LOCALIZED SEPTORIA LEAF BLOTCH LESIONS IN WINTER WHEAT FLAG LEAF DO NOT ACCELERATE APICAL SENESCENCE DURING THE NECROTROPHIC STAGE

Abstract

Mycosphaerella graminicola, the most damaging disease of wheat in Northern Europe, induces yield or grain quality losses usually related to green area losses. This work aims at clarifying whether induced senescence is due to an acceleration of apical senescence and thus to a modification of the leaf nitrogen remobilization rate. The effect on apical leaf senescence of a restricted diseased leaf area was investigated. The experiment involved four winter wheat cultivars varying in their susceptibility to four M. graminicola isolates. Flag leaves were inoculated at a dose representative of field epidemics. As soon as symptoms appeared, dynamics of apical senescence (Sapi) and around the inoculation zone (Ssep) were measured by repeatedly taking digital photos until senescent areas merged. Ssep trends were fit to a logistic function whose parameters depended on cultivar×isolate interactions. Disease severities ranged from 0 to 24% total leaf area. Apical senescence followed an exponential pattern in control and inoculated leaves. Sapi was twice as great in inoculated shoots as in controls, even when no disease occurred. The relative rate of Sapi depended on the cultivar, but no isolate effect was detected despite wide variations in Ssep extent. Paired comparisons between inoculated and control leaves showed that the relative rate of Sapi was not increased by inoculation. It is concluded, that over a wide range of Septoria disease, apical senescence was not accelerated in inoculated leaves as compared to control. Results suggest that the disease did not modify the regulation of leaf N remobilization rate.