Mustard has not traditionally been a crop with major disease issues for Saskatchewan growers. However, the potential for disease is influenced by the crop rotation, pathogen population and weather conditions. Therefore, scouting for disease is important to:
- Accurately identify disease symptoms before the disease becomes well-established in mustard and economic losses occur; and
- Determine the effectiveness of integrated pest management strategies.
If the field is less than 100 acres, check a minimum of five sites and if the field is greater than 100 acres, check a minimum of 10 sites. Be most diligent scouting fields at greater risk to disease that include:
- Fields that were planted to infected or poor quality seed;
- Fields that have a short crop rotation (including canola or mustard) or are adjacent to infested crop residue from the previous season; and
- Fields that were planted to a susceptible crop variety.
In addition, check for areas in the field that are potential hot spots for disease development:
- Areas of the crop that may be heavier seeded or have increased fertility (e.g. headlands);
- Areas where moisture may have accumulated (e.g. hollows or near fence lines); and
- Areas where plants received damage from wind-blasting, drought, herbicide injury, frost, hail or other stresses.
Symptoms may occur in patches, be limited to field edges or be scattered across the whole field. That is why it is important to determine the full extent of the problem by scouting the entire field. Watch for discoloured plants and/or small discoloured spots on the leaves. Stop at each site and look down within the crop canopy, remove some plants and closely inspect the leaves and roots. Inspect both the top and bottom of leaves. Use a magnifying glass to help distinguish small spots and to look for tiny chew marks or shredding, which could indicate damage caused by insects. Remember to rub small spots with your thumb—if they come off easily, they likely are not disease. Inspect the roots. Root rot usually results in stunted growth and the plants may be light green, yellow, or brown in colour. Plants with root rot will pull easily from the soil. Look for lesions on the crown region or on roots. Also examine roots for swelling and the presence of clubroot galls which may appear as rotten or decomposing root tissue later in the growing season.
Other factors can cause symptoms that may be mistaken for disease. Problems can be diagnosed or eliminated depending on the pattern of symptoms in the field. Linear and/or repetitive symptoms that are not spreading are more likely to be related to an abiotic factor (non-living), mechanical patterns (seeding, old swath row) or overlap/miss of a chemical application. Widespread/even damage is often related to environmental stress and may include dry soils, waterlogged soils, high temperatures, frost, hail and strong winds causing sandblasting. Record weather events on a calendar for future reference, and scout fields within two days following an extreme environmental occurrence to determine the effects on the crop. Random symptoms or focal points that appear to be spreading are more likely related to disease, and may have been introduced by seed, soil-borne pathogens, airborne spores, or insect vectors. However, herbicide residues and nutrient deficiencies may also be random, and if the focal point is related to a high or low spot in the field, symptoms may be due to stress.
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