A comparison of web blight epidemics on common bean cultivars with different growth habits

Abstract The progress of web blight caused by Thanatephorus cucumeris on common bean ( Phaseolus vulgaris ) “Carioca” cultivars with different growth habits was compared in a three-season study. Field trials were conducted in 2004/2005, 2005/2006 and 2006/2007 in a naturally infested area, with 10 susceptible cultivars from group I (determinate bush, upright plants: cvs. Iapar 81, BRS Horizonte and FT Magnifico); group II (indeterminate bush plants: cvs. BRS Pontal, and Perola) and group III (plants of indeterminate prostrate growth: cvs. BRSMG Talisma, Apore, BRS Requinte, Carioca, Carioca Precoce). Severity of web blight was assessed weekly and the resulting progress curves were fit to the monomolecular model. Overall disease impacts were estimated by the area under the disease progress curve (AUDPC), disease progress rate and disease onset. No significant interaction between AUDPC and planting season was detected. Cultivars BRS Horizonte, FT Magnifico and Iapar 81, with upright architecture, short guides, high pods and closer branches presented the lowest AUDPCs. The highest AUDPCs were found among indeterminate lodged plants with long guides and low pods (cvs Apore and Carioca Precoce), while indeterminate bush cultivars were fitted in an intermediate rank. Differences in disease epidemics among the three groups were confirmed by intercept and disease progress rates of linearized models. Such parameters were significantly lower for upright genotypes, probably because they result in late contact between neighbor plants and consequently delay the establishment of conducive microclimate and mycelial bridges for web blight development. Yields were strongly reduced due to the extremely high disease levels that prevailed at all three seasons. Nevertheless, a negative correlation between grain yield and web blight severity was significant in 2006/2007. Despite the fact that the number of cultivars is too small to generalize responses on escape to web blight, results evidenced that choice of plant architecture is a useful strategy for web blight management.