Pea canopy architecture affects microclimate and ascochyta blight development

Ascochyta blight (Mycosphaerella pinodes) development on pea is affected by leaf wetness duration (LWD) and temperature (1). The aim of this study was to investigate the effects of canopy architecture and development on microclimate and on ascochyta blight development in pea canopies with contrasting architectures under field conditions.A split-plot experiment was conducted at Le Rheu, France, in the spring of 2009 and 2010 with three pea cultivars (Athos, Antares and Gregor), each sown at two densities (80 and 40 seeds/m²) plus a third density in 2010 (30 seeds/m²). LWD and air temperature (Ta) were estimated respectively with leaf wetness sensors and thermocouples at the bottom and middle height of each canopy. Two mesoclimate parameters (LWD and Ta) were recorded with sensors placed at 1.50 m above the soil level. For each cropping season, microclimatic conditions prevailing during wetness episodes were compared according to the presence or absence of rainfall (rainfall and dry periods respectively).Differences were observed between meso- and microclimate parameters only on LWD but not on Ta. During rainfall periods, LWD was longer inside the canopy than outside (more than 3 to 10h daily). Conversely, during dry periods, LWD became shorter inside the canopy after canopy closure (decreased of 1 to 7h daily). In the denser canopies, these differences appeared first at the bottom. During dry periods, LWD due to dew was longer in the middle than at the base of the canopy, with an average daily LWD of around 4h that decreased during canopy development. When the canopy was completely closed, no LWD was recorded at the base of the plants. Moreover, cultivar Gregor with the greatest leaf area index (LAI) provided lower LWD than the two other cultivars. A plant density effect was also observed in 2010 with longer LWD (more than 2 to 6h daily) in the canopies with the lowest density for each cultivar as soon as canopy was closed. During rainfall periods, no leaf wetness distribution patterns were observed inside the canopy whatever the cultivar or the plant density with an average daily LWD of around 15h. In 2009, longer LWD were observed at the base level for each cultivar sown at the greatest density from two weeks before canopy closure (more than 2 to 5h daily). Regarding the effect of microclimate on disease development, we observed that during dry periods, temperatures were too low during wetness periods to favour disease development and correspondingly, a prediction model adapted from Magarey et al. (2) showed that infection periods occurred only during rainfall periods.Denser canopies limit water evaporation and prevent dew formation on leaves while the lowest canopy densities favour dew formation. Therefore the microclimates of denser canopies are the more favourable to disease development and led to greatest disease severities.