Optimizing Fruit Size and Preventing Bitter Pit in Apple Crops with Early Season Interventions

Calcium applications:

  • Apply 4-14 pounds of actual calcium per acre each season to help prevent bitter pit and cork spots. This amount is equivalent to applying 15-50 pounds of calcium chloride (CaCl2) or distributing 2-8 pounds of CaCl2 per cover spray.
  • For apple varieties that are particularly susceptible to bitter pit, such as Honeycrisp, Golden Delicious, and York Imperial, it is advisable to use the maximum recommended concentration of calcium.
  • Initiating calcium applications early in the season, starting at the Pink stage of bud development, is more beneficial than applications later in the season.
  • Foliar applications of calcium nitrate are not advised for Delicious and York apple varieties, as they can induce symptoms similar to cork spot.
  • In orchards with significantly low calcium levels, applying calcium sulfate (gypsum) in a band beneath the trees at a rate of 3 tons per acre can improve calcium content in the leaves and fruit. However, it may take two years or more to observe positive outcomes.
  • Avoid applying calcium chloride under conditions that slow drying, such as in the early morning, as it can harm the foliage. This is especially important for apple varieties sensitive to calcium chloride, including Idared and Golden Delicious.
  • A deficiency in boron can hinder the movement of calcium within the tree, potentially affecting fruit quality.

Boron applications:

  • Boron plays a crucial role in the development of flowers and the setting of fruit. A lack of boron can adversely affect both yield and the size of the fruit.
  • Furthermore, boron is necessary for the transportation of calcium within the plant. A deficiency in boron can lead to disorders related to calcium deficiency, such as bitter pit and cork spot in apples.
  • To avoid these problems, it is recommended to apply 0.5-1 pound of boron per acre, which is equivalent to applying 2.5-5 pounds of Solubor per acre.
  • Boron should be applied either at the pink or bloom stage of flower development, mixed in the tank with calcium chloride, or 7-10 days following the fall of the petals.

Prohexadion calcium:

  • Applying prohexadione calcium (PC), found in products like Kudos and Apogee, early in the season at the Pink stage can significantly reduce the incidence of bitter pit and blossom blight, as well as decrease the risk of shoot blight.
  • For optimal results from PC treatments, consider the following guidelines:
    • Apply PC at a rate of 6 ounces per acre.
    • PC’s effectiveness decreases in environments with high pH levels or in water with a high concentration of calcium carbonate (hard water). To counteract this, add ammonium sulfate (AMS) to the spray mixture.
    • Avoid combining PC with calcium or boron in the spray mix, as interactions may diminish efficacy.
    • Enhance the performance of PC applications by including a surfactant in the tank mix. This helps in better distribution and adherence of the product on plant surfaces.
    • Should the surfactant cause foaming, incorporate an anti-foaming agent to mitigate this issue.
    • Both Kudos and Apogee are approved for application at the Pink stage for apples, indicating a targeted timing for use.
    • Research by Sherif in 2019 (unpublished) suggests that applying PC concurrently with or before thinning agents like 6-BA or NAA does not compromise the effectiveness of thinning treatments.

Urea applications

  • Applying urea to the foliage during the bloom period (at a rate of 3 pounds per 100 gallons of water) and subsequently at petal fall and the first cover spray (5-6 pounds per 100 gallons of water) can significantly support cell division. This practice is especially beneficial for Gala apples. Such applications become critical when the primary, or king blooms, which typically produce the largest fruits, are lost to frost damage.
  • The foliar application of urea at the time of bloom can also improve fruit set by prolonging the period of effective pollination, enhancing the chances of fertilization and subsequent fruit development.
  • It’s important to note that foliar-applied nitrogen, while beneficial for fruit set and sizing, does not substitute for soil-applied nitrogen fertilizers. Unlike ground applications, foliar-applied nitrogen does not move into the tree’s woody structure but is instead directly utilized for the development of the fruit.

Fruit Russeting in Golden Delicious and Scarf Skin in Gala:

  • Fruit russeting and scarf skin in apples is often associated with high humidity during the first 30-40 days of fruit development.
  • To address this issue, apply 2-4 applications of GA4+7 (such as ProVide 10SG or Novagib 10L), starting from petal fall and continuing at 7-10 day intervals.

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