Molecular And Virulence Characterisation Of Phaeoisariopsis Griseola And Reaction Of Bean Cultivars To Races Of The Angular Leaf Spot Pathogen
Nyokabi, W I
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Amongst the legumes grown in Kenya, common bean (Phaseolus vulgaris L.) is ranked as the most popular in both production and utilization. Bean production is, however, constrained by several factors, which include angular leaf spot. For effective management of this disease, it is important to understand the biology of the causal agent (Phaeoisariopsis griseola), including the genetic factors that determine its virulence. A survey to determine the current status of prevalence, incidence and severity of angular leaf spot (ALS) of common bean was conducted in diverse agroecological zones (AEZs) in Embu, Kakamega, Kiambu, Machakos and Taita Taveta districts. The survey revealed that ALS was prevalent in all the districts and was recorded in 89% of the farms visited. Disease incidence and severity were high (mean values of 49.6% and 21.4%, respectively) and they varied significantly (P ::s 0.05) among districts, farms, AEZs and different altitudes. One hundred isolates of P. griseola were characterised into 44 physiological races based on the response of twelve bean differential cultivars, indicating tremendous virulence diversity in the pathogen. The isolates were grouped into three virulence categories, namely Andean, Afro-Andean and Mesoamerican, based on the range of differential cultivars that they infected. Eight of the races were Andean, 12 were Afro-Andean and 24 were Mesoamerican. A study of the genetic diversity of P. griseola isolates using group-specific primers and Amplified Fragment Length Polymorphism (AFLP) markers differentiated the isolates into Andean and Mesoamerican groups, corresponding to the two common bean gene pools. Significant polymorphism was observed with all the AFLP primer combinations used, reflecting a wide genetic diversity in the P. griseola population. Greater diversity was detected within (90.7%) than among (9.3%) the pathogen populations. Afro-Andean isolates were more genetically related to Andean isolates (genetic distance D 0.0095) than to Mesoamerican isolates (D = 0.0269). The reaction of selected bean cultivars was assessed against the 44 races of P. griseola identified in Kenya. Bean cultivars exhibited varying reactions to the races and there was a significant difference (P :s 0.05) in the level of ALS severity. None of the bean cultivars evaluated was resistant to all races of P. griseola. Thirteen cultivars were resistant (grade 1 to3) or moderately resistant (grade 4 to 6) to at least 40 (91%) of the races. All the resistant or moderately resistant cultivars were of the small-seeded bean types that are not popular with farmers, whereas the commonly grown large-seeded cultivars were generally susceptible. Variation in aggressiveness of P. griseola isolates on susceptible bean cultivars was evaluated based on incubation period, lesion 'size and density, rate of disease development, disease severity, area under disease progress curve (AUDPC) and rate of in vivo and in vitro sporulation. These parameters varied significantly (P :s 0.05) among the isolates as well as between virulence and genetically defmed groups of the pathogen. Disease severity was negatively correlated (r = -0.434) to the incubation period and positively correlated to AUDPC (r = 0.840), rate of in vivo and in vitro sporulation (r = 0.411 and r = 0.441, respectively) and lesion density (r = 0.499). Mesoamerican isolates were more aggressive than the Andean and Afro-Andean isolates. The results of this study are invaluable to bean breeding programmes as they indicate a wide virulence and genetic diversity in the angular leaf spot pathogen. This diversity should be taken into consideration when developing and deploying bean cultivars with resistance to ALS.