ASHS 2024 Conference

Effective fertilizer management and accurate nutritional analysis are critical for healthy tree growth and maximum yield. Citrus trees perform best in the optimum range of each nutrient, and a deficiency or excess can result in poor tree growth and yield. Accurate fertilizer management decisions for citrus trees begin with proper leaf sampling. To accurately assess tree nutrition status, the appropriate time and procedure for collecting leaves for nutrient analysis must be followed. Historically, nutrient management for healthy citrus trees involved annual leaf sampling from July to August, with subsequent year fertilization plans based on single nutrient analysis. Currently, nearly all commercial Florida citrus orchards are affected by Huanglongbing (HLB), a systemic bacterial disease that affects all citrus varieties and rootstocks, causing extensive tree health decline. Studies suggest that a good nutrition program can improve the tree health and production of HLB-affected trees. However, the key is to supply the nutrients that are deficient or low. This widespread prevalence of HLB needs new approaches for managing fertilizer programs by regular leaf sampling to identify tree nutrient status and requirements. Thus, this study focuses to improve and revise the leaf nutrient sampling guideline for HLB-affected trees by determining the optimal time for leaf sampling and adjusting fertilization for increased production and tree health. Key objectives include assessing the frequency of leaf sampling required per year to capture the tree's nutritional status effectively and adjust fertilizer accordingly to establish the correlation of leaf nutrient concentration with the yield, fruit drop, and canopy density. For this study, spring and summer emerging flushes were tagged on HLB-affected ‘Hamlin’ and ‘Valencia’ mild and severe sweet orange trees in a central Florida orchard over two years. The leaves from tagged branches were collected every 3 months for leaf nutrient analysis and the results of both spring and summer leaves were used to adjust the fertilization plan accordingly. It was found that the spring emerging flush was optimal in adjusting the fertilizer for the highest yield compared to the summer emerging flush. With the use of precision fertilization, the improvement in tree canopy density and fruit yield was more obvious in mildly HLB-symptomatic trees compared to severely symptomatic trees. Our results demonstrate the growers should be performing nutrient sampling quarterly and use this information to precisely manage fertilization and improve tree productivity.

Citrus greening, or Huanglongbing (HLB), poses a severe threat to Florida’s citrus industry, impacting tree health and yield. Due to the lack of a known cure, growers employ various strategies to manage its effects, including increased nutrient application. However, in regions like the Indian River District (IRD), characterized by poor soil fertility and organic matter content, nutrient retention is challenging. Consequently, there is growing interest in using cover crops to enhance soil fertility. Despite this, there is limited published data on their effectiveness in the IRD. To address this gap, a collaborative study was initiated in Ft. Pierce, Florida, involving a commercial grower and the University of Florida. A three-year field trial, employing a randomized complete block design with four blocks, included ‘Star Ruby’ grapefruit trees grafted on ‘US-942’ rootstock, ‘Bearss’ lemon trees on ‘Sour Orange’ rootstock, and ‘OLL’ sweet orange trees on 'US-942' rootstock. Two treatments were applied: conventional (without cover crops) and experimental (with cover crops). Cover crops were planted bi-annually (winter and summer), grown, and terminated at each season’s end. Soil nutrient content, organic matter, microbiome diversity, moisture, and temperature, as well tree and root growth parameters were measured seasonally. After three years of cover cropping, significant differences in soil nutrient content, organic matter, and microbiome diversity were observed between the conventional and experimental treatments, suggesting potential impacts of cover crops on soil characteristics. However, there were no noticeable effects on tree growth or physiology. This study is ongoing and aims to provide a comprehensive understanding of the long-term effects of cover cropping on soil parameters in HLB-affected citrus groves, informing sustainable management practices in citrus cultivation.

Huanglongbing (HLB) causes a steady decline in tree health. Part of this decline includes root dieback which limits their capacity to take up water. For this reason, affected trees tend to be more susceptible to drought stress. This raises a significant concern during the dry season (Oct-May) when trees are largely dependent on supplemental irrigation for water. Unfortunately, most growers continue using irrigation schedules that were optimized for healthy trees. We hypothesized that irrigating more frequently, but in smaller doses would provide more opportunities for uptake and improve water relations in HLB-affected trees. The control treatment received the standard practice of irrigating every other day for 2 hours (12 gal/hr). The experimental treatment received water every day, 3 times a day, for 20 minutes (12 gal/hr). The two treatments received the same amount of water over the course of a week, but the experimental treatment received water more often. Treatments were initiated prior to flowering in January 2022 and were continued for two years. Tree water status improved in the experimental trees as reflected in higher mid-day leaf water potentials than in the control. This suggests the experimental regime was better able to maintain tree water status than the conventional method. In the second year, flowering was more synchronized in the experimental treatment. The control treatment saw two peaks in bud production with the latter one being consistent with a drought stress-induced flowering event. The experimental trees also saw an increase in fruit set in both years. Trees receiving the experimental and control treatments dropped a similar proportion of their crop load during June Drop and preharvest fruit drop in year 1 and 2. Finally, the experimental treatment resulted in significantly higher yields on average than the control in both year 1 and year 2 (72% and 200%, respectively). Altogether, more frequent irrigation improved tree productivity.

