1. Please state project objectives and what work was done this quarter to address them:
Objective 1: Determine effects of lowered soil pH on CLas populations and root physiology including internal root apoplast and vascular tissue pH.
We got our experiment in rhizotrons completed in September 2023. Sample processing and data analysis is under way. Root tracing and quantification of root traits will be completed in the next quarter. We should get the data processed and published in the next quarter or immediately following project completion.
Objective 2: Field test multiple acidification materials including organic acids for tree response CLas suppression, nutrient uptake, and root and vascular pH changes. In this quarter, we continued root sampling and soil pH determinations and applied final round of the treatments. We also finalized canopy determinatiions for trees used for the study.
2. Please state what work is anticipated for next quarter:
We will collect fruit yield and juice quality data in March/April 2024 to finalize objective 2 milestones.
We will get one article prepared and published in a refereed journal.
3. Please state budget status (underspend or overspend, and why):
The budget is on track and meeting the project milestones.
1. Please state project objectives and what work was done this quarter to address them: Objective 1: To compare different rates and annual use patterns of trunk-injected OTC on late-season (Valencia) and early-season (Hamlin) sweet orange tree and their effects on health, yield, and fruit quality over a period of 3 years in a commercial citrus production environment.Trees were harvested and fruit quality was assed in December 2023. We measured less fruit drop and increased yields (18-32%) for injected trees compared with non-injected trees, but yield differences were only significant at p = 0.12. In general, the highest OTC rate (1.65 g/tree) and the 75% rate (0.825 g/tree) resulted in the highest yield (32% increase). The highest pound solids and percent juice were measured for the trees that received the highest rate (p < 0.001). Please note that the highest rate (1.65 g/tree) was administerd by split-injections (0.825 g each on two opposite sides of the trunk). Fruit size and juice color were also improved by the injections. A subset of fruits was collected and processed for OTC residue analysis by the National Science Lab in Gastonia.Leaves were collected for CLas analysis (in progress) and tree ratings were performed. Trees that received the highest OTC rate are the densest and show the least foliar HLB symptoms compared with the controls. Obective 2: To compare spring with late summer injections in Valencia orange trees to determine if OTC effects on yield can be enhanced, and the harvest window extended, through timing of injections.Canopy ratings were performed and fruit drop was monitored. Injected trees look denser; trees that received the highest OTC rate are the densest and healthiest looking trees. 2. Please state what work is anticipated for next quarter: The Valencia trial will be harvested, fruit will be collected for fruit quality assessment and OTC residue analysis. Tentative harvest time is March 2024. 3. Please state budget status (underspend or overspend, and why):The budget is as expected.
1. Please state project objectives and what work was done this quarter to address them: The zinc formulations were obtained from TradeMark Nitrogen Inc. and Santra’s Lab to support this year’s field trial. We continued foliar sprays applied every 6 weeks at two different rates (400 and 800 ppm Zn). Trials were conducted on both Hamlin and Valencia oranges. -Tree health continued improving by foliar application of Zn and that particulate Zn fomulations improved tree health more than soluble Zn (Zinc Sulfate). We harvested Hamlin trees on January 12 in Southwest FL. We measured a significant increase in yield with Fertizink treatment (246 boxes/acre), irrespective of the dosage used, as compared to controls (191 boxes/ acre). Nuzink also increased yield (219 boxes/acre) but this difference was not statistically significant at p<0.05. These results are coincident with last season's (were we had hurricane Ian as an external factor), but then only 800 ppm Fertzink increased yield in Hamlin. Next season we will need to further determine if both concentrations have the same effect on yield, as this will be important for economics in grove management. -We also monitored fruit drop, and it was significantly reduced by 30% on average in all treatments until December in Hamlin. From December to January, fruit drop was the same in all treatments including control fruit. This suggests that in the absence of a significant increase in internal quality (internal quality was not affected by the treatments and Brix were around 9) it may be judicious to harvest in early December instead of January and increase yield even more. -We have extracted all samples for auxin analysis and we have started to determine auxin content in peel and abscission zones. - Nuzinc and Fertizink at 800 ppm Zn were evaluated to determine what percentage of Zn is soluble and insoluble. Nuzinc was determined to have 87.01% of Zn in ionic form and 12.99% insoluble Zn, while Fertizink had 99.43% insoluble Zn. These results might explain why there is a difference in performance between Nuzinc and Fertizink. -We have treated seedlings with Zinc sulfate, Nuzinc or Fertizink to measure the rainfastness, track the absorption and translocation of Zinc seven days after treatment. The tissue digestions has been performed but quantification is still pending. 2. Please state what work is anticipated for next quarter: In the next quarter we will harvest Valencia and continue auxin determination. Moreover, the difference in Zn rainfastness, absorption and translocation will be quantified. 3. Please state budget status (underspend or overspend, and why): on track
