Scouting for cereal diseases: Risk and the need for a fungicide application

Originally Tweeted June 16, 2019 by @PCDMN

1. As wheat and barley crops progress concerns regarding disease switch from seed and seedling issues to mid-season leaf disease issues.  Scouting is key to identifying emerging leaf disease issues and to assessing the need for an in-crop fungicide.

2. When scouting for weeds, 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.

3. Around flag leaf emergence, look at the 3rd leaves down from the heads. Are they clean or do you notice disease symptoms?  Note that 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).

4.  You can look at incidence, which is the number of leaves or plant structures with symptoms OR you can look at disease severity which is the extent of disease on a particular plant part.  For cereal leaf diseases focus on severity (% leaf area diseased).

5.  Make sure the symptoms are actually caused by disease and not damage (burning, scalding, etc.) due to herbicides or foliar-applied fertilizers.  On the 3rd leaves from the head, if you see more than about 1-2% leaf area affected then you have a potential risk developing.

6.  Also keep in mind what the weather conditions are like.  Moderate temperatures and frequent rainfall increase the risk, especially with varieties that are not resistant to disease.

7.  If you notice significant levels of disease during weed scouting and increased severity on the third leaves from the head, a fungicide may be needed prior to head emergence, i.e. at or just before flag leaf emergence.

8.  If leaf disease levels are low during weed scouting and on the 3rd leaves from the head, 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.

9.  If you have significant leaf disease development at stem elongation to flag leaf emergence and you are also concerned about fusarium head blight, then more than one fungicide application may be needed, i.e. around flag leaf emergence plus again after head emergence.

10.  When using a fungicide, especially with multiple applications, always try to implement strategies to limit the risk of fungicide resistance development.  Only spray if needed, limit the number of applications, and where possible use products with multiple modes of action.

11.  Here are some standard area diagrams showing 5, 10, 15, 25, 35%, and 50% of the leaf area affected with scald in barley. INSERT “STANDARD AREA DIAGRAMS” FOR CEREALS.

12.  Here are some standard area diagrams showing 5, 10, 15, 25, 35%, and 50% of the leaf area affected with the tan spot/septoria complex in wheat.  Even for experienced plant pathologists, it can be very difficult to tell them apart and a lab diagnosis may be needed. INSERT FIGURE HERE.

13.  Here are some standard area diagrams showing 5, 10, 15, 25, 35%, and 50% of the leaf area affected with the spot-form of net blotch or spot blotch in barley.  Symptoms are similar and you may need a lab diagnosis to tell them apart. INSERT FIGURE HERE

14.  Note that the level of crop disease needs to be put into perspective.  Trace to low levels of disease = limited impact on crop productivity and/or quality; thus a fungicide may not be needed and if used can result in a neutral or -ve economic benefit.

15.  Note that the presence of crop disease needs to be put in to perspective.  Moderate to high levels of disease = reduced yield and grain filling, grade reductions, etc.; thus a fungicide application is needed and can contribute to a +ve economic benefit.

16. A diagrammatic representation of typical cereal leaf spot symptoms. INSERT FIGURE HERE.

17. A diagrammatic representation of leaf symptoms not typical of cereal leaf spot diseases. INSERT FIGURE HERE.