Research from Ontario and New York has looked at the effectiveness of pre and post bloom boron applications in apples. Boron is often applied at these times to reduce cold temperature damage in blossoms, and help improve fruit set and quality.

In Ontario, Dr. John Cline, University of Guelph, looked at the effect of boron on yield and quality in 'Empire' apple orchards. The study evaluated the timing of soil and foliar sprays of boron (pink bud, flowering and petal fall, early and mid-June, and at post-harvest). The boron sources applied were Solubor (soil and foliar) and Botrac (foliar). Treated trees had significantly higher leaf and fruit boron levels when compared to the untreated trees. Although B levels, regardless of product, timing or application method, all treatments showed no beneficial effects on total yield, fruit quality or fruit set.

The research from New York studied pre-bloom boron (Solubor), zinc (Zn-EDTA) and urea applications. The products were applied separately or in combinations.

Based on these projects and other research observations, keep the following points in mind when evaluating the effectiveness of pre and post bloom B applications.

Pre-bloom boron

  • When the previous season's leaf boron levels are adequate, treatments were not effective. In Ontario the level was 22 ppm or higher. In New York, it was 35 ppm.
  • The New York research did not observe a clear relationship between winter severity or observed winter injury and response to pre-bloom nutrients. It suggests that the previous season's growing conditions during bud development may determine whether or not trees will respond to these treatments.
  • Pre-bloom applications have little effect on increasing current mid-season leaf B levels.

Post-bloom boron

  • In Ontario if the previous season's leaf boron level was 22 ppm or higher, treatments were not effective.
  • Post bloom B is more effective in preventing B fruit disorders and increasing current mid-season leaf B levels.

Boron and calcium (Ca)

  • If the soil is marginally deficit in B but well supplied in Ca, applying additional B can promote Ca uptake into fruit.
  • In soils with adequate B levels no additional benefit in Ca uptake can be expected.
  • In some cases pre-bloom B may increase early season Ca uptake in fruit.
  • Post bloom B seems to have little influence on Ca uptake in fruit

Soil boron

  • Roots require B as well, so ensure adequate soil B levels. Foliar applied boron is not translocated to the roots and will not supply the roots adequately. Soil test boron levels (hot water soluble) of 0.5–1.0 ppm B are probably adequate.

Boron application guidelines

  • The Ontario research supports the current OMAFRA guideline for two foliar applications of 200 g actual B/ha in early-mid June of Solubor when a deficiency has been confirmed through leaf analysis.
  • New York research concluded that treatment would be most effective where cropping is expected to be light due to routinely sparse production or following cold injury.