Researchers examine nutrient costs for HLB-infected trees

03/21/2014 11:04:00 AM
Bob Rouse and Fritz Roka

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.

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