Evaluation of resistance to bacterial blight in Brazilian cassava germoplasm and disease-yield relationships

Cassava, an important source of carbohydrates in many developing countries, is affected by several diseases including bacterial blight, caused by Xanthomonas phaseoli pv. manihotis (Xpm), responsible for considerable losses in production. A 3-year split-plot design experiment was conducted to evaluate the resistance of 83 Brazilian cassava genotypes to Xpm under field conditions, inoculated or not with bacterial suspension (10(8) CFU mL(-1)). The relationship between disease progress and yield traits was also studied. Disease severity was evaluated using an ordinal 0 (no symptoms) to 8 (maximum degree of the disease) scores. At 11 months after planting, the following traits were measured: plant height, above-ground yield (measured as the total weight of leaves and stem), fresh root yield, dry matter content in the roots, and dry root yield. Based on the calculated disease index (DI) and the area under the disease progress curve (AUDPC) values, the accessions were grouped into different classes of resistance/susceptibility. There were 16 accessions classified as moderately resistant (MR), 14 as moderately susceptible (MS), 41 as susceptible (S), and 12 as extremely susceptible (ES). Complete resistance to bacterial blight was not found. Despite the significant reduction in above-ground and root yield average for the inoculated plants, when compared to non-inoculated, there was no correlation between the severity (DI) or disease progress (AUDPC) and the agronomic performance of cassava genotypes. The selection of the best 10 accessions should be a priority, considering that the agronomic performance could increase the above-ground yield, fresh root yield, and dry root yield by 22.17%, 109.61%, and 114.64%, respectively.