Manganese toxicity (Mn) in Canola has become a current issue due to the warm and dry conditions and late season rain which have been ideal for it’s prevalence in southern crops, although the potential for damage is not known.
Under normal autumnal conditions, Mn toxicity is not a problem as moisture, the chemistry of the soil and the soil microorganisms interact to decrease the availability of the trace element.
“Manganese is a trace element nutrient which is taken up by the plant, but its availability in the soil is affected by both the chemistry and the biology of the soil,” Dr Sergio Moroni, Lecturer, School of Agriculture and Wine Sciences, Charles Sturt University, Wagga Wagga said.
A researcher at the Graham Centre for Agricultural Innovation, Dr Moroni has studied the effect of Manganese toxicity on Canola and has noted the current prevalence among crops which he attributes to the very dry autumn which has interrupted the normal biological and chemical Mn cycle.
“During the hot summer the Mn slowly increases, especially in the top soil layer, and normally decreases through autumn … it is a yearly cycle,” he said.
“We’ve seen a dry and warm autumn and the microorganisms and soil chemistry have not had enough time to decrease the Mn so we end with high levels when the crop is sown and Canola is very susceptible to it.”
Dr Moroni noted with the late change in the season, the soil microbes are starting to decrease the Mn, but not quickly enough for the early emerging plants.
“Generally, the first two leaves turn yellow [from the toxicity], but as the plant grows and Mn slowly declines the new leaves do not show Mn toxicity symptoms.
“That is why you see it earlier in the season; and the other possibility is you might have Mn that has accumulated at greater soil depth and thus Mn toxicity symptoms may show up later on in the season as the roots grow deeper, but it is seldom seen.
“The third time that Mn toxicity can be a problem is when you see water logging which will reduce the availability of oxygen to allow the soil microbes to do their work and Mn starts increasing again.”
Manganese is a trace element nutrient which is taken up by the plant, but in the soil its availability is affected by both the chemistry and the biology of the soil- Dr Sergio Moroni
Dr Moroni said it is not beyond reasonable doubt to see Manganese toxicity around flowering time, although it is highly unusual and at this moment little is known whether the toxicity affects yield.
“We don’t know and because of the variability of Mn in the soil we cannot set up an experiment to test the affect on yield from Mn toxicity,” he said.
“But it only stands to reason that a plant affected by toxicity has less resources and any stress will set the plant back a little bit.”
Dr Moroni said it is possible there could be a yield penalty, and although there is no hard evidence, anecdotally with less resources available to the plant it cannot develop a deeper root system.
The lack of a more robust root system could very well be significant at the end of the season when the plant needs the extra moisture available at a deeper level.
If farmers are seeing yellowing of leaves in their Canola crops it could indicate Manganese toxicity but there is no cure for it.
“It indicates too much Manganese, not a deficiency,” Dr Moroni said.
“Plant tissue analysis would determine if it is indeed Mn toxicity, as symptoms of sulphur deficiency are similar.”
If producers have any doubt about the status of their Canola crops, Dr Moroni recommends samples of affected plants along with samples from non-affected plants are sent to their preferred laboratory for analysis.
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