Effect of Some Fungal and Bacterial Organisms on the Growth of Cowpea (Vigna unguiculata (L.) Walp) Seedlings

Aims: Cowpea [Vigna unguiculata (L.) Walp] is a legume widely consumed in Africa. The effect of eight organisms viz: Fusarium oxysporum, Aspergillus niger, Botryodiplodia theobromae, Rhizopus stolonifer, Pseudomonas sp., Corynebacterium sp., Micrococcus sp. and Xanthomonas sp. on the growth of Vigna unguiculata seedlings was determined.

Methodology: Spore suspension of each fungal organism was prepared from pure cultures grown on Potato Dextrose Agar (PDA) plates and the bacteria used were obtained from slants. Fungal spores were standardized with the help of a haemocytometer slide and gelatine (0.1%) was used as a sticker. Using serial dilution method, 0.1ml of each of the isolated bacteria was collected from the 10-3 dilution and sprayed on the young seedlings using the run-off method. The cowpea seedlings were separately inoculated with each organism at the three leaf stage, three weeks after planting. Seedlings were artificially inoculated by spraying the adaxial surface of the leaves until water-soaked spots were obtained. The experiment was allowed to stand for 2 months and the leaf number, root length, shoot length and total seedling height of the cowpea seedlings were determined.

Results: Symptoms observed on seedlings were: stunted growth, drying of leaves, few fibrous roots, yellowing of leaves, wilting, necrotic lesions, leaf spot, darkening of leaf veins and blight. Most of the test organisms were pathogenic to V. unguiculata causing varying degrees of damage. Fusarium oxysporum caused the most deterioration on cowpea seedlings when compared to the other treatments.

Conclusion: This study has demonstrated the ability of culture filtrates of pathogenic micro-organisms to express symptoms in seedlings and transmit diseases to healthy seedlings.