2018 ASHS Annual Conference

The Florida citrus industry has been devastated by the bacteria Candidatus Liberibacter asiaticus (CLas), the probable causal agent of greening (HLB). Blotchy mottled leaves, phloem plugging, root depletion, misshapen fruit, severe defoliation and fruit drop are some of the HLB symptoms in infected trees. Micronutrients play an important role in conferring plant disease tolerance. Manganese (Mn) protects the cells against the damaging effects of free radicals, and activates enzymes involved in: nitrogen metabolism, flavonoid, and indole acetic acid pathways. Boron (B) is required for sugar transport, carbohydrate metabolism, cell wall synthesis and cell structure. Our hypothesis is that manganese and boron applied in excess of current recommendations as controlled released fertilizer (CRF) can mitigate HLB symptoms in field established trees. Commercially established 'Vernia' grafted onto Rough Lemon trees were divided into eight (8) supplemental CRF nutrition treatments (two and four times the recommended doses) in a randomized complete block design in St. Cloud, FL. Treatments were composed of 6 trees in duplicate as follows: 1) No extra nutrition (control), 2) Harrell's – St. Helena Mix (H) 0.9 kg per tree; 3) Harrell's with 32g of Florikote Polymer Coated Sodium Borate (B) per tree; 4) Harrell's with 90g TigerSul® Manganese Sulfate (Mn) per tree; 5) Harrell's with 32g of B + 90g of Mn per tree; 6) 180 g of Mn per tree, 7) 64 g of B per tree, and 8) 180g of Mn + 64g of B per tree, applied every 6 months, for 5 years. Leaf nutritional analyses were done in March and September 2017, as well as qPCR for CLas in December. Low bacteria presence, meaning higher CT values were observed in trees that received four times the recommended dosage of manganese (180g Mn). No significant differences in juice characteristics, canopy volume and trunk section area were found between control and 180g Mn treated plants. Soil and leaf nutrients B, K, Mn and Zn were significantly different among treatments at various times in the study. We conclude that citrus trees supplied with excessive amounts of Mn bacteria populations are suppressed by its therapeutic effect. This response was not observed when Mn and B were combined in overdose, suggesting an antagonistic effect from B. When overdosed, Mn might restore biological functions and tree tolerance lost from nutritional imbalances caused by HLB. Further studies are needed to elucidate which pathways differ from overdosed and conventionally fertilized trees.