Efficacy of host resistance, seed sorting and antifungal plant extracts in management of angular leaf spot of common bean (Phaseolus vulgaris l.)

Abstract

Common bean (Phaseolus vulgaris L.), is the most important legume in pulses category of Kenya’s agricultural commodities and is second only to maize in importance as a food crop. Among the bean diseases angular leaf spot (ALS) caused by Phaeoisariopsis griseola is a major constraint to production in the tropics and subtropics, and causes considerable yield losses. The effectiveness of the available methods for control of angular leafspot, which include cultural practices, use of chemicals and resistant varieties is limited by the ability of the pathogen to survive in plant debris for a long period of time, high costs of fungicides, lack of expertise and the health hazards involved, and the high pathogenic variability occurring in P. griseola. Therefore, there is need to develop an effective and alternative disease control approach. The strategy most likely to be effective in the control of angular leaf spot is integrated disease management utilizing resistant varieties, natural fungicides and appropriate cultural practices. In this study 10 bean lines were evaluated for resistance against P. griseola under glasshouse conditions. Some of the bean lines were resistant or moderately resistant to various races of ALS pathogen. Bean lines KAB 02-83, KAB 02-9, KAB 12-75 and KAB 02-84 were resistant or moderately resistant to P. griseola races 63-63, 63-39 and 63-55. Eight indigenous plants (Azadirachta indica, Aloe vera, Allium sativum, Warburgia ugandensis, Urtica massaica, Lippea javanicum, Tephrosia vogelii and Prunus africana) were tested for their antifungal properties against the angular leaf spot pathogen using paper disc diffusion method. Allium sativum showed large inhibition zones on the growth of P. griseola culture and reduced the rate of defoliation when sprayed on inoculated bean plants vii in the glass house. Seed-to-seedling transmission of P. griseola was investigated in order to devise appropriate disease control measures and was found not to be important in the development of ALS. Seed sorting is therefore not effective in management of angular leaf spot. The results of this study indicate that A. sativum has antifungal properties against P. griseola and can be used as a natural fungicide. Bean lines identified to be resistant to ALS in the glasshouse should be tested under field conditions to ascertain their levels of resistance and recommend them to farmers as an important component of ALS management.