Disease Risk, Spatial Patterns, and Incidence-Severity Relationships of Fusarium Head Blight in No-till Spring Wheat Following Maize or Soybean

The first large-scale survey of Fusarium head blight (FHB) in commercial wheat fields in southern Brazil was conducted over three years (2009 to 2011). The objectives were to: (i) evaluate whether increased FHB risk is associated with within-field maize residue; (ii) determine the spatial pattern of FHB incidence; and (iii) quantify the relationship between FHB incidence and severity. FHB was assessed in a total of 160 fields between early milk and dough. Incidence ranged from 1.0 to 89.9% (median = 25%) and severity from 0.02 to 18.6% (median = 1.3%). FHB risk was neither lower nor higher in wheat following maize than in wheat following soybean. Only 18% of fields were classified as having aggregated patterns of FHB-symptomatic spikes. A binary power law description of the variances was consistent with an overall random pattern of the disease. These results conform with the hypothesis that FHB epidemics in southern Brazil are driven by sufficient atmospherically-transported inoculum from regional sources. The incidence-severity relationship was coherent across growing season, growth stage, and previous crop; one common fitted curve described the relationship across all observations. Estimating severity from incidence may be useful in reducing the workload in epidemiological surveys.