An evaluation on intensity of infection of Pseudocercospora leaf spot disease of cowpea cultivars (Vigna unguiculata L. Walp), with respect to infector rows, dates of inoculation and cultivars, grown under field conditions in Northeast Thailand

This investigation was carried out at the Experimental Farm, Faculty of Agriculture, Khon Kaen University, Khon Kaen, Thailand to evaluate intensity of infection of Pseudocercospora Leaf Spot disease of cowpea cultivars, grown under field conditions with the use of Yasothon soil series (Oxic Paleustults). The experimental design used was a strip split plot design with four replications. The experiment consisted of three factors, i.e., with and without infector rows (Factor A), with and without inoculation and inoculated dates, i.e. none inoculation, inoculated at days 15 and 30 after sowing of seeds in main plots (Factor B) and three cowpea cultivars of KVC7, KKU25 and IT81D-1228-14-1 (Factor C). The results showed that the use of infector rows of cowpea plants (KKU25) being sown at 15 days before sowing seeds of cowpea cultivars in main plots gave a similar intensity of disease as those infector rows being sown at 30 days before sowing seeds of cowpea cultivars in main plots but significantly higher than without infector rows. Pathogenic disease being inoculated to the cowpea plants at 15 days after the sowing of seeds in main plots gave significantly higher intensity of disease than none inoculation. The establishment of infector rows at 15 days ahead of the sowing of seeds in main plots together with an inoculation of the disease at 15 days after the sowing of seeds in main plots gave the best results in evaluating amount of the infected disease. Thus infector rows surrounded the main plots are required whenever trials on disease evaluation are taken place.