Influence of Bacterial Blight on different phenological stages of cowpea

Abstract

The cowpea bacterial blight is a common disease, but some of its epidemiological parameters have not been totally elucidated. The objective of this study was to determine the influence of the inoculation of the bacterial blight pathogen (Xanthomonas citri pv. vignicola - Xcv) at different phenological stages of the cowpea, as well the effect on its yield components, and the transmission of the bacterium to the seed. Under greenhouse conditions, plant defoliation was observed on the cowpea cultivar BRS Bragança inoculated with 107 CFU of Xcv/mL at the four phenological stages, - VC (plants with primary leaves at nodes 1 & 2), V3 (3rd trifoliolate leaf unfolded at node 5), R2 (50–100% of open flowers), and R3 (one pod at maximum lenght). There was a difference in the relative area under the disease progress curve (RAUDPC) between the phenological stages and the reduction of some production components, such as dry seed mass. The pathogen was isolated from seeds produced in infected plants in all phenological stage, with the highest values ​​of CFU per gram of seeds in the V3 and R3. Under field conditions, two experiments were conducted using the cv. BRS Vita-7 (resistant) and cv. BRS Bragança (susceptible) inoculated with Xcv at the V3 and R2. The disease was more severe in cv. BRS Bragança, with the RAUDPC being higher in plants inoculated at V3 with a reduction of dry seed mass in inoculated R2 and inoculated R3 when compared to their uninoculated counterparts across two growing seasons. Regardless of the phenological stage in which the plant was inoculated, it was possible to detect Xcv in cv. BRS Bragança and cv. BRS Vita-7 seeds. The highest values of CFU per gram were recorded in samples from cv. BRS Bragança inoculated at V3 stage. Results on the presence cowpea's seeds infected by Xcv show that bacterial blight disease is highly seed transmitted and demonstrate the current need to avoid additional spread of the pathogen to new areas where there are favorable climatic conditions in Brazil or to disease-free countries that can result in considerable losses. Thus, the disease management should be used based on seed certification programmes, realizing sanitary tests of cowpea seed to avoid the spread of Xcv and the maintenance of disease-free seed stocks.