The influence of severity and time of foliar damage on yield and fruit quality in apple (Malus domestica Borkh.)

Leaf damage/defoliation caused by pests or phytotoxic damage following chemical application can result in loss of leaf area at any time during the growing season. Four trials were conducted to examine the impact of severity and time of foliar damage on fruit set and quality in apple. Trees tolerated light to moderate leaf damage throughout the season with no significant effect on fruit set and thus crop load. Where 75% or more of the leaf surface was lost, fruit set was reduced and fruit quality was affected. Complete loss of leaves up to three weeks after full bloom (wAFB) resulted in very low or no fruit set, but from 9 wAFB crop load was not affected by leaf damage. Loss of terminal shoot leaves had no effect on fruit set, but spur leaves were critical early in the season with the impact lessening from 6 wAFB. This study has demonstrated that low levels of foliar damage have little effect on fruit size or quality, but high levels can impact on both set and quality. Fruit quality was reduced following leaf damage during the period 6–12 weeks after full bloom which is the critical period for cell division in the fruit as well as flower initiation for the following year. High levels of leaf damage also reduced fruit size in spite of reduced crop load levels, most likely the result of a reduction in carbohydrate assimilation during the cell division phase of fruit development.