NISKU, Alta. – As more farmers have adopted zero tillage, minimum tillage and other direct seeding methods during the past two decades, it has generated questions about increased disease risks.
Jeannie Gilbert, a cereal disease researcher at Agriculture Canada’s Cereal Research Centre in Winnipeg, said there’s no simple answer to which diseases increase with reduced tillage.
“There are some diseases much more associated with reduced tillage, like stubble borne diseases and soil borne diseases. Those that are wind borne, like rusts from the States, and viruses from aphids, are not really affected by tillage practices.”
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She said tillage changes soil moisture and temperature, which affects the biological activity of soil microorganisms. Direct seeding, with minimum or zero tillage, may favour some pathogens and limit others.
Gilbert said the common leaf spot pathogens that cause tan spot, septoria leaf blotch, stagonospora nodorum blotch and spot blotch all overwinter on crop residue and they can react differently.
A study in southern Manitoba found that tan spot was more prevalent in zero-till fields, septoria leaf blotch and stagonospora leaf blotch more prevalent in conventional-till fields and spot blotch was present equally in both tillage systems.
“With spot blotch, we found equal amounts. It didn’t matter whether you were in conventional tillage or no till. Spot blotch was there. We don’t know if it causes a great deal of yield loss or not,” she said.
“This fungi also causes black point on the seed and common root rot, so it’s a very prevalent species. We have a lot of them around.”
Gilbert said studies in Saskatchewan and Ontario reported higher tan spot levels in reduced tillage systems and higher levels of leaf blotch in conventional till.
“Tan spot is the one disease that is really associated with zero till. We find it only on wheat. It doesn’t affect barley or oats. It appears that the older stubble produces more inoculum than the fresh stubble. So it won’t be in the year after you had the disease, but if you grow wheat again, you’ll see tan spot,” she said.
“Your rotation may be wheat, something else, then back to wheat. It’s possible the stubble from two years before may provide you with the inoculum, if you’ve got the right conditions. You might end up with tan spot in your zero till. Tan spot really depends on the environment.”
Gilbert said a U.S. study has reported that higher levels of leaf spot diseases are associated with zero-till plots in years with higher than normal precipitation.
Leaf spot diseases were consistently reduced by application of adequate nitrogen.
Net blotch in barley was found to be more severe in zero-till fields than minimum or conventional-till fields, especially when barley was planted back into barley stubble. But most studies in Saskatchewan concluded that leaf spot diseases in wheat and barley are less affected by tillage than by weather conditions, nutrient status and rotation.
Glyphosate may also play a role in the level of leaf diseases in cereals. Gilbert said the rapid breakdown of plant tissue may provide nourishment for pathogens and result in an immediate increase of inoculum shortly after glyphosate is applied.
She said one researcher suggested a period of up to eight months between fall burndown and spring seeding, to manage the “green bridge” for these pathogens, plus a period of two to three weeks between spraying and seeding.
Gilbert said broadleaf and cereal crops are susceptible to rhizoctonia and pythium root rots. The only effective management is a period with no plants in the field.
Rhizoctonia is more severe when wheat and barley are seeded directly into cereal stubble, than into a conventionally tilled seedbed.
“The disease is more associated with dry conditions, when plants are under moisture stress. That’s when the (pathogens) move in. Saskatchewan studies indicate that under reduced till or no till, this disease is reduced.
“One of the reasons could be that you get an improved and more even moisture in soil which is under no till.”
She said in one study, the use of a modified narrow opener point that disturbs the soil beneath the depth of seeding consistently reduces the level of root damage. This was the result of enhanced root growth in the loose soil, which compensates for loss of root mass due to disease.
Other studies suggest common root rot was reduced by zero till, while wilts from fusarium increased.
Gilbert said fusarium graminearum, the pathogen that causes fusarium head blight, readily survives in crop residues. But it has been difficult to evaluate the effects of tillage practices on its development and severity.
“FHB is an environmentally driven disease. If the weather conditions are right, you’re probably going to get it. If Alberta’s temperature rises a little bit, and if your rainfall increases a bit, you could be in for fusarium (like in Manitoba),” she said.
Wheat and barley are most suceptible to fusarium during the flowering stage and because Manitoba normally receives more rain in July than other prairie provinces, when the crop is in flower, the fungus can more readily attack. Winter wheat is less suceptible because it flowers in early June when cooler temperatures limit fusarium’s advance.
One study of different tillage methods left 10 percent, 30 percent and 60 percent cover of stubble on the soil surface.
Gilbert said there was little difference between the chisel plowed plots and the direct-seeded plots. While plots worked with a mouldboard plow showed significantly less fusarium developed in the subsequent crop, overall levels of infection were still high.
“Whether you had 30 percent or 60 percent (stubble cover on the soil) made absolutely no difference. You got the same level of disease. Tillage wouldn’t be a way to control the disease. I don’t think you’re going to get more or less fusarium head blight through zero-till.”
She said the population of fusarium graminearum decreases as host tissue decomposes. While it may persist for a longer period on the debris in no-till fields, one study concluded that weather conditions are more influential than tillage practices in the development of the disease.
