Manganese toxicity found in canola

Manganese toxicity in a canola crop has been detected for the first time in Western Australia.

The problem is common in Eastern States crops.

The distinct symptoms of spotted edges and yellowing between leaf veins and edges were noted by a Midwest grower, which prompted Elders agronomist Belinda Eastough and Department of Agriculture and Food WA Research Officer Martin Harries to investigate further.

"Low soil pH, acid soil, increases plant available manganese in soil causing higher levels than usual to be absorbed by plants leading to toxicity," Mr Harries said.

Tissue samples and soil pH tests carried out on affected plants and paddocks showed results consistent with manganese toxicity.

"In one paddock, lime had been top dressed prior to sowing, meaning the pH at the root zone differed quite a bit," Ms Eastough said.

"We tested the pH of soil around the roots of plants and the results showed plants with roots in soil of pH below 4.5 were unhealthy while those growing in soil pH of about 6.5 were really healthy."

The difference in manganese leaf tissue concentration between healthy and unhealthy plants was 1287mg/kg.

The critical level indicating too much manganese in leaf tissue is about 800mg/kg and some unhealthy plants spiked at 1736mg - more than double.

Mr Harries said it is unlikely manganese toxicity will become a major problem in Western Australia but does act as a timely reminder pH levels have a large contributing effect on successful canola growing.

"As part of the DAFWA Focus Paddock Project we have been testing soils to 90cm depth and it has been interesting to see about 20 per cent of canola planting has occurred where the top soil pH is above 5.0 in calcium chloride, while pH below 10cm has been below the recommended pH of 4.8," he said.

"This is occurring because canola is replacing lupins as break crops on many soil types including the acidic sands that would traditionally go to lupin." DAFWA research officer Chris Gazey said a much broader survey of pH across WA has shown that soils historically showing appropriate pH are becoming acidic and it is a prime example of the importance of effective pH management.

"It is not surprising symptoms such as manganese toxicity develop on these soils as they acidify," he said.