Leaf spot diseases are an important production concern in cereal crops such as barley and wheat. Disease risk can vary from year to year depending on weather conditions, crop stage, variety susceptibility, crop rotation, and the presence of pathogen inoculum. Dry conditions may slow disease development early in the season, but periods of rainfall, high humidity, and prolonged leaf wetness can quickly increase the risk of disease establishment and spread.
An integrated management approach is important for reducing cereal leaf disease risk. Crop rotation and variety resistance are useful tools, but they may not provide complete protection. Rotations often need to be long enough to reduce pathogen carryover, and not all varieties have resistance to every important leaf disease. As a result, fungicide application may become an important management option when disease risk is high.
Regular scouting is essential for making informed disease management decisions. Early-season scouting can help identify the first signs of leaf disease and provide an indication of whether disease pressure is increasing. Continued monitoring from stem elongation through flag leaf and head emergence is especially important, as protecting the upper canopy leaves is critical for maintaining yield potential. Fungicide decisions should be based on disease presence, crop stage, weather conditions, variety susceptibility, and overall field risk.
- Exceptions occur with cereal rust diseases, especially stripe rust, where a herbicide timing application can be more useful. This is likely due to the biotrophic nature (i.e. they derive nutrients from living host tissues) of the rust pathogens and that initial rust inoculum typically comes from outside the crop, especially for spring cereals
- In contrast, cereal leaf spot diseases (scald, net blotch, spot blotch, tan spot, septorias) are necrotrophic in nature (i.e. the derive nutrients from dead plant tissue), while initial inoculum typically comes from old crop residues in the same field where the crop is growing
- Fungicide movement to the rust pathogen in living host tissues may be more extensive versus through dead plant tissues in the case of necrotrophic leaf spot pathogens. In addition, cereal leaf spot inoculum is continuously produced from old crop residues and thus can infect any new leaves that emerge after spraying has occurred
- 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, i.e. upper canopy leaves aka “The Money Leaves” (term coined by Nick Poole, Field Applied Research (FAR) Australia)
- 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/delay pathogen development and sporulation, but only for about 2-3 weeks
- After this period the leaf spot pathogens can resume growth and sporulation and thus contribute to subsequent disease development
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