1. Please state project objectives and what work was done this quarter to address them: The zinc formulations were obtained from TradeMark Nitrogen Inc. and Santra’s Lab to support this year’s field trial. We continued foliar sprays applied every 6 weeks at two different rates (400 and 800 ppm Zn). Trials were conducted on both Hamlin and Valencia oranges. -Tree health continued improving by foliar application of Zn and that particulate Zn fomulations improved tree health more than soluble Zn (Zinc Sulfate). We harvested Hamlin trees on January 12 in Southwest FL. We measured a significant increase in yield with Fertizink treatment (246 boxes/acre), irrespective of the dosage used, as compared to controls (191 boxes/ acre). Nuzink also increased yield (219 boxes/acre) but this difference was not statistically significant at p<0.05. These results are coincident with last season's (were we had hurricane Ian as an external factor), but then only 800 ppm Fertzink increased yield in Hamlin. Next season we will need to further determine if both concentrations have the same effect on yield, as this will be important for economics in grove management. -We also monitored fruit drop, and it was significantly reduced by 30% on average in all treatments until December in Hamlin. From December to January, fruit drop was the same in all treatments including control fruit. This suggests that in the absence of a significant increase in internal quality (internal quality was not affected by the treatments and Brix were around 9) it may be judicious to harvest in early December instead of January and increase yield even more. -We have extracted all samples for auxin analysis and we have started to determine auxin content in peel and abscission zones. - Nuzinc and Fertizink at 800 ppm Zn were evaluated to determine what percentage of Zn is soluble and insoluble. Nuzinc was determined to have 87.01% of Zn in ionic form and 12.99% insoluble Zn, while Fertizink had 99.43% insoluble Zn. These results might explain why there is a difference in performance between Nuzinc and Fertizink. -We have treated seedlings with Zinc sulfate, Nuzinc or Fertizink to measure the rainfastness, track the absorption and translocation of Zinc seven days after treatment. The tissue digestions has been performed but quantification is still pending. 2. Please state what work is anticipated for next quarter: In the next quarter we will harvest Valencia and continue auxin determination. Moreover, the difference in Zn rainfastness, absorption and translocation will be quantified. 3. Please state budget status (underspend or overspend, and why): on track
1. Please state project objectives and what work was done this quarter to address them: The zinc formulations were obtained from TradeMark Nitrogen Inc. and Santra’s Lab to support this year’s field trial. We continued foliar sprays applied every 6 weeks at two different rates (400 and 800 ppm Zn). Trials were conducted on both Hamlin and Valencia oranges. -Tree health continued improving by foliar application of Zn and that particulate Zn fomulations improved tree health more than soluble Zn (Zinc Sulfate). We harvested Hamlin trees on January 12 in Southwest FL. We measured a significant increase in yield with Fertizink treatment (246 boxes/acre), irrespective of the dosage used, as compared to controls (191 boxes/ acre). Nuzink also increased yield (219 boxes/acre) but this difference was not statistically significant at p<0.05. These results are coincident with last season's (were we had hurricane Ian as an external factor), but then only 800 ppm Fertzink increased yield in Hamlin. Next season we will need to further determine if both concentrations have the same effect on yield, as this will be important for economics in grove management. -We also monitored fruit drop, and it was significantly reduced by 30% on average in all treatments until December in Hamlin. From December to January, fruit drop was the same in all treatments including control fruit. This suggests that in the absence of a significant increase in internal quality (internal quality was not affected by the treatments and Brix were around 9) it may be judicious to harvest in early December instead of January and increase yield even more. -We have extracted all samples for auxin analysis and we have started to determine auxin content in peel and abscission zones. - Nuzinc and Fertizink at 800 ppm Zn were evaluated to determine what percentage of Zn is soluble and insoluble. Nuzinc was determined to have 87.01% of Zn in ionic form and 12.99% insoluble Zn, while Fertizink had 99.43% insoluble Zn. These results might explain why there is a difference in performance between Nuzinc and Fertizink. -We have treated seedlings with Zinc sulfate, Nuzinc or Fertizink to measure the rainfastness, track the absorption and translocation of Zinc seven days after treatment. The tissue digestions has been performed but quantification is still pending. 2. Please state what work is anticipated for next quarter: In the next quarter we will harvest Valencia and continue auxin determination. Moreover, the difference in Zn rainfastness, absorption and translocation will be quantified. 3. Please state budget status (underspend or overspend, and why): on track
