Features Herbicides Seed & Chemical
CSI: Crop Symptom Investigation: diagnosing manganese toxicity

The field problems presented to Emile deMilliano, a certified crop advisor and manager of Agronomic Services with Viterra at Fort Saskatchewan, Alberta, was a head-scratcher. Symptoms of poor canola growth had been reported in a few fields in northern Alberta in 2009 around Mayerthorpe, and a few more in 2010. Whereas most showed minor symptoms, one field in particular at Lac La Biche in 2010 had about one-third of the field affected.

The symptoms showed up around the three to four leaf stage of Roundup Ready canola. In general, deMilliano says the symptoms included chlorosis of leaf margins, and cupping of leaves was evident. However, chlorotic mottling of the entire leaf (leaves) and overall stunting of plant growth were apparent in areas most severely affected. “Our first intuition was a sulphur deficiency, but we ruled that out,” says deMilliano, who worked with Rigas Karamanos, Viterra’s manager of Agronomy Services in Calgary. “As we worked through the diagnostics, it was certainly one of the most unique challenges we have encountered.”

Sulphur (S) was ruled out on the Lac La Biche field because a sufficient amount of ammonium sulphate was applied (more than 20 lbs of actual S per acre) in 2010. According to deMilliano, the field history indicated canola had been planted at least every two years and more than 20 lbs of S per acre in the form of ammonium sulphate were applied each time. The rainfall for the spring growth, four inches, was spread out across the two months of spring growth. This amount of precipitation was not adequate to leach the sulphur out of the rooting zone, says deMilliano.

Another clue in this classic case of crop diagnostics appeared when the field did get rain later in the spring, the crop came back to life, and while vegetative parts of these plants continued to exhibit symptoms, flowering and pod set appeared relatively or reasonably normal. The crop set seed, and matured normally. On the other hand, severe sulphur deficiency is evident with prolonged flowering, flower sterility and poor pod set.

Herbicide residue was also ruled out, after looking at herbicide application records. Next, deMilliano ordered up full soil and tissue test analyses, comparing unaffected and affected areas of the Lac La Biche field. “I wanted to look at all the parameters and make comparisons. I was looking for a correlation between symptoms and possible reasons,” explains deMilliano.

DeMilliano and Karamanos then hit the books, doing a literature search on possible causes. As they narrowed down the search, aluminum (Al) and manganese (Mn) toxicity became possible causes.  Karamanos also looked at symptomology of manganese toxicity in Australia, where it is common.

Soil pH provided another clue. The poor areas had strongly to very strongly acid soils, and at these levels, soluble forms of aluminum, iron and manganese can form. Below pH 5.5, aluminum and manganese can increase to toxic levels.

The symptoms tended to point towards manganese toxicity, as aluminum toxicity restricts root growth, phosphorus uptake and translocation within the plant. Manganese toxicity causes chlorosis of leaf margins and cupping of leaves in canola, explains
deMilliano.

Defining the manganese toxicity problem
The evidence continued to pile up and point to manganese toxicity. DeMilliano’s literature research brought out more evidence, as shown in these excerpts:

• The problem of predicting whether or not a soil will be Mn toxic is, however, rather complicated. Information is required as to the crop grown and the soil Mn levels which will cause toxicity for that crop as well as the initial level of Mn available in the soil and, equally important, how this level will change during the season.
• Soil Mn levels were well correlated with pH and tissue Mn levels. Manganese toxicity symptoms were observed at tissue Mn levels of approximately 1,000 ppm in beans, 550 ppm in peas, and 200 ppm in barley.
• High manganese levels resulting from low pH and extreme  environmental conditions can also affect canola growth through reduced photosynthesis.  Although plants appear to overcome the toxic effects of manganese as the season progresses, the effect of reduced photosynthesis on final yields is still unknown.
• Depending on species, levels greater than 300 to 500 ppm in the mature leaf tissue indicate toxicity.
• To date, there has not been an empirical evaluation of the  impact of high soil Mn on canola yield under field conditions.  Normal levels of Mn content of canola with and without Mn  toxicity symptoms are about 400 mg kg-1 and greater than 1500 mg kg-1, respectively.
• High Mn is prevalent in acid soils and varies widely during the season, making it difficult to predict when or where it will occur and how high the soil Mn concentration will be.  These preliminary results hinted at the possibility of canola yield being reduced by a decline in leaf area index (LAI) induced by Mn toxicity.
• Soil pH was the best measure of predicting Mn status of soybean growing on acid soils. Toxic concentrations of Mn did not accumulate in soil or in leaf tissue at pH levels > 5.5.

Case closed
Based on the evidence, deMilliano and Karamanos believed manganese toxicity was the culprit. DeMilliano explains in a summary that “areas that appeared most affected during the vegetative stage (severe chlorosis and significant stunting), cumulatively amounted to a low percentage of the total acreage in the field. Areas with mild to moderate symptoms accounted for a larger percentage (maybe 10 to 15 percent of the field) but revealed fewer symptoms through the flowering and pod stages. This may be due to a ‘dilution’ of Mn concentrations with additional rains and slight changes in pH. The yield impact on these areas is more difficult to quantify but I suspect is far lower than initially expected.”

To correct the manganese toxicity problem, liming would be one tactic aimed at increasing soil pH.  However, the cost can be prohibitive, especially with debatable returns given the lack of information on yield loss due to Mn toxicity in canola in Western Canada. DeMilliano hopes to continue to monitor the situation, to assess yield loss, and determine the impact on rotational cereal crops. “Solving a case like this takes a process of elimination. You look at all the factors and try to eliminate the possible causes,” says DeMilliano. “While the process might be obvious, being able to compare good and bad areas of a field forms an excellent basis for investigating many crop diagnostic cases.”