Cultural, Morphological and Pathogenic Variability among Fusarium oxysporum f. sp. phaseoli Causing Wilt in French Bean (Phaseolus vulgaris L.)

Aim: This study aimed at evaluating cultural, morphological and pathogenic variability among Fusarium oxysporum f. sp. phaseoli strains isolated from French beans.

Study Design: Cross-sectional study.

Methodology: The French beans showing Fusarium wilt symptoms were obtained from different fields in Kabaa irrigation scheme in Machakos County, Kenya. The diseased plants were washed and cut into 5 mm pieces which were surface sterilized before plating on Fusarium-selective medium. They were incubated for 10 days at 25-26°C. The developing colonies were transferred on Potato Dextrose Agar (PDA), Carnation Leaf Agar (CLA) and Spezieller Nahrstoffarmer Agar (SNA) media for cultural and morphological characterization. Pathogenicity was assessed using on French bean ‘Amy’ variety.

Results: From 84 Fusarium isolates obtained, 18 were confirmed to be F. oxysporum from which 8 were confirmed to be F. oxysporum f. sp. phaseoli (Fop). Variations existed cultural and morphological variations among the 8 isolates. Isolates showed luxuriant, moderately luxuriant and scanty aerial mycelial growth. Mycelial texture was either fluffy or fibrous. Isolate Fop8 had the highest growth at 85 mm at 7th day followed by Fop3 (84 mm) and Fop6 (84 mm) while Fop7 had the least growth at 71 mm. The colony colour was purple, pink or white. Microconidia sizes ranged from 8 X 3.0 to 10 X 3.4 µm while macroconidia ranged from 28 X 3.8 to 42 X 4.2 µm. Macroconidia had 3 septa. The microconidia were abundant and aseptate. Chlamydospore formation was terminal and intercalary, occurring in singles and in pairs. Although all the 8 isolates were pathogenic on ‘Amy’ French bean variety, their pathogenic potential was significantly different (P< 0.01). The most pathogenic isolate was Fop03, followed by Fop06 and Fop07 at means of 97.0, 92.4 and 92.0%, respectively. The least pathogenic isolate was Fop05 with a mean of 65.9% pathogenicity.

Conclusion: The isolates in this study are culturally and morphologically varied and have high pathogenicity on French bean.