1. Please state project objectives and what work was done this quarter to address them: Specific objectives are: 1) To determine the right timing for Zn and K treatments to minimize fruit drop.Harvest in Hamlin was done in early January. As a reminder, just like last year we performed treatments at three different dates: on early June 2023, a second set of treatments by the end of July, 2023, and a third set by early September 2023.Yields from plots treated with Zn and Zn+K in September were significantly higher than non-treated controls. Our yield numbers were 165 boxes/ acre in nontreatted controls, 224 boxes/ acre in Zn-treated trees, and 275 boxes/ acre in Zn+K treated trees. These differences were statistically significant at p<0.05. 2) To determine effects of GA3 and 2,4D applications on fruit retention when applied at different times during fruit development.This quarter we harvested the Hamlin block, unfortunately we did not see any significant differences, although GA and 2,4 D both had about 20 lb/tree more fruit (equivalent to 300 boxes per acre versus 265 boxes per acre in control), showing a positive increase trend. The block was harvested on November 20 by the grower, which we believe is exceptionally early. Therefore, any benefits that GA and 2,4 D may have on fruit drop reduction were not observed as the grower picked the fruit before fruit drop accelerates. Currently, we continue to monitor Valencia trial, we anticipate harvesting it in March 2024. 3) To develop a strong and proactive outreach program. - Alferez F. Terapias nutricionales para incrementar la producción de citricos cultivados en condiciones endémicas de HLB (nutritional therapies to increase citrus fruit yield under endemic HLB). International IST (Consultoria y Capacitacion, University of Tamaulipas, Mexico, On Zoom). November 10, 2023. 2. Please state what work is anticipated for next quarter: Harvesting of Valencia will be performed in March for both objectives 3. Please state budget status (underspend or overspend, and why): Budget continues on track
1. Please state project objectives and what work was done this quarter to address them: The objectives of this project are: 1, To study the effect of Brs on priming immunity on young, newly planted trees. This will allow to know for how long immune response will last after Br application, so we can adjust timing (number of applications).After 1 year (with a monthly application), some trees (around 20%) are still HLB-negative. Denser foliage has consolidated in a denser canopy with less leaf drop in winter. Spring flush in Br treated trees has already started in early February and is highly syncronized, following the trend we observed during Summer and Fall flushes. This confirms our previous observations that flushing occurred at the same time in Br-treated trees, and will allow better planification of insecticide sprays. 2, To determine the best time of application (frequency) to achieve maximum protection against pests and disease in newly planted trees. We have started to get data showing that even though immunity is maximum by 30 days, it is still significantly high 60 days after treatment as compared to controls. 3, To determine the effect of Br application on advancing fruit maturation in both Valencia and Hamlin. Treatments started in September on Hamlin. We performed two different sets of treatments: biweekly treatment and only once treatment. Last season, there were no differences between biweekly treatments and only once treatment in terms of yield and quality. This season, we wanted to confirm this. We have been following quality every 15 days, including internal quality and external color development. We started to see a significant increase in Brix with Br treatment performed in mid-November for Hamlin, just like last year. In this case, we had 9.2 Brix in Br treated fruit as compared to 8.4 in controls at harvest. Ratio was again greatly increased,15 in Br-treated as compared to 11.8 in controls. External color is also increased. Together this is encouraging, as we are confirming results from last year in Hamlin. Interestingly, we had also a mild 25% increase in total yield at harvest. These trees were harvested in January 12. We started treatments in Valencia on January. Outreach: -OJ BREAK Jan 17, Lake Alfred. IPC’s and Brassinosteroids to Prolong Health and Improve Fruit Yield and Quality in Newly Planted Trees Under HLB 2. Please state what work is anticipated for next quarter: We will continue treatments in Valencia until harvest. We will perform juice analisis as we did last year in both Valencia and Hamlin to determine the composition of sugars and acids. 3. Please state budget status (underspend or overspend, and why): Spending continues on track.
