Influence of plant stage and organ age on the receptivity of Pisum sativum to Mycosphaerella pinodes

On spring pea, ascochyta blight (Mycosphaerella pinodes) frequently appears at the plant base on yellowing stipules and disease scores are higher on basal parts of the plants than on the uppermost parts. In order to investigate the relationship between pea plant growth stage and/or organ age, and ascochyta blight on whole plants and detached stipules and pods, two experiments were conducted in 2009 and 2010 under controlled conditions. This study showed that plant stage does not influence receptivity to ascochyta blight before the appearance of the first visual signs of senescence (beginning of yellowing). When stipules were green, regardless of the plant stage, no differences of receptivity was observed (except for the youngest stipule(s) at the top of the plant). Plant stage only had an effect on receptivity by the way of the importance of the visual senescence of the organs and visual senescence has a more predominant effect on plant receptivity than the plant stage considered alone. An effect of the nodal position, linked with the stipule age, was observed on whole plant inoculation, with a decreasing disease severity gradient from the base to the top of the plant which was explained by a greater receptivity for the basal/older stipules which had begun yellowing. On detached stipules, a higher receptivity was observed as soon as visual senescence had been observed. On pods, during their filling, few symptoms appeared and severity was important as soon as they began to yellow. The effect of ascochyta blight on induced senescence of the infected stipules was also observed.