By: Juanita Popenoe, Commercial Fruit Extension Agent

Citrus trees affected with Citrus Greening (HLB) decline in growth, resulting in reduced yields. Root growth is greatly reduced and branch dieback is typical as the infection spreads. This bacterial disease currently has no cure, so growers and researchers are trying many things to overcome the symptoms of the disease. Severe pruning can sometimes rejuvenate trees damaged from cold. Foliar nutrient application has been used to get the nutrients to the leaves when root function is diminished. Growers were using these two treatments to try and overcome HLB symptoms although there was no research to prove effectiveness.

Experiment

UF researchers tested pruning and foliar nutrition to try and resuscitate HLB-weakened trees. Infected fifteen-year-old ‘Valencia’ orange on ‘Swingle’ rootstock were severely pruned back to main scaffold branches for comparison to unpruned trees. Between 2010 and 2015 foliar nutrients were sprayed on both pruned and unpruned trees during growth flushes. Three enhanced nutritional foliar treatments were compared to a control standard foliar nutritional application on the trees. The enhanced foliar nutrition treatments were the “Boyd cocktail” (a mixture of micro- and macronutrients), Fortress (a commercial micronutrient package) sprayed with potassium nitrate, and Fortress sprayed with urea. Trees were pruned in February 2010 before the spring flush.

Results

Pruned trees grew longer shoots than unpruned trees in the year after pruning. Fruit yields from pruned trees never surpassed the yields from unpruned trees, although yields were similar in some years. Therefore severe pruning was not a cost-effective treatment through the first five years after pruning. The rapid regrowth of pruned trees indicates that less severe pruning may result in similar regrowth with less impact on yield. Enhanced foliar nutrition treatments provided some yield increases, especially in the early years of the trial, but they were not cost effective.

The full article may be found at: Rehabilitation of Huanglongbing-affected Citrus Trees Using Severe Pruning and Enhanced Foliar Nutritional Treatments. HortScience 52(7):972-978. 2017. By: R.E. Rouse, M. Ozores-Hampton, F.M. Roka, and P. Roberts.

UF/IFAS Citrus Greening Resources

Photo credit: http://aggie-horticulture.tamu.edu/citrus/pruning/hedging.jpg

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