Editor's Note: A special thanks to Dr. Monica Ozores-Hampton for helping coordinate the Immokalee Report.

The Orange Hammock citrus grove has become well known for use of the Boyd Mix, a combination of nutrients and SAR (systemic acquired resistance) products applied foliarly to greening-infected trees. Combined with an Asian citrus psyllid control program and a soil-applied dry fertilizer program, the Orange Hammock trees have remained healthy and productive for nine seasons. 

In 2008, a trial using various combinations of the Boyd Mix was established at the University of Florida Institute of Food and Agricultural Sciences’ Southwest Research and Education Center in Immokalee, Fla., to investigate which components contributed the most to production and ultimately to grower returns. The trees were 4-year old Hamlin sweet orange on Swingle citrumelo rootstock. PCR leaf analysis confirmed that all trees were positive for huanglongbing, or HLB, in 2008 and remained positive throughout the trial. 

Timing of treatment applications was synchronized during the year with the vegetative growth flushes—spring flush [March], summer flush [June], and the fall flush [September]. Visual observations of tree condition and expression of nutritional deficiencies and HLB symptoms were made during the year. 

Observations included foliage canopy color and general tree health. Each year, fruit was harvested and average yield by treatment was reported as pounds per tree. Yields were converted into boxes [90-pound box] and scaled upwards to an acre by assuming a tree density of 150 trees per acre. Table 1 ranks the treatments by projected cumulative production and the top seven treatments were not significantly different.

Juice quality as measured by U.S. Department of Agriculture fruit maturity factors determined with an official commercial FMC juice extractor indicated no significant differences among treatments.

It was observed that the control trees and trees receiving foliar treatments without micronutrients exhibited symptoms of manganese and zinc deficiencies throughout the year and some chlorotic leaves including HLB blotchy mottle.

The trees receiving treatments with micronutrients rarely expressed symptoms of micronutrient deficiencies during the growing season. Trees receiving both macro and micronutrients appeared greener and healthier than other trees. 

All trees showed moderately visible HLB symptomatic leaves during fall and winter. The trees receiving the complete complement of macro- and micronutrients remained mostly free of HLB symptomatic leaves during the growing season with only minor symptomatic leaves during the winter.


Nutrient treatments pay

Five year cumulated totals of production, material costs, and net income above foliar material costs are summarized in Figure 1. Treatment 2 produced more than 1,300 boxes with a foliar mix of macro- and micro nutrients along with phosphorous acid. 

The control treatment (#10), a grower standard before the advent of greening, produced only 468 boxes during the five-year study period. The most expensive treatment was #1, the Boyd Mix, costing nearly $550 per acre of material costs each year.

Multiplying annual costs by five years and subtracting from fruit revenue provides an estimate of cumulative gain in grower income. To estimate fruit revenues, one needs to make some assumptions about pound solids, delivered-in fruit prices and harvesting costs.

In our example, we assumed that every box of Hamlins produced 5.6 pound-solids per box and that the grower would receive a delivered-in price of $1.70 per pound solids.

Pick, roadside and haul costs were assumed to be $3 per box. Treatments 4, 2 and 7 generated the most income above foliar material costs. Treatment 4 generated nearly $6,900 of cumulative income, or nearly $4,500 more income than the control treatment (#10). Returns increased by $6,521 and $6,015 for treatments 2 and 7, respectively.


Dr. Bob Rouse is an associate professor of citrus horticulture at the Southwest Research and Education Center in Immokalee. He can be reached at rrouse@ufl.edu or 239-658-3400. Dr. Fritz Roka is an associate professor and economist in the Food and Resource Economics Department based at the SWFREC. He can be reached at fmroka@ufl.edu.


Table 1. Projected cumulative production over 5 years by treatment.

Treatment Rank by Production

Cumulative production

Treatment #


box/ac (5 years)











Micro+ Macro +H3PO3+ SAR                           









Micro + KNO3+H3PO3






Macro +H3PO3



Control: liquid micros w/summer oil


Assumption: 150 trees per acre

Descriptions: KNO3 is potassium nitrate spray grade, Macro is macronutrients as liquid 3-18-20, 2Micros (MnSO4, ZnSO4, Na2MoO4, B (sodium borate),  H2O2 is hydrogen peroxide, H3PO3 is K-phite 7LP (Plant Food Systems, Zellwood, Fla.) phosphorous acid, SAR is SAver (Plant Food Systems) is potassium salicylate (salicylic acid), and Serenade ASO is Bacillus subtilis (Agraquest Inc., Davis, Calif.).


Figure 1. Yield in boxes per acre, fertilizer materials cost, and income by treatment.

Researchers examine nutrient costs for HLB-infected trees

Assumption: 150 trees per acre