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Nutrient Management of HLB Affected Round Orange and Grapefruit

At UF/IFAS, we are working on finding solutions for Florida’s citrus growers.  This is a summary of one project made possible by state legislative funding for the UF/IFAS Citrus Initiative during the 2018-19 funding cycle. It documents how we are making progress and providing Florida growers with reasonable, pragmatic solutions to successfully grow citrus in this new age of citrus greening.

Researchers: Kelly T. Morgan, Soil and Water Sciences, Davie Kadyampakeni, Soil and Water Sciences, Rhuanito Ferrarezi, Horticultural Sciences

IMPACT: Increased rates of Ca and Mg applied three times per year increased tree canopy growth. Below-ground root growth was also increased which is important in improving nutrient uptake to help rehabilitate HLB-diseased trees. The study also found that by enhancing rates of Mn and Zn overall, the health and growth of HLB-diseased trees could be improved even further. In addition to improving fertilization practices, adjusting tree planting density can help improve overall fruit yields.

The bacterium responsible for citrus greening disease does much of its initial damage below ground, reducing the host tree’s overall fibrous root density by 30% to 70%. This root loss makes it harder for infected trees to acquire water and nutrients from the soil. Because virtually all Florida commercial citrus groves are now affected by citrus greening disease, also known as Huanglongbing or HLB, the state’s citrus growers often find that they need to apply nutrients in smaller amounts, at more frequent intervals, than they did in the days before HLB. Research has also found that applications of Calcium (Ca), Magnesium (Mg),  Manganese (Mn) and Zinc (ZN) must be applied to improve tree growth, root development, and yield.

Many growers now spray their groves with liquid fertilizer mixtures containing macro- and micronutrients to supplement the use of dry granular fertilizer applied  to the soil. To help orange and grapefruit producers better understand the pros and cons of fertilizer sprays, soil and water scientist Kelly Morgan of the UF/IFAS Southwest Florida Research and Education Center led a research project investigating how fertilization rate and tree spacing influenced fruit size and yield. The project involved HLB-positive orange trees planted at Southwest Florida Research and Education Center in Immokalee and Citrus Research and Education  Center in Lake Alfred and grapefruit trees planted at Indian River Research and Education Center at St. Lucie County. Experiments were conducted using ground applications of macronutrients (Ca and Mg) and foliar and ground applications of micronutrients (Mn and Zn) on separate groups of trees. Experiments at Indian River Research and Education Center conducted the same experiments on grapefruit trees planted in single rows and staggered rows.

Results after one year of application indicated that the soil and foliar fertilizer application rates had no significant effect. However, applications of Ca+Mg and micronutrients for three years increased canopy and root growth. Researchers have secured funding to continue the study for two more years to determine
whether treatment effects change over time. Experiments on high density grapefruit indicated that single row spacing provided the greater yield than staggered rows.

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