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Using information from their analysis INRA scientists in Versailles-Grignon have reconstructed the most probable migration patterns and datelines for invasion by the plant pathogen. They said their work has shed new light on the process of biological invasion by which the disease spreads, and identified potential new opportunities for controlling the rot.

At a global level, Leptosphaeria maculans, the fungus that causes collar rot in plants of the crucifers family (which includes cabbage, turnip, oilseed rape, mustard and horseradish), is responsible for major yield losses of between 5 and 20 percent, or even the local destruction of entire plots. 

The INRA scientists said pathogens affecting wild plants can adapt to agricultural plants in the same family (referred to as host jumping), which are susceptible as they are commonly cultivated on large surface areas. This causes the emergence of new plant diseases.  These pathogens are then spread by human activity to new areas. 

Based on their knowledge of the way these diseases occur and are spread, the scientists attempted to develop a number of recommendations aimed at predicting the potential of the disease to spread, preventing this spread and reducing the impact in the event of its introduction to a new region.

A fungus originating from North America?

The INRA scientists have analysed over 450 strains of Leptosphaeria maculans from eleven different parts of the world (including Europe, the Americas and Australia). They found that these strains belong to genetically isolated populations that correspond to their geographical origins (Europe, Australia, USA, Chile and Canada). 

According to the scientists, different climatic conditions and other variables which may lead to changes, e.g. shorter oilseed rape developmental cycle in Canadian climate, affect the type of fungus in the region. From this starting point, the INRA researchers used simulations to determine the most probable scenarios for migration.

Looking back, they surmised the fungus probably originated in the USA. It is though the first known outbreaks of the disease were at the beginning of the 20th century in cabbage plants.  It then probably spread to Australia and Eastern Canada about a hundred years ago, and then the provinces of central Canada and Europe 50 years ago.

The development of appropriate resistance strategies

The possible future scenarios identified reveal the risk posed by the fungus. Certain fungi studied display long-distance migration, while some can only be disseminated over a distance of a few metres.  The researchers said this would have to be taken into account in developing strategies for disease control and selecting varieties for resistance to strains of the disease.

They said Canada will have particular difficulty implementing control measures due to the reproduction patterns, genetic diversity and virulence of fungus in the region. Central Canada, a major producer of oilseed rape, will prove particularly troublesome.

Europe, in contrast, will have an easier ride. Here sustainable management strategies for varietal resistance can be expected to be met with more success. INRA scientists warned that policy makers in these different global regions should avoid implementing the same strategies which, due to the diversity of the disease, will prove less effective in some areas than others.

They warned that current trade and agricultural practices have created the ideal conditions for the fungus to continue its spread over large distances. The scientists said, “The globalisation of trade and the homogenisation of cultivated species are particularly favourable to this type of dissemination.”

Although not thought to be present across large parts of Asia, scientists said monitoring is increasing in China, the world’s largest global oilseed rape producer, particularly because traditional Chinese varieties are highly susceptible to the fungus.

* It’s all about sex.  Phoma Lingam is the anamorph of Leptosphaeria maculans, which means that it’s the name used for the disease when it’s reproducing asexually.  So they are the same thing.

For completeness L. maculans is referred to as the teleomorph stage of the life cycle (reproducing sexually).

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