In today’s competitive commercial stone fruit market, fruit size and actual packout volume per acre are critical components for grower profitability. Because hand thinning can be the largest preharvest cost, what factors can be manipulated to positively influence fruit size and packout at a reasonable cost?
The first factor is genetic. Some cultivars have higher flower bud numbers or bloom density than others. Also, some cultivars are genetically “largefruited” while others are “small-fruited.” Research has demonstrated that this latter genetic difference is reflected in the cell number per fruit. Largefruited cultivars may actually have more than twice as many cells per fruit as small-fruited cultivars. In fact, these cultivar differences can actually be observed in the flower buds the preceding fall! More cells per fruit are capable of being expanded during the final swell phase of fruit growth, so fruit will be larger. Therefore, careful consideration of cultivars and their genetic size potential is one way to ensure optimal performance.
The second factor is cultural. Cultural practices to manipulate crop load can occur before, during, or after bloom. Spring flower production is largely dependent on carbohydrate reserves that were laid down in the tree the previous fall. At bloom, only a minimal leaf canopy exists and there is competition between many tree organs for the limited reserve carbohydrates. Too many flowers/developing fruitlets competing with other organs for reserves will limit cell division, the actual cell number per fruit, and final fruit size. Therefore, to maximize the effect on final fruit size, thinning (selective removal of flowers or fruits) should be done as early as possible.
Reduction of flower bud numbers prior to bloom is risky because deepwinter freezes or spring frosts may adversely limit the total number of flowers. Research utilizing fall-applied gibberellic acid (ProGibb, Valent USA) or ethephon or dormant season applications of soybean oil or Dormex (hydrogen cyanamide, Dormex Co. USA) has successfully reduced spring flower bud numbers. The main problem in using these products is the consistency and reliability of results in commercial orchards across cultivars, locations, and seasons. On the other hand, research using dormant pruning to selectively remove fruiting shoots or heading fruiting shoots by 50% has been demonstrated to reduce the amount of hand thinning necessary without adversely affecting fruit size.
During bloom, flowers can be removed physically or chemically. This type of early thinning allows you to see what flowers are available to be pollinated and develop into fruits. Physical removal of flowers by hand, using brushes or mechanical rope “curtains,” can be quite reliable and effective. Flowers can also be removed by airblast application of various caustic chemicals or surfactants such as ammonium thiosulfate (Thio-Sul, Tessenderlo Kerley). Some growers have had success with chemical bloom thinning but results often vary because of many factors including how the trees are trained, the stage of bloom, relative humidity and temperature, concentration of chemical used, volume of water, the use of surfactants, etc. It is hardly an exact science where extreme over or under-thinning is more the rule than the exception.
The most conservative approach is to “wait to see what you’ve got” after bloom in terms of actual fruit set after pollination. Unfortunately, by this time, the maximal potential impact of thinning on final fruit size is reduced because the final cell number per fruit is almost set. Post-bloom treatments include hand or mechanical removal of fruits, chemicals, and pruning. Hand removal is reliable but expensive.
The key consideration is the actual number of fruit per tree rather than spacing on limbs or number of fruit per shoot. Mechanical limb or tree shakers can be used but damage to bark may predispose trees to insect or disease infestation. With mechanical thinning, the largest fruit are often removed and touch-up hand thinning is necessary. Post-bloom chemical thinners are inconsistent. Pruning to limit the number of fruiting shoots per tree can be particularly effective if simple tree designs are utilized (i.e., Kearney perpendicular V).
Finally, it cannot be overemphasized that thinning should be conducted with great care. Consideration of cultivar, timing, and method of thinning will help to ensure results that maximize profit potential.
This column by Dr. Desmond R. Layne, “Managing Crop Load to Maximize Profitability” appeared in the April 2007 issue of The American Fruit Growermagazine on page 60.