Effect of crop age on primary gradients of late leaf spot ( Cercosporidium personatum ) on groundnut

Artificial sources of late leaf spot were established by inoculation of one plant at the centre of several groundnut plots differing in age. First‐generation dispersal and resulting primary gradients of disease were then compared. The gradients were corrected for source strength, spontaneous infection (primary gradient), and interplot interference (first‐generation dispersal). The shapes of the gradients for spore dispersal and of the gradients of disease corresponded for each age of groundnut plot, but differed between the ages. The slopes of gradients were steeper in the older plots. Spore dispersal and the resulting disease gradients were not isodiametric, gradients being most protracted in a north‐to‐north westerly direction. The results suggest that the strength of the sources was reduced by abscission of infected leaves in the sources of the younger plots, whereas accessibility of the canopy to the transported spores was reduced by increasing canopy density in the older ones.