The disease citrus greening (HLB, Huanglongbing) continues to decimate Florida’s citrus industry, resulting in the lowest yields since the 1940s. With no cure or tolerant rootstock/scion combinations available, growers must manage the disease. Although increased fertilizer applications have been shown to improve tree health, the additional inputs are expensive and decrease operational profitability. As a result, interest has grown in improving soil fertility parameters such as organic matter, with the goal of reducing inputs and increasing horticultural sustainability. Despite the recognized potential of organic matter, little research has been undertaken to establish the optimal contents required to improve citrus root characteristics. The objective of this study was to answer two specific questions: what are the amounts of organic matter needed to significantly affect citrus root growth and physiology? And how does the incorporation of organic matter influence nutrient availability? To answer these questions, a six-month completely randomized greenhouse experiment was conducted to measure the impact of different soil organic matter contents on potted citrus trees. Six different treatments consisting of different organic matter contents (0% - control, 1%, 2%, 3%, 5%, and >10%) were created by mixing different amounts of locally sourced compost with sand. One-year-old ‘US-942’ (Citrus reticulata x Poncirus trifoliata) rootstock plants (n = 6 per treatment) were grown in the different mixtures for six months under controlled greenhouse conditions. During the study, soil moisture, plant height, and stem diameters were assessed every other month. At the conclusion of the study, total plant and root biomass, root nutrients, average root length, and average root diameter were measured. Results from the study indicated that higher organic matter contents (>2%) led to significantly increased plant biomass and stem diameter, and increased root growth. Significant results were also found at the soil level, where increased organic matter contents led to increased nutrient retention and increased root uptake of nutrients. Additional investigation is needed to better understand the advantages of increasing organic matter content, even by marginal percentages, utilizing grafted citrus trees of various cultivars in field trials. Such trials would provide insights into the practical implications of these findings within the industry.

Citrus in key citrus production states in the U.S. have been heavy hit from insect borne and disease pressures. The bacterial disease Huanglongbing (HLB) has had devastating impacts on the citrus industry in Florida. Despite having this threat to the Texas industry as well, the most recent negative impact to decrease citrus acreage has been due to harsh climatic events. In February 2021, citrus groves in south Texas were hit with icy weather that exposed orchards to prolonged freezing temperatures for several hours. The impacts of this one event reduced citrus orchards in south Texas by approximately 25% (over 2,800 ha or 7,000 acres). Climate variability has increased grower awareness to the potential for more frequent extensive drought and freezing winter periods. To better evaluate how to rehabilitate freeze damaged citrus trees, a two-year field study was created aimed at improving tree root health and soil conditions. Compost amendment (CA) was broadcast applied as well as application underneath the tree canopy of grapefruit trees in combination with varying nitrogen fertilizer rate application. Three fertilizer treatments: T1 (base rate = 112 kg N ha-1), T2=1.5x and T3=2x the T1 base rate were evaluated in a randomized complete block design with (11.2 Mg CA ha-1) and without CA in Rio Red grapefruits. Results indicated that root abundance significantly increased under composted trees compared to non-composted trees. Composted trees led to higher yields within 18 months following the freeze event compared to non-composted trees. The impacts of this research may aid Texas citrus growers and other producers looking to improve in-field management strategies that may assist in preparation for post-freeze weather events and extended seasons of water scarcity.

As the atmospheric levels of CO₂ continue to rise, the concentration of carbon sources available for the plants also increases photosynthesis. The rise in atmospheric CO₂ is associated with global temperatures, resulting in an array of plant responses. Studies evaluating plant response to CO₂ treatments of 350 plant species show that plant responses are highly variable and species dependent, with the majority of studies being conducted on vegetable and grain crops, leaving a gap in understanding how tree crops respond. Mandarin (C. reticulata) trees are an economically important crop produced in California subject to climate change. This study used nursery trees of cv. Tango budded on C35 rootstock to determine the effect of elevated CO₂ exposure (400 ppm ambient vs. 800 ppm elevated) at elevated temperature (28°C ambient vs. 45°C elevated) to identify the physiological and metabolomic plant responses associated with each treatment. A longitudinal investigation was conducted over eight weeks in growth chambers (Conviron A1000, Winnipeg, CA) utilizing four treatment groups: ambCO2/Tamb (Control: CO2 400 ppm/Temperature 28°C); eCO2/Tamb (elevated CO2 at 800 ppm/Temperature 28°C); ambCO2/eTemp (CO2 at 400 ppm/Temperature 45°C); and eCO2/eTemp (CO2 at 800 ppm/ Temperature 45°C). Every two weeks, four replicates were collected from each treatment group. physiological measurements, photosynthesis response surveys, and metabolomic analysis were performed. Phenotypic measurements such as plant height, branches, leaf area, and leaf count showed that plant growth was impacted by treatment. Plant height, dry weight, and leaf count were significantly lower in the ambCO2/eTemp treatment as compared to the control (ambCO2/Tamb) as well as the other two treatments, indicating heat stress. Under elevated CO2 levels, trees were able to exhibit similar growth behaviors as the control treatment even under heat stress, signaling that the trees under eCO2 could compensate for a higher burden on carbon balance under heat stress. eCO2/Tamb trees accumulated significantly higher starch than all other treatments, whereas trees under both eTemp treatments showed significantly higher soluble sugars while significantly lower starch levels. This indicates that the trees under supplemental CO2 accumulate higher photoassimilates when they are free from heat stress, and the trees under heat stress exhibit starch conversion to soluble sugars as a stress response. Metabolomics analysis using Nuclear Magnetic Resonance (NMR) will provide valuable insight into the impact of treatment groups of eCO₂ and eTemp impact on Mandarin trees.