1. Please state project objectives and what work was done this quarter to address them: 1. To determine how many leaf nutrient sampling per year are required to effectively capture the tree nutritional statusand adjust fertilizer accordingly.2. To establish the relationship of leaf nutrient concentration with yield, fruit drop, and canopy density3. To determine how the leaf nutrient (all 14 nutrient) levels change in the tree throughout the year.4. To evaluate how the leaf age affects the leaf nutrient status. In this quarter we harvested the hamlin tree, monitored fruit drop, final leaf sampling was done for nutrient analysis and leaf characteristics documentation. Other activities included data analysis as we have a massive data set. The results from nutrient profile comparison of fruting and non fruiting branch show that fruting branch have severe deficiency of macro nutients as the fruit matures whereas the micronutrient depletes in non fruiting branch as the season progresses. Thus, we have initiated a small scale trial to fertilize the trees based fertilizing trees on nutrient levels in fruiting branch versus non fruiting branch. It is intersting to find that in many countries across the world, nutrient analysis is performed on fruiting branch as the growing fruit is major sink of nutrients. Thus, we believe it is imporatnt for FLorida citrus industry to know if fertilizing based on nutrients in fruiting leaf can benefit fruit production on HLB-affected trees. 2. Please state what work is anticipated for next quarter: 1. Data analysis and interpretation 2. Developing recomendations for nutrient analysis 3. Please state budget status (underspend or overspend, and why): The budget is being spent as per the plan where major funds have been used for nutrient anlaysis.We have also been able to hire staff for proper analysis and interpretation of data. 4. Please show all potential commercialization products resulting from this research, and the status of each: Changes in leaf sampling recommedation will happen as a result of this research. We anticipate to have recommendations by december 2024 including whether to sample fruting or non fruting leaves.
Soil samples were taken on November 27 and the majority of IPCs removed due to tree size. Leaf samples were collected from all trees and sent to SWREC for HLB determination (pending), trunk diameters and tree heights were measured. Nematodes and fibrous roots were recovered from the soil samples. The IPCs increased (P<0.001) the fibrous root weight more than two-fold (0.115 vs 0.055 mg/g soil), the 12-month growth of trunk girth by 81% (662 vs 365 cm3) and 12-month increase of tree height by 3-fold (19.8 vs 6.5 inches) compared to non-covered trees. Vydate had no significant effects on the tree growth in 2023 although root weights were 20% and 14% greater, and trunk girths 7% and 6% greater in IPC vs non-covered trees, respectively. To date, the cumulative sting nematodes were reduced by just 30% during this trial which is not typical of other reports of trials we have conducted. A difference between this and previous trials is that we generally apply Vydate to trees in small plots using handheld sprayers or herbicide applicators. Here, the material is injected into dedicated irrigation lines and distributed via microjets. In October we tested the hypothesis that the distribution of chemicals applied via microjet is significantly less uniform than if sprayed directly on soil. Sodium bromide was applied under 3 trees via micro-jets and soil collected at different depths and distances from the emitters, immediately and periodically for one week. The bromide was leached from the soil samples and measured. Analysis of the chemical distribution is ongoing, but a second trial measuring the deposition of water and entomopathogenic nematodes (EPN) in the wetted zone during a one-hour irrigation period is informative (Figure 1; see MS Word document). The water/EPN deposition was measured by placing canisters at different distances and directions from the emitters on several trees. Soil at distances of 1 or 5 feet from the emitter received 75% less water than at 3 feet. The deposition of EPN was even more skewed than that of water because (as with chemicals) they were injected during about 15 minutes of the hourlong irrigation cycle and had less opportunity to recover from wind directional changes. These plots were partially covered by tenting plastic sheeting over the trees; the variability would be more extreme in an open field. The data suggest that there is greater opportunity for nematodes to avoid encountering a fatal dose of the nematicide in soil treated with micro-sprinklers than with a directed spray pattern from equipment such as herbicide applicators. The project continues and will evaluate effects of water pressure and emitter configuration on the deposition patterns. The effects of the Syngenta product were also measured during this quarter. The overall suppression of sting nematodes was unaffected by an adjuvant and so the treatments with and without adjuvant were combined for analysis. When the high and low rates were applied once annually in the spring, sting nematode cumulative density during two years was reduced by 55% and 36%, respectively, compared to controls, but the differences were not significant. When the high rate was applied twice annually (in spring and fall), the sting nematodes were significantly reduced by 74%. Again, the Vydate reduction (37%) was not significant.
