Top Crop Manager

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To spray or not to spray?

With the increase in interest in fungicides and a number of products on the market, there are several factors growers should consider when deciding whether a fungicide application will benefit their farm.

April 20, 2009  By Top Crop Manager

Use a spray decision checklist to guide spray decisions.
  Photo and graphics courtesy of Bayer CropScience. 


With the increase in interest in fungicides and a number of products on the market, there are several factors growers should consider when deciding whether a fungicide application will benefit their farm.

“First and foremost, scouting is by far the most practical way of determining the extent and severity of a plant disease problem,” says Dr. Randy Kutcher, plant pathologist at Agriculture and Agri-Food Canada’s research centre in Melfort, Saskatchewan. Kutcher presented an integrated disease management approach at the Western Canadian Crop Production Show in Saskatoon in January. The presentation at the Crop Production Show was developed with his colleague Dr. Kelly Turkington, plant pathologist at Agriculture and Agri-Food Canada’s Lacombe, Alberta, research centre. Over the years, Kutcher and Turkington have collaborated on many research projects and grower education initiatives on the impact of plant diseases in Western Canadian field crops.


“There are a number of things that growers should take into consideration when scouting,” explains Kutcher. Scouting just prior to the crop growth stage recommended for fungicide application is important to help make the decision to spray, but equally important is scouting later in the season to evaluate the fungicide decision, including leaving an untreated check strip. 

In addition, it is especially important to identify disease problems for future years, particularly since growers are now often shortening crop rotations. In cereals, Kutcher recommends a second scout during kernel formation, before the upper leaves have senesced, while in canola he advises checking for incidence of sclerotinia during the week of swathing. Crop scouts can be hired for these later-season scouts if growers are faced with the usual pre-harvest time crunch.

For an intensive survey, Kutcher advises growers to scout 10 locations in a field and collect 10 plants at random for a total of 100 plants. He recommends identifying the diseases and determining their incidence (percentage of plants infected) and/or the severity (amount of tissue damage on the plants) depending on the crop and disease.           
In cereals, leaf spotting diseases will likely infect all plants evaluated so it is important to measure the severity, the amount of tissue infected on particular leaves or over the whole plant. For canola, diseases such as sclerotinia stem rot and blackleg usually do not infect every plant, so incidence (percentage of plants infected) is a useful measure, and if growers would like to be more precise, then evaluation of the damage to each stem (severity).
Kutcher also stresses good record keeping, saving this useful information to help monitor and manage diseases on each field over the years.


The Disease Triangle
Kutcher cites the disease triangle as a useful tool to consider when assessing the threat that plant diseases pose. It is the interaction of host, environment and pathogen, which are the factors that contribute to disease initiation and its severity.

Kutcher notes that for some diseases such as blackleg and sclerotinia stem rot of canola and fusarium head blight of cereals, which can readily move to adjacent fields, the risk of an outbreak may be influenced by field location. If these diseases were severe in the adjacent field in the previous season, this is an added risk to the crop.

Conversely, cereal rusts are a special case, as rust largely originates from epidemics that occur in the US Midwest. Outbreaks in Kansas and Nebraska in May can be harbingers of disease on the Canadian Prairies in late June and in July.

Field scouting, while an important tool in the toolbox, is not the only one. The degree of resistance in the varieties or hybrids, the length of crop rotation and use of foliar fungicides are also key.

Aside from the agronomic environment in a field, knowing what has occurred and what is happening in a particular field is critical to making the fungicide decision.

Farmer-run trials help determine fungicide return on investment
For the past few years, farmers across the Prairies have put fungicides to the ultimate field test to determine their performance and payback. In doing so, they are taking Kutcher’s advice to leave a test strip to check the fungicide’s effectiveness.  

Demonstration Strip Trials (DSTs), co-ordinated by Bayer CropScience technical reps and run by farmers at 36 locations across the Prairies, compared wheat, barley and canola acres treated with Bayer CropScience fungicides with untreated acres.

This comprehensive side-by-side trial program in Western Canada was designed to measure product performance across a variety of environmental and local conditions. Each DST location has four to 12 strips in a field-scale-sized trial, replicated twice. Strips are sprayed side-by-side under identical soil, moisture, climatic conditions and agronomic practices to provide meaningful results.

In 2008, wheat trials consisting of four to 12 strips at 17 locations compared the core treatments of Folicur fungicide or Stratego fungicide, both from Bayer CropScience, to untreated wheat. Folicur helped deliver six percent better yield than untreated checks when applied at the critical flag leaf stage, while Stratego outyielded the untreated wheat by five percent. “The bottom line is that fungicide treatments helped to protect the yield potential of those wheat crops from leaf diseases that varied from location to location and environment to environment,” says Troy Basaraba, market development specialist with Bayer CropScience. “When applied at the right timing and rate, it will, on average, pay to use a fungicide.”

At six locations in 2008 where barley was tested, the barley treated with Stratego saw a yield advantage of six percent compared to untreated, or 80 bu/ac compared to the check at 73 bu/ac average.

Stratego is a foliar fungicide composed of two active ingredients with different modes of action (trifloxystrobin and propiconazole) to combat leaf disease in cereals and has been used for a number of years by Prairie producers. Folicur is another foliar option with the active ingredient, tebuconazole, used by wheat growers against leaf diseases and fusarium head blight.

In DST locations where fungicides were tested on canola, two different fungicide treatments applied at 30 percent bloom also showed a yield advantage compared to untreated canola. Proline fungicide treated canola produced yields of 52 bu/ac in the trials, 4.6 percent above the untreated yield of 49.8 bu/ac. Rovral Flo fungicide has a similar result, yielding better than untreated in five locations. “With the investment growers make in canola seed we believe protecting their yield potential with a foliar fungicide against the impact of sclerotinia is a wise investment,” says Basaraba. 

Proline, with the active ingredient prothioconazole, provides curative, protective and eradicative action, particularly in controlling sclerotinia, one of the most serious pathogens in canola.

The Canola Council of Canada’s sclerotinia checklist is a good reference for growers looking to decide whether to “spray or not to spray.”


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