Effect of pea plant architecture on spatiotemporal epidemic development of ascochyta blight (Mycosphaerella pinodes) in the field

The effect of pea canopy structure on epidemics of Mycosphaerella pinodes was investigated in four spring pea cultivars (Bridge, Obelisque, Solara and Athos) in two field experiments. These cultivars had similar levels of susceptibility to ascochyta blight and presented different architectural features (branching, standing ability and stem height). Stem height ranged from 77 to 95 cm in 1999 and from 72 to 92 cm in 2000, while leaf area index (LAI) ranged from 3·8 to 5·1 in 1999 and from 3·8 to 4·7 in 2000. Internode lengths varied from 4·6 to 6·0 in 1999 and from 3·8 to 4·7 in 2000. Mean distance between nodes in the canopy (MDN) ranged from 12·2 to 15·3 cm in 1999 and from 11·2 to 13·9 cm in 2000. Canopies with different architecture differed in disease progression on stipules and mainly on pods. The levels of disease on stipules and pods were strongly correlated. Moreover, if disease was considered as a function of stipule height up the stem, large differences in vertical disease distribution were observed between cultivars. Three architectural features acted on disease development: cumulative LAI above the node on which disease was assessed (LAIcum) and large MDN favouring disease development, and large internode length reducing disease severity. Modifying LAI distribution and plant organization could be one way to reduce the impact of ascochyta blight, by direct or indirect effects on environmental and dispersal conditions.