In the high disease severity years, the mean leaf-spot disease severity of the unsprayed check was 73.2%, which was reduced to 32.0% by fungicide application. There were differences in disease severity among fungicide treatments with the highest severity observed by a single fungicide application at anthesis (41.0%), the lowest with the dual application (20.2%), and intermediate severity for the flag leaf application (34.7%). Only the synthetic fungicides reduced leaf-spot disease severity compared to the untreated control; the biological fungicide (B. subtilis) did not. Overall, tebuconazole reduced severity to 36.1%, and prothioconazole + tebuconazole to 27.6%. The FHB severity was ≤4.0% and no differences were observed among treatments.
Yield increased by 12.4% with synthetic fungicide application (average of the three fungicide treatments) for the high disease site years (Table 1). The unsprayed check had a mean yield of 3676 kg ha-1 and the fungicide treatments 4132 kg ha-1. Yield of the flag leaf treatment (4010 kg ha-1) was less than the dual application treatment (4246 kg ha-1). There was no difference between the flag leaf and anthesis treatments (p = 0.2343) or the anthesis treatment and the dual application treatment (p=0.2987). Yield differences were not detected between the synthetic fungicides. There was a yield decrease of 9.1% with the biological fungicide (3346 kg ha-1) compared with the unsprayed check (3676 kg ha-1).
In low disease severity years, the mean leaf spot disease severity of the unsprayed check at the low disease locations was 16.2%, which was reduced to 6.6% with the application of fungicide (Table 2). Leaf spot severity was slightly lower in the dual fungicide treatment (5.0%) compared to application at either flag leaf (7.1%) or anthesis (7.6%). The FHB severity was similar to the high disease severity site-years (≤3.9%) and no differences were detected among treatments. Thousand kernel weight was also lower when fungicide was applied at flag leaf stage (38.7 g) compared to anthesis stage (39.1 g) or to the dual fungicide treatment (39.3 g), while the anthesis and dual treatments did not differ. There were no statistically significant differences in yield, TW or protein content among any of the treatments at p = 0.05.