MANAGING TREE VIGOUR AND FRUITFULNESS IN DECIDUOUS ORCHARDS
Vigour appears directly related to the available volume of soil suitable for root growth. Irrigation levels that replace less than evapotranspiration lower vigour as a consequence of developing water stress in the tree and by reducing the volume of wetted soil. Under Regulated Deficit Irrigation the trees are water stressed during periods of rapid shoot growth and fully irrigated during rapid fruit growth, usually for 5–8 weeks before harvest. Relating the level of irrigation to seasonal stages of shoot and fruit development results in a high degree of control over vigour while maintaining or increasing yield. Pruning can control tree size, canopy shape and density. However pruning also increases shoot vigour with negative effects on fruitfulness. The growth retardant Paclobutrazol controls vigour and increases fruit size in stone fruit, cherries being particularly sensitive. Activity in apple and pear is less predictable with undesirable side effects on fruit shape. In stone fruit both the degree and duration of growth retardation are dose related. This suggests that while high dosage can virtually eliminate extension growth, precise treatment with small doses eg. by feeding through the drip irrigation system, will result in short term control over vigour that could be used to manipulate canopy development.