Proc. Amer. Soc. Hort. Sci. 53: 141-142 (1949)
Effect of Boric Acid Sprays Applied During Bloom Upon the Set of Pear Fruits
U. S. Department of Agriculture, Wenatchee, Wash.

FOR no apparent reason certain fruit species or varieties frequently fail to set as many fruits as desirable for satisfactory yields. In the Northwest, Anjou pears bloom heavily but characteristically set relatively light crops. Degman (4) has applied sprays containing different growth regulating chemicals to Anjous during the bloom period but failed to obtain any increase in fruit set. Several investigators (1, 2, 3) have reported that boric acid or borax added to the sugar medium increased both the percentage of pollen germination and the pollen tube growth for certain plant species. Blaha and Schmidt (3) reported that the addition of 0.0005 per cent boron to the culture medium resulted in a much higher germination of cherry and pear pollen, but no increase was obtained with plum and prune pollen. Eaton (5) has presented evidence to indicate that at least one of the functions of boron in plant nutrition is intimately related to the formation of plant hormones. Reports by these investigators serve to indicate the background for initiating experiments to determine the effect of boric acid sprays applied during the bloom period on the set of Anjou pears.


In 1948 two Anjou orchards in the vicinity of Wenatchee, Washington, were selected for experimental treatment. In both orchards four limbs (200 to 300 blossom clusters per limb) located on the south side were selected on each of 10 trees. When 40 to 60 per cent of the blossoms were open, boric acid sprays at concentrations of 5, 25, and 125 ppm of boron were applied to individual limbs on each tree. The fourth limb on each tree served as a control. Fruit set records were obtained on an individual limb basis by counting the blossom clusters prior to treatment and the number of fruits remaining after the June drop.


The data presented in Table I indicate a consistent increase in fruit set with increase in concentration of boron applied. In both orchards the 125 ppm treatment and in the Griffith orchard the 25 ppm treatment resulted in a significantly greater fruit set than the control.


Spray Treatment Number Fruits Per 100 Blossoming Spurs
Griffith Orchard Peterson Orchard
Control 20  38 
    5 ppm Boron 26  47 
  25 ppm Boron 29* 53 
125 ppm Boron 32* 60*
*Significantly greater than the control at 5 per cent point.


Under the conditions prevailing in this experiment it seems evident that boric acid sprays significantly increased the set of Anjou pears. In speculating an the role of boron in this instance there seem to be at least three possibilities, all of which may he more or less interrelated: (a) stimulation of pollen germination and pollen tube growth; (b) stimulatory effect of the boron on formation of plant auxins during and immediately following the bloom period; (c) correction of an incipient or temporary deficiency of boron.

It is commercial practice in the area where these studies were performed to make soil applications of boric acid at periodic intervals. In 1947 both of the experimental orchards received a soil application of boric acid at the rate of 30 pounds per acre. Although no visible boron deficiency symptoms have ever been observed in these orchards, the decisions to make such applications were based on the fact that occasional boron deficiency has occurred in the general area where the orchards are located. Analysis of fruit and leaves at harvest time (1948) showed the boron content of both tissues well within the luxury range. Whether or not there was an incipient deficiency during the bloom period is a matter of speculation. If boron within the tissue of the pear tree is relatively immobile, a temporary deficiency of this element could possibly have been a limiting factor in fruit set The spring of 1948 was abnormally cold and wet, a condition that might possibly have restricted the supply of boron to young developing growth provided there was no boron reserve available within the tissues for retranslocation. If this hypothesis in correct, boric acid sprays could hardly be expected to increase the set of fruit in a season more favorable for boron absorption from the soil.

More extensive work is in progress to determine various aspects of response to boric acid sprays covering a wide range of fruit species, varieties, and conditions.


  1. ARK, P. A. Further studies on walnut blight. Diamond Walnut News 31, No. 2:4. 1949.
  2. BERLING, V. V. Influence of major and minor elements on pollen germination is plants. Compt. Rend. (Doklady)  Acad. Sci. USSR 32,  6:439-442. 1941.
  3. BLAHA, J., and SCHMIDT, J. The effects of boron on the germination of pollen in fruit trees. Sbor. Cesk. Abad. Zem. 14 No. 2: 186-192. 1939.
  4. DEGMAN, E. S. Unpublished data. U. S. Department of Agriculture, Medford, Oregon.
  5. EATON, FRANK M. Interrelations in the effects of boron and indoleacetic acid on plant growth. Bot. Gaz. 101:700-705. 1940.