The Role of Nonhost Cereal Seeds in the Epidemiology of Cowpea Bacterial Blight (Xanthomonas axonopodis pv. vignicola (Burkholder) Dye)

Abstract

Cowpea (Vigna unguiculata (L) Walp.) production in Nigeria is mostly undertaken under mixed cropping systems with cereal crops. To determine the role of nonhost cereal seeds in the epidemiology of cowpea bacterial blight, farmer-saved maize, millet and sorghum seeds were collected from nine cowpea producing states and assessed for ability to harbor and transmit Xanthomonas axonopodis pv. vignicola. The pathogen was extracted consistently from millet seed samples from all the states, except Kebbi state at concentrations ranging from 0.2-3.3 x 10 2 CFU mL -1 . All maize and sorghum seed samples tested negative for the pathogen. By comparing direct seed plating, seed washing alone and seed blending pathogen extraction assays, it was found that pathogen extraction efficiency was significantly (P<0.05) enhanced when seeds were blended. Emerging maize, sorghum, millet and cowpea seedlings harbored populations of the pathogen up to 3.3 x 10 7 CFUmL -1 in the first week of seedling emergence. There was no significant variability in the pathogen population sizes on cowpea and nonhost seedlings, indicating that tissue colonization at this stage of growth was epiphytic. Pathogen population kinetics over a 49-day sampling period showed a gradual decline over time to levels that did not differ from those of untreated control in maize and sorghum. However, pathogen populations in millet tissues remained high on Day 49. Xanthomonas axonopodis pv. vignicola was transmitted systemically from inoculated cowpea, millet and sorghum (but not maize) seeds to seedlings, to harvested seeds at maturity. Susceptible cowpea plants grown together with the inoculated cereal crops were not cross-infected. The results of this study demonstrated that seeds of nonhost cereal crops could serve as passive reservoirs for Xanthomonas axonopodis pv. vignicola. Such epiphytic pathogen populations could serve as inoculum sources for bacterial blight in cowpea, leading to significant economic losses.