Improving Fruit Set and Yield of the 'Hass' Avocado with Potassium Phosphate or Potassium Phosphite Applied During Bloom

Continuing Project; Year 4 of 4

Project Leader: Carol J. Lovatt (909)-787-4663
e-mail: carol.lovatt@ucr.edu
Department of Botany and Plant Sciences, UC Riverside

Cooperating Personnel: Larry Summers, Jian-Yu Fu, Anwar Ali, Grant Klein



Benefit to the Industry

At the end of this research, we will be able to tell avocado growers the optimal potassium and phosphorus fertilizer formulation(s), best application technique (soil vs. foliar), and a time of application based on the phenology of the tree to use to increase fruit size, cumulative yield and reduce alternate bearing.


Objectives

The main objective of this project is to improve yield in the 'Hass' avocado by supplying nutrients that might be limiting during fruit set due to competition between individual setting fruit and between setting fruit and developing vegetative shoots at the time they are most needed but are not adequately supplied by the roots or from reserves in the leaves or branches.

In this project we are compare the efficacy of applying potassium phosphate versus potassium phosphite to the canopy of 'Hass' avocado trees during bloom to increase yield and net return to the grower in comparison to trees receiving potassium phosphate or potassium phosphite to the soil via irrigation.


Experimental Plan and Design

The research orchard is located in Somis, California. The trees are 11-yr-old 'Hass' avocado on Duke 7 rootstock. The experimental design is randomized complete block with 20 individual tree replicates per treatment. All treatments are being applied at the cauliflower stage of inflorescence development. This stage of inflorescence development was determined by Salazar-Garcia et al. (1998) to be the time of gametogenesis, i.e., pollen, ovule, and egg formation and confirmed through field research to be the optimal time to apply foliar nutrients (Jaganath and Lovatt, 1995) to improve fruit set.

Treatments are the following: (1) control; (2) phosphourous acid based fertilizer, 0-28-26, at 2.86 quarts/acre to the soil; (3) phosphoric acid based fertilizer, 3-18-18, at a rate of 4.44 quarts/acre to the soil; (4) foliar-applied phsophorous acid based fertilizer, 0-28-26, at a rate of 2.86 quarts/acre; and (5) foliar-applied phosphoric acid, 3-18-18, at a rate of 4.44 quarts/acre. All treatments are applied at rates that provide the same amount of P and K per treatment (1.6 lbs P2O5 and 1.6 lbs K2O per acre). We also tested soil and foliar applications of potassium phosphite, 4-30-8, keeping the N and P levels similar to the potassium phosphate, 3-18-18. Foliar applications are made with a "Herbie" sprayer to simulate helicopter application.

Forty spring flush leaves from non-fruiting terminals are collected at chest height around each data tree in September for nutrient analysis. The leaves are immediately stored on ice, taken to UCR, washed thoroughly, oven-dried, ground and sent to Fruit Growers' Laboratories for analysis of potassium, phosphate and/or phosphite, and all other nutrients, to make sure no other essential nutrients are limiting.

Harvest data include total kg fruit/tree and the weight of 100 randomly selected individual fruit/tree, which are used to fruit size to calculate packout per tree, evaluation of internal fruit quality, and a cost/benefit analysis of each treatment.

All data will analyzed for significance at P0.05 by analysis of variance and repeated measure analysis using SAS.


Summary

We have leaf analyses and yield and fruit quality data for three harvests. In all years of the study, the treatments had no significant effect at the 5% level on leaf nutrient status, fruit quality, fruit size or yield. In year 1, total lb. fruit per tree and total number of fruit per tree were both numerically higher for trees receiving foliar applied potassium phosphate (3-18-18) or foliar applied potassium phosphite (4-30-8) than the control trees or other treatments (Table 1). The yields of the trees receiving soil-applied fertilizers were lower. In addition, trees receiving foliar potassium phosphate (3-18-18) or potassium phosphite (4-30-8) tended to yield more large size fruit (packing carton sizes 60, 48 and 40). In year two, the best treatment was foliar-applied potassium phosphite (4-30-8) (Table 2). The two-year cumulative yield data is presented in Table 3. In year 2 the orchard was analyzed for Phytophthora. In the 60 samples analyzed, no Phytophthora was detected.

The year three yield was devastated by the freeze. More than half of the data trees in the experiment had no fruit. Foliar potassium phosphite (0-28-26) was the best treatment (data not shown).


Table 1. Effect of three foliar and two soil K and P fertilizer treatments on yield and fruit size of the 'Hass' avocado after one year.

      Fruit of sizes 64+48+40
Treatmentz       Lbs. fruit/tree       No. fruit/tree       lbs/tree       no./tree
Control 71       132       66       122  
Foliar-applied
  Potassium phosphate		  
  (3-18-18)       79       152       73       137
  Potassium phosphite  
  (4-30-8)       79       145       73       132
  (0-28-26)       73       133       68       123
Soil-applied
  Potassium phosphate  
  (3-18-18)       57       104       53       97
  Potassium phosphite  
  (0-28-26)       64       118       60       110
P0.05       NS       NS       NS       NS

zAll treatments were standardized to provide 1.6 lbs. P2O5/acre.


Table 2. Effect of three foliar and two soil K and P fertilizer treatments on yield and fruit size of the 'Hass' avocado after two years.

      Fruit of sizes 64+48+40
Treatmentz       Lbs. fruit/tree       No. fruit/tree       lbs/tree       no./tree
Control 59       134       45       90  
Foliar-applied
  Potassium phosphate		  
  (3-18-18)       51       107       38       75
  Potassium phosphite  
  (4-30-8)       68       155       50       101
  (0-28-26)       57       126       44       89
Soil-applied
  Potassium phosphate  
  (3-18-18)       62       139       46       92
  Potassium phosphite  
  (0-28-26)       61       137       46       92
P0.05       NS       NS       NS       NS

zFoliar treatments were standardized to provide 1.6 lbs P2O5/acre. Soil treatments were standardized to provide 22.5 lbs. P2O5/acre


Table 3. Effect of soil- or foliar-applied potassium phosphate and potassium phosphite fertilizer formulations with and without nitrogen on yield of 'Hass' avocado. Treatments were applied at the cauliflower stage of inflorescence development.z

Treatment       Cumulative
lbs. fruit/tree/2 years		       Cumulative
fruit no./tree/2 years
Control       130       264
Soil-applied  
  3-18-18 as PO4       121       241
  0-28-26 as PO3       126       253
Foliar-applied  
  3-18-18 as PO4       132       276
  0-28-26 as PO3       128       264
  4-30-8 as PO3       148       303
P0.05       NS       NS

zSoil treatments applied to provide 22.5 lbs. P2O5/acre and foliar treatments applied to provide 1.6 lbs. P2O5/acre.