The effect of plant extracts on anthracnose of Phaseolus vulgaris L. and Vigna unguiculata (L.) Walp

Abstract

Anthracnose is one of the serious diseases of cowpea (Vigna unguiculata L. Walp) and common bean (Phaseolus vulgaris L.) caused by the Colletotrichum fungi. The disease is prevalent is small holder farmers’ fields due to the scarcity and high cost of the synthetic fungicides. This study was conducted with the main aim of improving food security and income of the smallholder farmers by increasing legumes, P. vulgaris and V. unguiculata thereby increasing production and improve food security and income of smallholder farmers. Investigations involved in vitro bioassaying for antifungal activities of the crude extracts on Colletotrichum lindemuthianum (Sacc. and Magn.) Bri. and Cav. and Colletotrichum dematium (Fr.) Grove var. truncata field isolates and evaluating the effect of crude plant extracts seed treatments on seed germination, emergence and control of anthracnose disease of common bean and cowpea. Furthermore, ultra-structural changes of plant extracts treated and efficacy of foliar application of extracts. The in vitro study showed that Allium sativum L., Agapanthus caulescens Spreng., Carica papaya L. and Syzygium cordatum Hochst.ex Krauss extracts have good antifungal activities against both C. lindemuthianum and C. dematium. The low concentrations (5 mg.ml-1) of Syzygium and Agapanthus water extracts and acetone extracts of Agapanthus and Carica gave a high percentage of bean seed germination, emergence, short mean emergence time (MET) and were effective in controlling the anthracnose disease. The treatment of Agapanthus (both water and acetone) extracts also increased the shoot length and dry weight of the seedlings. The Allium acetone extracts (5 mg.ml-1) was the only treatment that gave good results with respect to germination percentages, MET, shoot length, leaf area and dry mass of cowpea. Five mg.ml-1 concentrations of Syzygium and Agapanthus water extracts and acetone extracts of Agapanthus and Carica have potential as seed treatments on bean. Allium acetone extract (5 mg.ml-1) was the only potential cowpea seed treatment that could be recommended to farmers as an alternative to the synthetic fungicide. Electron microscopy revealed that principle differences were observed in the cotyledon-embryo connecting tissues of seeds treated with Agapanthus, which had few cristae in their mitochondria than the cells from other treatments. The embryonic root cells of bean seeds treated with Agapanthus had coalescing protein bodies. The embryonic root cells of cowpea and bean treated with Syzygium had fewer lipid bodies as compared to the control and the Agapanthus treated seeds. Bean plants that were foliar treated with the 15 mg.ml-1 concentrations of Allium water, Agapanthus water, Carica water, Agapanthus acetone, Carica 5 and 15 mg.ml-1 acetone, Syzygium 5 mg.ml-1 acetone extracts and the combinations (2.5 mg.ml-1 + 2.5 mg.ml-1) of Allium + Agapanthus, Allium + Carica, Agapanthus + Syzygium and Carica + Syzygium extracts registered low anthracnose (C. lindemuthianum,) disease severity and high leaf area. The cowpea plants treated with 15 mg.ml-1 water extracts of Agapanthus and the combinations of Allium + Agapanthus, Agapanthus + Carica and Agapanthus + Syzygium extracts recorded low cowpea anthracnose (C. dematium) disease severity, highest leaf area and dry mass. The study revealed that A. sativum, Agapanthus, C. papaya and S. cordatum plant extracts have antifungal activities and can be used as alternative seed treatments and foliar fungicides against the anthracnose diseases of legumes (cowpea and common bean) instead of synthetic fungicides without causing any negative effect on seed germination, emergence, ultra-structure of seeds and plant growth. Copyright