The landmark trial of a South West farmer accused of contaminating his neighbour's crops with genetically modified canola has wrapped up in the Supreme Court.

Organic farmer Steve Marsh is suing his neighbour Michael Baxter for $85,000 in damages after GM canola allegedly blew from Mr Baxter's crop onto Mr Marsh's land in 2010.

Mr Marsh claims the contamination caused him to lose his organic certification on more than half his property for almost three years.

In the Supreme Court today, Mr Marsh's lawyers argued Michael Baxter showed negligence because he could reasonably have foreseen the risks of the seeds being spread by swathing his crops.

Mr Baxter's lawyers told the court yesterday their client was simply exercising his right to grow a crop that is judged safe and legal by the State Government.

The case has been closely watched by farmers and anti-GM activists as it may have wide-reaching implications on the production of genetically modified crops in WA.

Mr Marsh's campaign has been financially backed by the Safe Food Foundation.

The foundation's director Scott Kinnear said the organisation had invested at least $750,000 in the lawsuit.

The Pastoralists and Graziers Association have confirmed they set up a fund to provide financial backing to Mr Baxter.

A moratorium was placed on the commercial cultivation of GM crops in WA in 2004. The order was enacted by the then Labor government under the Genetically Modified Crops Free Areas Act 2003.

The moratorium was lifted prior to the 2010 season, and the number of farmers growing GM canola has risen.

There were 317 farmers cultivating GM canola in 2010.

This jumped to 325 in 2011, 350 in 2012 and 406 last year.

In 2013 almost 17 per cent of all canola sown in WA was a genetically modified variety, according to figures provided by the Department of Agriculture and Food.

Judge Kenneth Martin is expected to hand down his verdict in the coming months.