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Key points about floral biology, pollination and fruit set in WA38

Written by Jenny Bolivar, WSU Extension ITT, Jun 4, 2021.

For a formatted printable inforgraphic pdf download. Floral biology and pollination in WA38

En Espanol download Biologia floral y polinizacion en WA38

Floral Biology

  • In general, there are 6 blossoms per bud: a king flower (central)
    surrounded by 5 Lateral flowers. WA38 King flowers bloom 48-72
    hours earlier than lateral flowers.
  • In 2020, WA38 king flowers bloom starts by the third week of April
    (Rock Island, Wenatchee).
floral biology graphic

Pollination (results from 2019 study)

  • Apple flower requires cross-pollination from two different compatible cultivars to set fruit.
  • Effective pollination period (EPP) is the time window when a pollination results into a successful flower fertilization and fruit set. In WA38, preliminary data shows an EPP of 2 days for king flowers.
    EPP is influenced by stigmatic receptivity, pollen tube growth and ovule longevity.
  • Stigmatic receptivity is the time when stigma is receptive (sticky) and support adhesion of the pollen grain and the subsequent pollen germination. The stigmatic receptivity in WA38 was 9 days.
  • Pollen tube growth rate is the length of time for a pollen tube to grow and reach the ovule. In WA38, Granny Smith pollen tubes reach the ovules 7 days after pollination.
  • Ovule longevity is the duration of ovule’s viability. In WA38, ovule longevity started to declined after 9 days.
  • Overall, it was found that the first 2 days after the flower opens (anthesis) are critical for a successful pollination in WA38.
cross pollination Granny Smith and WA38 graphic


  • Is a tree with pollen that is compatible with a commercially interesting apple cultivar (in this case WA38), and therefore, used as a source of pollen.
  • A good pollinizer has a fast pollen tube growth rate inside WA38 flower and good fruit set.
  • Effective pollinizers evaluated for WA38 are Snowdrift, Indian Summer, Granny Smith, and Everest. Snowdrift pollen tube growth was faster in 2019, but there were no difference in pollinizers in 2020.
graphic representation of pollinizer tree

Fruit Development and Quality

  • WA38 is a self-thinning variety, meaning it will only retain 1-2 fruits per cluster.
  • Flower and fruitlet drop occurred from 3 days to 8 weeks after bloom.
    Fruit quality is not affected by the pollen source (pollinizers mentioned above).
  • Good size and shape fruit was correlated with 9-10 seeds per apple.
  • WA38 king fruit tends to be more spherical, heavier and numerous in seeds than the lateral fruit.
fruitlet and fruit graphic


Serra, S., and Musacchi, E. (2021). Pollination, Flower Biology and Fruit Development in ‘WA38’ apples. Final Project


Jenny L. Bolivar-Medina
ITT Tree Fruit Horticulture Extension-WSU articles may only be republished with prior author permission © Washington State University. Republished articles with permission must include: “Originally published by Washington State Tree Fruit Extension at” along with author(s) name, and a link to the original article.

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