2022 moisture conditions in many Prairie regions are shaping up to be conducive for the development of leaf spot diseases in barley and wheat. Although crop rotation and variety resistance are useful strategies, rotations typically need to be at least two years between host crops. In addition, the variety being grown may not have a disease resistance package that covers all leaf diseases. As a consequence fungicide application becomes a key tool for managing cereal leaf disease risk.
Scouting is key to identifying emerging cereal leaf disease issues and the need for one or more in-crop fungicide applications. Initially when scouting for weeds, producers and consultants can note the presence and level of leaf disease. This gives you a heads up in terms of a developing issue and the potential need to spray a fungicide as the crop progresses from stem elongation to flag and head emergence. Although there may be interest in applying fungicide at a herbicide timing, research indicates it will likely be of limited value in terms of protection of upper canopy leaves.
See:
 |
| Barley 2-3 leaf stage |
Given the nature of fungicides, movement within plant is typically
limited, but with more mobile actives they are typically xylem mobile and thus
need to be applied directly to the plant tissues that need protection. Finally, fungicides do not generally have
good eradicant activity, especially on mature established infections that are
already 1-2 weeks old. Thus, fungicides
may not kill the leaf spot pathogens in well-established infections, although they
may suppress pathogen development and sporulation, but only for about 2-3
weeks.
 |
| Stylized representation of fungicide movement within a leaf |
Should producers decide to use fungicides at a herbicide
timing, if the disease risk remains moderate to high, they will need to spray
again from flag leaf emergence to just after head emergence to protect key
upper canopy tissues. Keep in mind that as the number of in-season fungicide
applications increases so does the risk of fungicide resistance development. Thus, if producers need to spray fungicide in
a crop more than once during the growing season they need to implement
strategies to limit the risk of fungicide resistance development. Only spray if needed, limit the number of
applications, rotate actives, and use products with multiple modes of
action. Here are some great resources in
relation to managing the risk of fungicide resistance.
https://manageresistancenow.ca/disease/dfs-how-to-manage-fungicide-resistance-in-your-crops/
https://www.gov.mb.ca/agriculture/crops/guides-and-publications/pubs/guide-crop-protection-2022.pdf (see page 453)
https://publications.saskatchewan.ca/api/v1/products/77706/formats/87089/download (see page 453)
https://bluebook.print3connect.com/Store/Site/Layout/Custom.aspx
As the crop moves towards flag leaf emergence, producers
and consultants can use the extent of leaf disease development in the lower to
upper canopy to gauge risk and fungicide need.
The absence or limited appearance of symptoms (e.g. <1% leaf area
affected or so) in the lower to middle canopy would generally indicate a low
risk and thus limited need for fungicide.
However, things can change as the crop moves towards head emergence, so
it is important to scout again. Following
head emergence if moderate disease levels are observed in the lower to middle
canopy (e.g. >5-10% leaf area diseased) there is likely an emerging risk and
need to spray a fungicide to protect key upper canopy leaves where most of your yield comes from (https://www.albertawheatbarley.com/the-growing-point/articles-library/where-does-my-yield-come-from?setcommission=alberta-barley).
When scouting for cereal leaf diseases correct
identification is critical. A variety of
issues may produce leaf symptoms that may be incorrectly identified as fungal leaf
diseases For example, general leaf
yellowing is not indicative of the cereal leaf spots (e.g. tan spot and
septoria in wheat, net blotch/scald/spot blotch in barley), but may reflect
nutrient deficiencies, drought/heat stress or perhaps root disease issues. Make sure the symptoms are actually caused by
disease and not damage (burning, scalding, etc.) due to insect feeding,
herbicides or foliar-applied fertilizers.
Physiological leaf spotting can be confused with cereal leaf spot
symptoms and are typically related to chloride deficiency and sun damage. For example, with sun damage in barley the
most severe symptoms are on the upper canopy leaves, while chloride deficiency
symptoms are found throughout the cereal canopy and may be associated with
particular cereal varieties. In contrast,
cereal leaf spot symptoms generally follow a gradient, i.e. most severe in the
lower canopy, but decrease as you move to the middle and upper canopies.
