Optimization of macronutrient and micronutrient concentrations in roots and leaves for Florida HLB-affected sweet orange trees

Abstract

Florida sweet orange production has declined largely due to Huanglongbing (HLB) disease. The causative agent, Candidatus Liberibacter asiaticus (CLas) bacteria, lowers tree performance by reducing water and nutrient uptake. HLB-affected trees have a fibrous root density loss of about 30 to 50%, which increases as HLB symptoms develop in the canopy. Currently, no guidelines exist for optimal nutrient concentrations in citrus roots. This study aimed to develop root nutrient and fertilization guidelines for HLB-affected sweet orange trees. The research was conducted on two sites with 5- to 6-year-old ‘Valencia’ sweet orange trees grafted on ‘Swingle’ rootstock. The two sites were representative of the different soil characteristics of the state of Florida, namely Ridge and Flatwood regions. Seven treatments were tested using a randomized complete block design. The Ridge soils site has a high density of trees of about 1359 trees ha−1 while the Flatwoods soils site of southwest Florida has a lower tree density of about 716 trees ha−1. The experiment began in May 2019 and concluded in May 2021. Foliar, root and soil samples were collected from both sites twice a year and nutrient concentrations were analyzed. No significant patterns were observed; however, differences in soil nutrients were found between sites. The obtained data represent trends that could become significant over a longer-term study which encompasses the two different citrus-growing regions in Florida. These lengthier trials are especially needed in the age of HLB in order to curb the declining citrus production.