Resistance/susceptibility of Faba Bean to Botrytis fabae: The Causal Agent of Chocolate Spot with Respect to Leaf Position

Plants have evolved different defense mechanisms to combat pathogen attacks. In this study, detached leaf assays were conducted to estimate the influence of leaf position (leaf age) on the development of chocolate spot disease, caused by Botrytis fabae, in the lower, middle and upper leaves of five faba bean (FB) cultivars with different levels of resistance/susceptibility. Two components of resistance in terms of lesion diameter and spores per lesion were used to evaluate the disease intensity. To evaluate plant defense response, oxidative burst and phenol-oxidizing enzymes were assayed. The results indicated that regardless of the resistance level of the FB cultivar, lower (older) leaves were more severely infected by the chocolate spot pathogen than upper (younger) ones. A comparison of the defense-response behavior of the FB leaves revealed that the chocolate spot pathogen induced lipid peroxidation and the production of reactive oxygen species, peroxidase, and polyphenol oxidase in leaf tissue during the FB-B. fabae interaction. The production of these defense compounds in leaves was not static but governed in time and extent by physiological maturity. Younger leaves exhibited significantly higher oxidizing enzyme activity and lower oxidative stress than older ones. These variations in the levels of defense compounds could explain the differences in leaf resistance. The extreme differences in disease development recorded in upper and lower leaves of all FB cultivars suggest that assessing resistance using leaves from the middle positions would be the most efficient and reliable for evaluating the resistance/susceptibility of FB plants to B. fabae.