Secrets to a Successful Avocado Bloom

By: Danny Klittich, PhD
April 26, 2021

Why Are My Avocado Trees Turning Yellow During Bloom?

Foreword

Walking an avocado grove during early spring can be a frustrating experience. The tree is starting to push flowers in parts of the canopy, however it remains cool so fruit set is likely a long ways off. The trees, in most cases, have a crop waiting for the market to hit just right to start picking.

We hope winter rains produce the size bump we want. And then there are those pesky yellow leaves showing at the ends of branches. Why is that? Is it a problem? That’s what I wondered last year so we did a little investigating. Here are our results.

Background

During flower development the goal is to produce robust flowers that are capable of setting and retaining fruit. Bud development under nutrient deficient conditions will lead to incomplete floral development, poor pollination, low fruit retention, and lost yield potential. It is common in California avocado orchards to see localized chlorosis at the ends of branches during bloom development. Chlorosis indicates that one or several nutrients are deficient in leaves resulting in irregular metabolism and chlorophyll synthesis.

During bloom development the bloom, itself, becomes a major sink for nutrients within the plant. In California production this generally occurs during cooler months when we tend to fertilize less, and soils are not providing much nutrition. This situation prompts the tree to reallocate mobile nutrients from vegetative tissues (i.e. roots and leaves) to developing reproductive tissues. This also gives us a lens to see what nutrients are necessary at high amounts during these periods and make management decisions to deliver them at appropriate times.

Figure 1. Cross-section of early flower development.
The Research

Recent work surveying tree tissues during bloom development pointed to several key nutrients that are used at high rates during bloom development. The work suggests their importance in early and late season fertility programs. The survey was conducted at three sites in the greater Santa Paula/Fillmore area including the Avocado Commission Pine Tree Ranch research facility and two cooperating growers. Leaf tissue samples were collected during late April bloom development. Samples were gathered from branches exhibiting chlorosis during floral development and from branches of similar developmental stage but having green, non-chlorotic leaves.

Samples were analyzed by Fruit Growers Labs and nutrient levels were compared using a paired t-test. Nitrogen, zinc, and iron all showed significantly lower leaf tissue content in chlorotic leaves. All other nutrients analyzed were equivalent between chlorotic and green leaves.

Discussion

Avocado bloom development is a metabolic energy intensive process with large nutrient demands. Our survey indicates that nitrogen and zinc, both mobile nutrients, and iron, were moved from leaves to adjacent floral development tissues.

Nitrogen is a key component of amino acids and proteins. Protein synthesis is a major limiting factor of pollination and early fruit retention. Early nitrogen applications, as well as, the efficacy of urea in bloom sprays to provided added nitrogen, is well document to improve yield (Lovatt 1999, Lovatt 2001). These data indicate that sufficient tissue levels of nitrogen are essential to support bloom development. An example of this is the change over time in tissue nitrogen targets. Over the past decade it has been recognized that fall tissue nitrogen percentages of 2.5-2.7% correlate with higher yields. This is an increment higher than the older bracket that ranged from 2.0-2.2% nitrogen. The stored nitrogen in leaves acts as a reservoir for bloom development. Therefore, higher fall tissue levels of nitrogen result in a better start to bloom.

Zinc is a common nutrition input in avocado to increase fruit size. Zinc is regularly deficient in many avocado orchards. This is especially true under high fruit load. Zinc is important for protein synthesis, enzyme production, and other physiological uses. Due to zinc’s chemisty it is a challenging nutrient to move through soil and into the root system making it difficult to manage in orchards. Chelated zinc sources are the best options for uptake as they move in soil and are readily taken up by trees. Iron is a micronutrient that we rarely apply in avocado production. However, in cool, wet soils iron is chemically tied up and is unavailable. Supplementing iron and zinc in early season bloom sprays is a simple application method to deliver needed nutrition to developing blooms.

Nutrient sufficiency and availability are key to maximizing fruit set and retention. Nutrient allocation is the driving force behind what we often anthropomorphize as tree choice in deciding which fruit are retained and which BBs are dropped. If a nutrient is deficient during any step between floral initiation and June fruit drop then that reproductive potential is lost and with it yield potential. Insufficient nitrogen to produce pollen equals flower abscission. Insufficient zinc to build enzymes in newly fertilized fruit equals abortion. Insufficient calcium to build cell walls during stem elongation equals fruit drop. Applying nutrition products during key developmental stages results in successful tree and fruit development and achievement of yield goals.

Bloom Application Details

A successful bloom starts the preceding year by growing a strong tree with bud wood that can support set, both physically and nutritionally. In the spring there are 3 key steps in our framework to improve yield potential:

Step 1 – Early Bud Development. This stage is marked by the cauliflower bloom development and floral elongation. Chemigate Nitrogen, zinc, iron, boron, and calcium.

- AN20 5 gallons per acre
- TriPlex Micro Flowable (Zn, Fe, Mn) 32oz per acre
- TriPlex Boron 0.5 pounds per acre
- Mainstay Calcium 0.5 gallons per acre

Step 2 – Bloom Spray. Floral tissue efficiently assimilates foliar applied nutrition. Pollination relies heavily on calcium for pollen tube elongation, nitrogen for amino acid production, and micronutrients. Supreme contains zinc and iron and amino acids to increase fruit set and reproductive energy. This application can be made from cauliflower stage (Figure 3) through full bloom. It can be tank mixed with ProGibb.

- Supreme 2 pounds per acre
- Lo-Bi Urea 6 pounds per acre
- Mainstay Si 0.25 gallons per acre

Step 3 – Fruit Set. Avocados usually set the majority of fruit in the late spring when nights warm to over 55oF. At this point weekly chemigations of the following blend will support fruit set and development.

- Mainstay Calcium 0.25 gallons per acre
- AN20 1 gallon per acre
- Rx Platinum 16 oz per acre
- TriPlex Micro Flowable 8 oz per acre

References

Lovatt, C.J. 1999. Timing citrus and avocado foliar nutrient applications to increase fruit set and size. Hort Technology. 9:607–612.

Lovatt, C.J., 2001. Properly Timed Soil-applied Nitrogen Fertilizer Increases Yield and Fruit Size of Hass' Avocado. Journal of the American Society for Horticultural Science. 126(5), pp.555-559.

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