It is essential to determine the need for fungicide applications. This involves checking the level of disease on 50 individual tillers randomly selected throughout the field. Fields should be scouted for powdery mildew at flag leaf emergence and boot stage (Feekes Growth Stage 8 and 10, respectively) and scout for Stagonospora leaf blotch and leaf rust at boot stage and after full head emergence. (See Figure 21.)
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| Figure 21. Scout wheat for disease at Feekes Wheat Growth Stages 8, 10, and 10.5. |
The upper two leaves on tillers and the glumes on heads contribute most to grain fill prior to flowering. (See Figure 22.) Thus it is important to keep these upper plant parts free of disease to avoid yield loss. Additionally, as the plant reaches growth stages beyond flowering, the glumes on the head become more important to grain fill than the leaves. This is why foliar applications of fungicides after flowering have little effect on yield.
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| Figure 22. Relative importance of wheat leaves and glumes on the heads to the production and translocation of photosynthates used to fill the grain. Intensity of coloration and size of arrows indicate relative amounts and direction of translocation. Illustration adapted from Wheat Documenta Ciba-Geigy, 1980. |
Disease thresholds are one percent (Figures 23-26) of leaf area affected on the leaf below the flag leaf up to boot stage (Feekes Growth Stage 8 through 10), and one percent leaf area affected on the flag leaf between head emergence to the end of flowering (Growth Stage 10.1 to 11) (Table 4). When these disease levels are present, a fungicide should be applied as soon as possible to protect leaf tissue before more becomes infected.
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| Figure 23. Percentage of leaf area affected by disease. |
| Table 4. Wheat Disease Thresholds for Fungicide Applications. | |||
| Wheat Growth Stage | Disease | Leaf* | Disease Level** |
| Flag leaf emergence (GS8)to Boot (GS10) | Powdery mildew | 2 | 2 to 3 lesions |
| Flag leaf emergence (GS8)to Boot (GS10) | Stagonospora leaf blotch | 2 | 1 to 2 lesions |
| Head emergence (GS10.1)to end of flowering (GS10.5.4) | Stagonospora leaf blotch | 2 | 1 to 2 lesions |
| Head emergence (GS10.1)to end of flowering (GS10.5.4) | Leaf rust | 1(flag) | 5 to 10 pustules |
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* Leaf number counted from top leaf (flag leaf = leaf 1) down on tiller. ** Disease level based on average of 30 to 50 tillers randomly collected throughout the field. |
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One percent leaf area affected roughly translates to two to three powdery mildew pustules (Figure 24), one to two Stagonospora nodorum blotches (Figure 25), or five to 10 leaf rust pustules (Figure 26).
