FACTORS INFLIMCING FRUIT SET OF APPLES 



Of the many crucial periods in production and marketing of apples (from the 

 flovirer bud to apple pie), the fruit-setting period is always a perennial source of 

 anxiety to the grower. There are many factors affecting fruit set some of which 

 can be controlled and others that cannot be controlled by the grovrer. Factors 

 such as (1) site, (2) varieties, (3) location, choice and sufficient number of 

 pollenizers, (h) nitrogen and (5) fungal, bacterial and insect injury are those 

 ;Thich can be controlled by the grower. ^Afhile on the other hand, the grovrer has 

 little or no control over ecological factors, namely temperature, rainfall, vj-ind 

 and humidity, Pollenizing insects also affect fruit set but cannot be classified as 

 either controllable or a non-controllable factor. Although the grovrer cannot pro- 

 vide favorable weather for bee flight, he can place hives of bees throughout his 

 orchard to insure better cross-pollenation. On the other hand, some years it may 

 pay the grower to take the bee hives out of the orchard after a day of good pollen- 

 izing weather to help prevent over-setting of fruit. 



Although the grower has little or no control over the ecological factors affect- 

 ing frioit set, it is these that vdll be discussed in this article in hope that it 

 Tfill increase grower understanding of factors affecting fruit-set. 



Temperature 



Temperatures belov/ UO°F during bloom not only prevent bee activity, but they 

 either hinder or prevent pollen germination. Ejcperiments vdth hardy fruit trees 

 have shovTO there is some pollen gerraination from UO°F to 50°F but that pollen tube 

 grovrth is greatly retarded. Optimum temperatures for pollen germination and pollen 

 tube growth are from 70°F to 80°F, 



Frosts just preceding or during bloom may reduce fruit-set considerably. 

 However, the temperatures that fl.ovj-er buds and developing fruits will endure 

 depend on many factors. The length of time the low temperature persists, the 

 tree vigor, the weather preceding the frost, the variety and the stage of bud 

 development all play a part in how much cold buds, blossoms, or fruits will stand. 

 The buds or fruit on a healthy vigorous tree will endure lower temperatures than 

 those on a weak tree -when other conditions remain the same, A frost follovdng a 

 period of favorable growing temperatures will cause more damage than one folloidng 

 a period of cold, cloudy weather as the sap concentration is lower and freezing point 

 higher during periods of warm sunshiny weather. 



The temperature 'which decid\ious fruits will endure at various stages of devel- 

 opment are given in Table I. ■ 



Table I. Temperatures Endured for 30 Minutes or Less By Decidous Fruits (Sheltered 



Thermometers) 



Stage of Development 



' Fruit '. Buds Closed But Full Bloom Small Green Fruits 



Showing Color . ■ 



__ ^_ _ , up 



Apples 25 ■ ■ • • 28 29 



Peaches 2$ 2? ' 30 



Cherries 28 - 28 ' 30 



Pears 25 28 30 



Plums 2$ 28 30 



