The effect of defoliation on phenological development and yield in cauliflower

Decision support models used to reduce inputs of pesticides require a precise modelling of the changes in assimilate production and distribution, but also of possible changes in developmental rates due to sub-optimal conditions. Changes in developmental rates, which shorten or prolong the growing time may aggravate or reduce the effects of sub-optimal conditions on yield. Therefore, the effect of defoliation on development of cauliflower was investigated in a number of field experiments. Reduction in curd growth due to defoliation during the curd growth phase was compensated by an increased duration of curd growth. It was estimated that defoliation during curd induction corresponded to an increase in temperature sum requirement for curd induction of 15 to 25 degree days, while defoliation during curd growth increased the temperature sum requirement with 12 to 60 degree days. Measurement of maturity by means of curd specific weight showed that defoliation delayed maturity. This implies that yield losses due to defoliation can to a large extend be avoided by postponing the harvest of curds. A model that would not include this effect on development of defoliation by pests and diseases would overestimate the need for pesticide application.