Here are some links to a PCDMN post on white flecking in
wheat, and Twitter posts and a Montana State University alert from earlier this
spring regarding differentiating physiological leaf spots from those caused by
cereal leaf spot pathogens.
https://prairiecropdisease.blogspot.com/2022/06/unusual-symptoms-in-alberta-barley-and.html
https://twitter.com/UtaMcKelvy/status/1532032900790530048
https://apps.msuextension.org/mtagalerts/htmlalert.cfm?alert=14927
https://twitter.com/pcdmn/status/1531762399669362689
 |
| Diagrammatic representation of typical cereal leaf spot symptoms |
 |
| Diagrammatic representation of leaf symptoms not typical of cereal leaf spot diseases |
Here are some links to PCDMN resources for cereal leaf spot identification:
Wheat:
- Speckled leaf blotch
- Parastagonospora/Septoria leaf and glume blotch
- Tan spot
- Fusarium head blight
- Fusarium head blight seed infections
- Bacterial leaf streak
- Cereal rusts
Barley:
- Net-form net blotch in barley
- Spot-form net blotch in barley
- Scald in barley
- Spot blotch in barley
- Fusarium head blight
- Fusarium head blight seed infections
- Cereal rusts
The use of standard area diagrams can help in terms of
determining the level of leaf spot development.
Here are some standard area
diagrams showing 1, 5, 10, 25, and 50% of the leaf area affected with various
leaf spot diseases in barley and wheat.
Note that symptoms of spot-form net blotch and spot blotch in barley can
be difficult to distinguish without laboratory testing of plant samples. In addition, differentiating tan spot from
the septoria complex in wheat can be challenging even for experienced plant
pathologists, and thus a lab diagnosis may be needed. However, all cereal leaf spot diseases result
in destruction of leaf tissues and thus affect grain yield and filling. The goal should be correct identification of
leaf spot disease issues and then assessment of the overall level, i.e.
severity.
It is important to put the level of disease into perspective. Trace to low levels of leaf disease from tillering to head emergence = limited risk, and thus a fungicide may not be needed, and if used can result in negative net returns. In contrast, if you notice significant levels of disease during weed scouting, and increased severity in the lower to middle canopy at flag leaf emergence (especially on the third leaf from the head), a fungicide may be needed prior to head emergence, i.e. at or just before flag leaf emergence. If leaf disease levels are absent or limited during weed scouting and at flag leaf emergence, you may be able to delay and only use one in-crop fungicide application after head emergence to provide leaf disease control and fusarium head blight suppression. Lastly, a producer may have significant leaf disease development at stem elongation to flag leaf emergence and are also concerned about fusarium head blight and post head emergence leaf spot development. In this situation more than one fungicide application may be needed, i.e. around flag leaf emergence plus again after head emergence. Keep in mind that when you need to use more than one fungicide application during the growing season, make sure to use strategies to limit the risk of fungicide resistance development.
Finally, keep in mind what the weather conditions are like as well as the disease resistance package in the cereal variety you are growing. Moderate temperatures, frequent rainfall and heavy dews increase the risk, especially with varieties that are not resistant to disease. In contrast, with warm dry conditions and where the soil and crop canopy are dry in the morning, the risk of leaf spot development is limited and a fungicide is likely not needed (see wet pants test below). Fungicide response is generally greatest for cereal varieties that are susceptible to moderately susceptible. In contrast, varieties that are moderately resistant to resistance are less responsive and may not need a fungicide application. One caveat is that a typical crop may be faced with multiple leaf disease issues. Although a variety may be resistant to one disease, it may be susceptible to other diseases, and thus a fungicide may be needed depending on weather conditions and in-season disease appearance.
Provincial cereal variety guides with disease resistance ratings:
https://publications.saskatchewan.ca/api/v1/products/103730/formats/131802/download
https://www.seed.ab.ca/wp-content/uploads/2021/12/cereals-dec21.pdf