RED DEER – The theory that calcium compounds can control sclerotinia stem rot in canola is based more on anecdotal than scientific evidence, but an Alberta researcher says it has a strong scientific rationale.
Stephen Strelkov, a researcher with the University of Alberta, told the recent Agronomy Update in Red Deer that calcium’s ability to control sclerotinia is believed to result from its ability to bind oxalic acid, a small molecule produced by sclerotinia sclerotiorum and other fungal pathogens.
Oxalic acid behaves as a non-specific toxin and is important for disease development. The amount of oxalic acid that is produced strongly relates to the virulence of the pathogen.
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Strelkov said strains that produce more oxalic acid produce more disease than strains that don’t produce oxalic acid.
“The exact role of oxalic acid in disease development is not completely understood, but it’s believed to result from its ability to remove calcium ions that are bound to pectins in the host cell walls,” he said.
“The calcium is bound to the pectin in the form of calcium pectate. The oxalic acid takes the calcium out of the cell walls, making them more susceptible to degradation by enzymes produced by the fungus. Oxalic acid also shifts the pH of the infected tissues lower, bringing it closer to the optimum for cell wall degrading enzymes.”
In other research, oxalic acid has been shown to block one of the defence mechanisms that host plants such as tobacco and soybean have against pathogens. Strelkov said it’s likely the same effect happens in canola.
Oxalic acid also has an effect on proper stomata closure, so plant stomata stay open abnormally. The plants use more water, resulting in the wilting response associated with sclerotinia stem rot.
“Given this importance of oxalic acid and its interaction with calcium, the question is, ‘if you add sufficient quantities of extra calcium to chemically tie up all the oxalic acid being produced by the pathogen, can you control the disease?’ “
Strelkov said preliminary greenhouse studies were conducted a number of years ago that looked at calcium chloride, calcium carbonate and calcium nitrate as foliar sprays to control stem rot.
“But they did this only in greenhouse studies, so the results should be taken with caution. They found all three compounds significantly reduced the pathogenesis of sclerotinia sclerotiorum. Calcium chloride and calcium nitrate had the strongest effects.”
In that study, the researcher’s hypothesis was that if enough calcium was added, it may be possible to prevent infection. However, Strelkov said little additional work has been done on the topic.
“Similar results have shown equally positive results in greenhouse studies in closely related fungi, but the results from field studies are more mixed. There’s not much hard data out there.”
Strelkov said a North Dakota State University study into foliar calcium’s ability to control white mould on dry beans found that calcium treatments had higher infection rates than fungicide treatments, but lower infection rates than untreated controls. He said the study concluded that calcium provided a viable alternative to control sclerotinia in the field.
“(Another) researcher from the same university looked at the use of calcium compounds for control of stem rot in canola and found the calcium compounds had no effect on disease severity,” Strelkov said.
“But I think there were problems with this study because neither did the fungicide treatment. I believe this was related to the timing of the fungicide application.”
Given the interest in the topic, Agricore United decided to conduct a preliminary field study in 2005 to look at using calcium for controlling sclerotinia. At the U of A, Strelkov became involved by helping evaluate disease symptoms and analyzing the results.
A similar study was under way with the Gateway Research Association in Westlock, Alta., and Agriculture Canada in Lacombe, Alta., but the two groups didn’t become aware of each other until it was too late to co-ordinate for 2005. In 2006, they plan to collaborate their studies.
Strelkov’s group looked at seven treatments: calcium carbonate at seven pounds per acre; calcium chloride at three gallons per acre; calcium carbonate and calcium chloride in combination; calcium carbonate with a half rate of Rovral; calcium chloride with a half rate of Rovral; the recommended rate of Rovral and an untreated check.
Each treatment was replicated four times, but was not randomized and has been done for only one year, so results should be taken with caution.
Treatments were applied at full flower, with plots rated in August for main stem and branch infection. No disease was found at the first site so no analysis was done there. The second site had a mild infection, but there was enough to do some analysis.
He said results from the calcium carbonate, calcium chloride and mixed treatments were not significantly different from the untreated check, but the half Rovral with calcium carbonate, half Rovral with calcium chloride and full Rovral treatments were significantly different from the untreated check, which the researchers had expected.
“In this preliminary study, there was no significant effect observed from calcium application, but the experiment was not properly randomized, disease pressure was low and we didn’t have a half Rovral treatment, which would have allowed us to see any potential synergistic interactions between the calcium and the fungicide.”
