know that more than 60 percent of current hybrids carry the same Rlm3 gene for blackleg resistance, which sets us up for a repeat of the disasters experienced by the canola-oilseed rapes growing areas of Australia and France.

Ironically, many of the seed companies that are resisting the release of this disease resistance information are the same ones who vehemently preach the virtues of rotating herbicides, especially if they have an alternative product.

A tool like this would be a great benefit to farmers and agronomists who want to use rotation effectively as a best management practice. The same principle could also be applied to clubroot resistance.

The next time you are talking to canola seed company representatives, ask them why information about the nature of the resistance in their canola hybrids is not readily available for your planning and implore them to pass this request up the chain. When a company does divulge this deep dark secret, show your gratitude by supporting them. You can change the industry.

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