June drop
One of the most often asked question this time of the year concerns june drop.  I decided to get Linda, my graduate student, to address this question as part of her Master thesis comprehensive exam .  Following is my question (open book) to her and her answer.  I hope her answer gives you a better understanding of this very important and complex phenomenon. 

Question:
June drop is an event that occurs about four to six weeks after bloom.  What is the physiology of June drop and what environmental, cultural and physiological factors influence it?  Growers would like to know, in advance, the extent of drop they will be encountering in order to assess their fruit set.  Are there any signs or symptoms that growers can use to estimate the amount of drop they will have?  Given the weather conditions we have had this spring, what would your estimate of the extent of June drop expected?

Answer by Linda Boccuzzo
Fruit set is whatever fruit is left on the tree after fruit and flower drop (Rom, 1992).  An early determination of fruit set is valuable piece of information for apple growers.  The amount of fruit set will determine the need/quantity for summer pruning and nutrition needed in the block.  Potentially, it may indicate the final quality of the crop (quantity produced and general size of apple).  As well, storage space, harvest labor and input costs into the block can be assessed earlier in the growing season.

Fruit set is dependent on the amount of "return bloom", proper pollination, and fruit drop (Faust, 1989).  Return bloom is necessary for good fruit set.  In apples, the blossoms should not be excessive.  As too many blossoms causes weak set (Dennis, 1996).  Apple return blossoms should be strong, with large spur leaves (Lord et al., 1979; Rom and Ferree, 1984). Induction of flower buds, like any other physiological process in the tree, requires proper carbohydrates, water, temperature, and hormones (Buban and Faust, 1982). Return bloom is dependent on the nutritional status of the tree, particularly N, B, and Zn (Rom, 1992).  Most importantly, nitrogen needs to be at optimal levels for flower formation (Dennis, 1996).  Also, sufficient time/weather for flower formation the summer prior; hot weather generally increases shoot growth and heavily competes with the bud for resources (Rom, 1992). Winter injury will damage the blossoms and reduce fruit set, also windy spring season may desiccate opening flower buds and reduce the vigor of the flower (Strang et al., 1980; Proctor and Schechter, 1992).  Hot weather at the time of bloom may also reduce fruit set, simply by being too hot for pollination (Dennis, 1996).
Pollination, is obviously essential to fruit set.  Since apple trees generally require cross-pollination, pollinator trees must be available when needed.  Pollination must occur during the effective pollination period (EPP) of the flower (Williams et al., 1984).  (EPP = time the ovary is receptive minus the time required for the pollen to travel down the style and fertilize the egg.).  Bees (the usual pollinators) must be available in the orchard and must be attracted to the apple blossom and not the weeds (i.e. dandelions) (Dennis, 1996).  It has been shown that bees do not work wet, cloudy weather (Williams et al., 1984).  Hence, such weather at bloom will probably decrease fruit set.

Fruit drop generally is agreed to occur in four waves in apples. The waves may overlap.
First wave:  Post-bloom drop caused by incomplete/damaged flowers as a result of last year's set.
Second wave:  Flowers (fruitlets) drop due to lack of pollination and application of chemical thinners).
Third and fourth wave:  Drop due to competition for resources (?) or further injury causes drop (June drop)  (Faust, 1989).
The third and fourth waves are often the most unpredictable and the largest.  It is theorized to occur for several reasons, which will be considered here.  Poor nutrition, plant hormones, winter injury, inter-fruit competition and weather have all been implicated in this drop.

HORMONES
In 1969, Crane claimed that fruit drop was a direct result of hormonal gradient of diffusate from the fruit.  He claimed that fruits that were stronger sinks for auxin, did not drop (Crane, 1969).  This indicated that there was some specific inter-fruit gradients/competition.  It was thought that fruits that had not set properly would lack the proper number of seeds and therefore be hormonally weaker.  Fruits that had the proper seed number would regulate the auxin sink (gibberellin : auxin interaction) (Crane, 1969; Westwood, 1978 ).  Seeds produce the gibberellins that trigger auxin production and theoretically, the formation of a stronger sink (fruit) (Westwood, 1978). 
The research done on the seeds has been source of conflict in apple research.  Weinbaum and Simons (1974b) in histological studies noted smaller seeds and different starch allocation in fruit that dropped versus fruit that did not.  However, gibberellins are not produced in fruit seeds until 5 weeks post-bloom and therefore cannot regulate the hormonal gradient, as had been suggested (Weinbaum and Simons 1974a).  It may be some combination/ratio of hormones that affect the drop, but the exact nature has not yet been determined (Westwood, 1978; Bangerth, 1990; Abruzzese et al., 1995) .
Ethylene produced when the second wave of fruitlets drop, may deter growth in locally growing fruit, weakening them and causing them to drop (Klein and Faust, 1978).  Ethylene synthesis inhibitors will prevent drop, but do not increase fruit quality in the crop (Greene, 1983).
Bangerth showed that applications of growth regulators (thinners etc.)  may alter the polar flow of auxin resulting in fruitlet drop (1990).  Bangerth could find no internal regulation of a hormone that would alter the flow as such (1990).   In this vein, many external growth regulators are applied to developing fruitlets to control drop.  Napthaleneacetic Acid (NAA), carbaryl and benzyl-adenine (BA) are currently used to influence fruit drop.  Although the mechanisms by which they work are unclear.  NAA is thought to influence source-sink relationships by promoting ethylene synthesis (Yang et al., 1980).  Carbaryl will work in conjunction with NAA.  BA, is a cytokinin that is thought to influence rapid cell division in persisting fruit.

