Influence of defoliation timing and severity on cauliflower earliness, yield and quality

In early spring, cauliflower may be subjected to damaging hailstorms which can severely damage foliage and delay subsequent curd formation. It is unknown the limit of foliar defoliation that is permissible before earliness, yield and quality are decreased. The objective of this experiment was to quantify the long-term effects on yield and quality of leaf defoliation gradients occurring 1) soon after transplanting, 2) mid-season, and 3) at early head formation near the end of the growing season with early and late maturing cultivars. Plants were defoliated with scissors removing 25%, 50% or 75% of leaf tissue early season (11 days after transplanting), mid season (large plants without any sign of head initiation), and late season (heads were approximately 1-2 inch diameter). Control heads were always the heaviest, but head weights differed by the time and level of defoliation applied. Early season defoliation at 25% reduced the final head weight by 5%, but this was not significant. Increasing the severity of defoliation from 50 to 75%, however, reduced head weight significantly from 9% and 12%, respectively. Defoliation during mid-season had a stronger effect, especially at the highest defoliation level of 75%. At 25 to 50% defoliation, head weights were reduced 11% and 10% in contrast to the controls, respectively, and these reductions were similar to those experienced at the same defoliation levels during early defoliation. However, defoliation at mid-season at 75% level, reduced head weight 23% in contrast to the control. Late season defoliation was not more detrimental and was similar to the mid-season defoliation. Cumberland was less affected by defoliation than Majestic, experiencing only a 160 to 319 kg/ha reduction in yield with 25 to 50% defoliation at all sampling periods. Majestic suffered a loss of 2,235 to 3,673 kg/ha at the same defoliation levels. Defoliation of 75% for Majestic and Cumberland reduced kg/ha 5,430 and 1,916, respectively. It is apparent that tolerance to defoliation differs by cultivar with some, like Majestic, having greater tolerance than Cumberland.