1. Please state project objectives and what work was done this quarter to address them:Objective 1: Using callose inhibitors to improve systemic uptake and reduce HLB symptoms.We repeated the field experiment on sweet orange ~8 years old (Citrus x sinensis) trees. We selected and labeled 48 trees according to 4 different treatments: (1) injected control (water), (2) Oxytetracycline (2 g per tree), (3) DDG (0.1mM) + Oxytetracycline (2 g per tree), (4) 3AB (0.1mM) + Oxytetracycline (2 g per tree). Then we located and designed the appropriate map for the experiment. Before applying treatments, baseline stomatal conductance data and data related to trunk diameter, canopy volume and canopy density was collected, and CLas titer and quantify callose level of each tree were measured in September 29th 2023. The trunk injection was employed (as described in Vincent et al. 2022) in October 3rd 2023. Following to the scheduled date, we collected eight mature leaf samples per tree to measure the CLas titer and quantify callose level of each tree 10-day (October 13th 2023) and 1-month (November 1st 2023) and 3-month (January 3rd 2023) after treatments. Furthermore, we collected stomata conductance data to assess the treatment impact on leaf health, and also measured the canopy volume and density in pretreatment (April 20th 2023) and 1st measurement (October 24th 2023). In order to minimize the variation and difference on applying each treatment, the injection flow rates of each four treatment (see above) were also measured on another extra 24 trees with 4 directional sites (a) north, (b) west, (c) east, and (d) south in October 31st 2023. Objective 2: Maintaining water-saturated injection site: Sealed trunk injection ports that stay functional by avoiding woundresponses.Field experiments for this objective are ongoing, testing the potential duration of single injection sites. We expect to have results next quarter. Objective 3: Targeted root delivery.Experiment began in January and is ongoing. 2. Please state what work is anticipated for next quarter:We had a serious problem with the health of the tree we are using. Those trees, while on UF propoerty, were not maintained well. We had some good p[reliminary results with the 3AB callose inhibitor, and we decided to make new injections in healthier trees, using 3AB and OTC. We will evaluate the trees for callose level and CLas titre bi-monthly starting in May 2024 (5-month after treatment) to evaluate the effect of the callose inhibitor on tree health and CLas infection. We will also measure the fruit drops via monthly data collection and the canopy volume and density via 6-month measurement to determine the effect of injections on tree health.3. Please state budget status (underspend or overspend, and why):Underspend- since it took some time to initiate the work proparly.4. Please show all potential commercialization products resulting from this research, and the status of each:We are still evaluating the effect of the callose inhibitor in the field trials
The third mass screen of germplasm for sting nematode tolerance in large sand-filled tanks was completed in January 2024. Sixteen rootstocks were compared by planting four replicates of each in each of 4 tanks (256 seedlings total) – two tanks infested with sting nematode and 2 that were not infested. The trial began in May and ran for 7 months, two months longer than the previous trials, to enhance the nematode pressure. The trial is noteworthy for comparing rootstocks created at USDA and UF to each other and to historical industry standards. Most of the rootstocks have been available for propagation for a number of years; however, only the historical industry standards were available for comparison the last time sting nematode tolerance was evaluated 4 decades ago. An initial trial in this project evaluated UF rootstocks, whereas USDA rootstocks were screened in the second trial. Here we look for differences among the rootstocks and for consistency with the previous trials. Nematode populations were evaluated, prior to plant recovery from tanks, finding more than 200 sting nematodes of all stages per 250 cm3 soil in each infested tank, and no nematodes were detected in non-infested tanks. Plants were carefully dug from tanks and photographed after soil was rinsed from roots. They were then separated (top, tap roots, fibrous roots), dried for 72 h at 70oC, and weighed. Roots appeared abnormally thickened when damaged heavily by the nematode which may mask differences if measured exclusively by mass (Fig 1). A rating system (1-10) was used to assign a score to each plant within a rootstock, relative to the best plant(s) (given 10) in the non-treated group of that rootstock. Each rootstock was assigned a tolerance index T, where T= mean rank of 8 infested plants/mean rank of 8 non-infested plants. Three investigators rated the plants independently, the T rankings were highly significantly correlated, and were averaged. Fig. 2 shows these subjective ratings (Tsubj)plotted against tolerance estimated as the mean fibrous root weight from infested tanks/mean fibrous root weight from non-infested tanks (Twt). Tolerance of the rootstocks was surprisingly similar within 1) those used historically, or 2) produced by USDA or 3) by UF. The four historical standard rootstocks had low T values measured visually or by mass suggesting that most newer releases confer some tolerance to the nematode. While USDA rootstocks tended to have higher Tsubj values than historical standards, this was not the case for Twt. UF rootstocks tended to have the most tolerant lines according to both measures. The results of the trial were consistent with previous results and field observations. Kuharske which is widely used for burrowing nematode control was heavily damaged by sting nematode in a 3-year trial to evaluate cover crops and nematicides. Swingle citrumelo was rated as susceptible to sting nematode in previous surveys. In this and our previous tank trial using USDA rootstocks, US812 and US942 and US1516 outperformed sweet orange or sour orange. Similarly, in this and the previous trial using UF rootstocks, UF1, UF5 and Orange 16 had the highest Tsubj and Twt scores. There were discrepencies, however, such as LB8-9xS13-15-16 which performed poorly in the first compared to this trial. The very small plant size of LB8-9xS13-15-16 compared to the better performing rootstocks may make relative differences between infested and non-infested plants more difficult to measure accurately. The rootstock screening trials have provided consistent evidence that newer rootstocks are somewhat more tolerant under these conditions than the previous generation of commonly used rootstocks. The widespread occurrence of sting nematode in Florida may be a factor driving grower adoption of newer rootstocks. This is somewhat encouraging; however, there is no evidence in these trials that an acceptibly tolerant citrus cultivar has been identified. We shall continue screening new cultivars with Kuharske and UF5 as standards for intolerance and tolerance, respectively.