NUTRITION
Due to the inconclusive nature of the hormone regulation of fruit drops, the competition for photosynthate was studied.  However, no amount of carbohydrate differences was seen in thinned versus un-thinned (highly competitive) fruit (Beruter, 1985). Also, Weinbaum and Simons histological studies that showed no difference in amount of starch accumulated between dropping and persisting fruit (1974a).  However, Carbon-14 studies done by Schneider (1978) indicate that selective translocation is responsible.
If there is a nutrition competition it is not obvious.  Abruzzese et al. demonstrated this on 'Gloster'/M9 (1995).  There were high levels of soluble sugars and sucrose in both.  (Sucrose is the preferred carbon source for developing fruit (Beruter, 1985).)  However, a lower content of potassium and hydrolyzable polysaccharides, (but higher levels of calcium,) was seen in abscised fruitlets.  They also noted that all abscised fruit had the proper number of seeds, however they were smaller than those of persistent fruit.  Abruzzese et al. concluded that inter-fruit competition was not the primary cause of fruit drop (1995).    However, proper fertilization is agreed upon to provide strong trees (Abruzzese et al., 1995; Dennis, 1996)

MECHANICAL INJURY
Ovule injury has been shown to cause drop resultant from physical/winter damage (Strang et al., 1980; Wilde et al. 1984).  In an attempt to re-create frost damage, Proctor and Schechter, used a needle to damage a portion of a flower blossom (1992).  Upon damage, there was severe drop noted.  The amount of drop appeared to be cultivar-specific in that 'McIntosh' dropped more fruit earlier and consistently throughout the season.  'Delicious' and 'Idared' had far less drop.  Although, the persistent fruit of all cultivars, was deformed in many ways, the L:D ratio remained the same, indicating attempts at normal growth patterns (Proctor and Schechter, 1992) . 

CLIMATIC CONDITIONS
Now, all of the above internal factors may play a role in drop.  None has been conclusively proven or disproven.  However, an external factor may also play the most important role.  Weather has been shown to have an effect on the physiology of the tree, as well as a direct correlation with drop.  Rom (1992) stated that cool, cloudy conditions limit fruit growth and set.  Three or four days of cloudiness the month post-bloom increased fruit drop.  Internal shading in the tree affected the drop similarly.  Also, flooding or extreme dry conditions post-bloom will increase drop (Rom, 1992).
The physiological basis of June drop has not yet been conclusively identified.  It may be a combination of all of the above listed factors, as no single one can be directly correlated with drop.  The best way to insure a good fruit set, begins with proper nutrition and light penetration the year before, prevention of over-cropping, along planting of cold-hardy varieties.  Irrigation should be used in the month after bloom if dictated by the weather.  Trees that are flooded regularly should probably be chemically thinned less, realizing that June drop will more severely affect those trees. 
The only physical symptoms of excessive fruit drop would be trees that were not healthy enough and therefore, set weak blossoms.  Also, if vigorous shoot growth was noted, resulting from excess nitrogen or excess dormant pruning, then a grower may be able to estimate higher drop.  If a grower wanted to estimate drop, they might take a tree and cut open fruit to compare seed sizes, but that may not reliably determine the drop.  Weak fruit has been seen to persist on trees.  Ethylene inhibiting sprays may prevent some drop, but will not guarantee quality fruit.

Given this year's consistently cool (40-66º F for most of bloom), cloudy and rainy weather (that reached flooding in areas), I would estimate that June drop would be rather extensive in the Burlington area.  The southern part of the state has had more sun and heat and therefore, I would expect fewer drops.  Also, due to last year's hot weather, I would expect less fruit set due to improperly set flower buds.  Pollination weather this year was less than favorable (cloudy, wet, cool), resulting in less fertilization.  This may account for more flower drop than usual. 
The only factor that I am unsure of is the effect of the warm temperatures (in the 80's (F)) that occurred immediately after tight cluster.  I don't think enough of the flowers were open at this point, so the heat may not have affected a large portion of the crop.  However, the flowers that opened earliest were probably the king blossoms, and damage of them would have the most detrimental effect on fruit set.  Bloom was not seen until, May 10, 2000 at the Horticultural Research Center (South Burlington, VT) and temperatures peaked on May 7, 2000 @ 81.9º F---so the time may be close enough to have damaged some of the king blossoms, but probably not excessively so.   (Full bloom was seen on May 15, 2000)
I think the cloudy, cool, wet weather since will have more of an effect on reducing the fruit set.

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