Scouting for disease and fungicide application timing

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By Shannon Chant, Regional Crops Specialist, Swift Current

Ministry of Agriculture

With the cool conditions and moisture in a lot of the regions in the last few weeks, diseases should be on everyone’s mind. For a disease to develop, the pathogen (the organism that causes the disease), a susceptible crop and the right environmental conditions need to be present. For most diseases, the amount of pathogen in the field can be reduced using crop rotation.  

When deciding if a fungicide application is necessary, you should also consider the cost of the fungicide and its application. If the expected net return from the crop is negative after the costs of fungicide application are added, an application should not be made.  

The proper scouting and fungicide application time will depend on the crop type and the disease.  

For cereal leaf-spotting diseases, scouting should begin before, during and after flag leaf emergence. The flag leaf and the upper leaves are responsible for 50 per cent or more of grain fill and are the most important ones to protect. The greatest yield response will occur if the fungicide is applied between flag leaf emergence and flowering. A fungicide application at the three to five leaf stage can be beneficial when a cereal crop is grown on cereal stubble, the canopy is moist and dense, disease symptoms are on the newest leaf, crop prices are good and an early application is followed up with a flag leaf application.  

The most important factor in the development of Fusarium head blight (FHB) is the weather. The disease is most likely to develop when plants are flowering, temperatures range from 15 to 30°C and high moisture is continuous for 48 to 60 hours. Because FHB has one infection cycle per year, it is too late to apply a fungicide once symptoms are observed.  

An application may be beneficial for FHB suppression if:

- the pathogen is established in the region;

- OR you have experienced FHB losses in the last two years;

- OR Fusarium graminearum (one of the pathogen causing FHB; results in DON production) is in a seed sample at greater than five per cent;

- OR the field is next to a field with infected residue or in a field that has had durum or wheat in the last two years;

- AND conditions have been wet and warm at the heading stage and are forecast to continue during flowering.

If conditions have been hot and dry at heading and will continue during flowering, a fungicide application may not be necessary. The fungicide must be applied at early flowering to protect the opening florets (small flowers in cereal heads).  

Sclerotinia is much worse and widespread during wet years. The decision to apply a fungicide to control this disease is based on soil moisture, weather conditions, crop stage and density and disease history. Similar to FHB, it is too late to apply fungicide when symptoms are observed. Fungicides should be applied to canola, flax and mustard between the 20 to 50 per cent flower stages to protect the petals from being colonized by the spores before they drop onto the rest of the plant below. For lentils, chickpeas and peas, fungicide application should occur at the beginning of flowering before the canopy closes. Additional fungicide applications for sclerotinia control can be made with some products. Always check the label for the fungicide you are using and do not exceed the maximum number of applications per season.

Fungicides for blackleg in canola should be applied at the rosette stage.

The timing of scouting in pulse crops depends on the disease. For aggressive diseases like ascochyta in chickpeas, scouting should begin early. Chickpea crops should be scouted for this disease from emergence to when the green colour of the crop starts to fade. Fungicide application timing varies so make sure to check the label. Many products allow for additional fungicide applications but do not make more applications than the maximum number stated on the label. For other diseases in pulse crops, scouting should begin at the early bloom stage. Fungicide application should also occur at this time. The risk for spread and infection of many pulse disease increases with rain and moist conditions.  

For more information on crop diseases and fungicide application, refer to the “Plant Disease Control: To Spray or Not to Spray” fact sheet available at our website or any regional office and pages 307 to 383 of the 2014 Guide to Crop Protection. Always follow label directions for any fungicide you are using and use high water volumes to ensure penetration into the crop canopy.

For more information or assistance with fungicide application questions, contact Shannon Chant at 306-778-8291 or your local agronomist.

Geographic location: Saskatchewan

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