The third mass screen of germplasm for sting nematode tolerance in large sand-filled tanks was completed in January 2024. Sixteen rootstocks were compared by planting four replicates of each in each of 4 tanks (256 seedlings total) – two tanks infested with sting nematode and 2 that were not infested. The trial began in May and ran for 7 months, two months longer than the previous trials, to enhance the nematode pressure. The trial is noteworthy for comparing rootstocks created at USDA and UF to each other and to historical industry standards. Most of the rootstocks have been available for propagation for a number of years; however, only the historical industry standards were available for comparison the last time sting nematode tolerance was evaluated 4 decades ago. An initial trial in this project evaluated UF rootstocks, whereas USDA rootstocks were screened in the second trial. Here we look for differences among the rootstocks and for consistency with the previous trials. Nematode populations were evaluated, prior to plant recovery from tanks, finding more than 200 sting nematodes of all stages per 250 cm3 soil in each infested tank, and no nematodes were detected in non-infested tanks. Plants were carefully dug from tanks and photographed after soil was rinsed from roots. They were then separated (top, tap roots, fibrous roots), dried for 72 h at 70oC, and weighed. Roots appeared abnormally thickened when damaged heavily by the nematode which may mask differences if measured exclusively by mass (Fig 1). A rating system (1-10) was used to assign a score to each plant within a rootstock, relative to the best plant(s) (given 10) in the non-treated group of that rootstock. Each rootstock was assigned a tolerance index T, where T= mean rank of 8 infested plants/mean rank of 8 non-infested plants. Three investigators rated the plants independently, the T rankings were highly significantly correlated, and were averaged. Fig. 2 shows these subjective ratings (Tsubj)plotted against tolerance estimated as the mean fibrous root weight from infested tanks/mean fibrous root weight from non-infested tanks (Twt). Tolerance of the rootstocks was surprisingly similar within 1) those used historically, or 2) produced by USDA or 3) by UF. The four historical standard rootstocks had low T values measured visually or by mass suggesting that most newer releases confer some tolerance to the nematode. While USDA rootstocks tended to have higher Tsubj values than historical standards, this was not the case for Twt. UF rootstocks tended to have the most tolerant lines according to both measures. The results of the trial were consistent with previous results and field observations. Kuharske which is widely used for burrowing nematode control was heavily damaged by sting nematode in a 3-year trial to evaluate cover crops and nematicides. Swingle citrumelo was rated as susceptible to sting nematode in previous surveys. In this and our previous tank trial using USDA rootstocks, US812 and US942 and US1516 outperformed sweet orange or sour orange. Similarly, in this and the previous trial using UF rootstocks, UF1, UF5 and Orange 16 had the highest Tsubj and Twt scores. There were discrepencies, however, such as LB8-9xS13-15-16 which performed poorly in the first compared to this trial. The very small plant size of LB8-9xS13-15-16 compared to the better performing rootstocks may make relative differences between infested and non-infested plants more difficult to measure accurately. The rootstock screening trials have provided consistent evidence that newer rootstocks are somewhat more tolerant under these conditions than the previous generation of commonly used rootstocks. The widespread occurrence of sting nematode in Florida may be a factor driving grower adoption of newer rootstocks. This is somewhat encouraging; however, there is no evidence in these trials that an acceptibly tolerant citrus cultivar has been identified. We shall continue screening new cultivars with Kuharske and UF5 as standards for intolerance and tolerance, respectively.
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