Some effects of canopy structure and microclimate on infection of tall and short wheats by Septoria nodorum

Abstract

The results of three kinds of field experiment were consistent with the hypothesis that the canopies of taller wheat varieties generate a less favourable microclimate for development of S. nodorum from standard inoculum. (1) Measurements of photosynthetic area per unit volume of space occupied by canopies showed that taller varieties had lower canopy densities than shorter varieties. (2) Less disease developed in artificially thinned canopies of tall and short varieties. (3) Less disease developed in plots raised on mounds and more developed in plots sunk in trenches, relative to plots at ground level.The results of three further kinds of field experiment were consistent with the hypothesis that reduced leaf surface wetness contributes to the tendency for taller varieties to be less infected by S. nodorum. (1) Continuous electrical measurement showed that leaf surface wetness (mainly dew) on two tall resistant varieties lasted for a shorter time than that on a short susceptible variety. (2) Instantaneous estimation of dew by weighing showed that less was deposited on taller and more resistant varieties than on shorter and more susceptible varieties. (3) For 10 sister lines from the same cross, selected only for their variation in height, there were correlations of duration of surface wetness with height (negative), amount of S. nodorum with height (negative), and amount of .V. nodorum with duration of surface wetness (positive). There was also genetic variation in surface wetness that was independent of height.Nevertheless, a microclitnatic effect is thought to be only one of several mechanisms that contribute to the tendency for taller varieties to be more resistant. In addition, there is much genetic variation in resistance that is independent of height.