Field variety trials are a simple but effective tool to test plant horticultural performance under different environmental conditions and enhance the commercial adoption of new cultivars. Large-scale, rapid implementation of HLB-tolerant cultivars depends on reliable data, and the Millennium Block is addressing the need of establishing field plantings to generate regional, updated information for the Indian River IIR) Citrus District. The project has two objectives: (i) Assess performance of new grapefruit cultivars with certain rootstocks under HLB endemic conditions in the IR district and (ii) Evaluate the influence of UFR and other recent rootstocks on grapefruit, navel, and mandarin in the IR in comparison to legacy/standard rootstocks.
Trials tested: T1) grapefruit cultivars on three rootstocks, T2) 36 rootstocks with Ray Ruby grapefruit as the scion, T3) 30 rootstocks with Glenn 56-11 navel orange, and T4) 30 rootstocks with UF-950 mandarin.
We planted 3,400 trees in Sep/2019, 1,100 trees in Aug/2020, and 400 trees on Jun/2021. Only grapefruit trees on UFR-8 are going to be planted in Fall since not ready yet. Controlled-release poly coated fertilizer was applied in Sep/2019, Jan, May and Sep/2020, Jan and May/2021. Irrigation controller, sand media filtration system and water flow meter were installed. The group met with the certified crop advisor to develop a spray program schedule based on time of year and conditions to be applied as determined by IPM scouting. Hoop boom was modified to spray young trees with higher accuracy, reducing the waste of agrochemical products. We created a location map and began production and distribution of QR tags to be used with scanner codes during data collection in the field.
Tree height, tree width in two positions (E-W/N-S), and trunk diameter were measured in three central trees from each experimental plot in Feb, July and Nov/2020, and Feb/2021, and canopy volume calculated. have been presented at the 2020 and 2021 ASHS annual conferences. The study is on schedule, and the following summary data reflect the best and worst treatments in each trial during the first 18 months of growth.
Tree size data from Feb/2021 show some scion-rootstock combinations are exhibiting significant differences in canopy volume. In Trial 1, Pummelette UF-5-1-99-2 grapefruit on US-942 was 6x larger (4.7 m3) than US 1-83-179 grapefruit hybrid on Sour Orange (0.8 m3) (P<0.0001). In Trial 2, Ray Ruby grapefruit on US-812 was ~3x larger (4.1 m3) than on Orange 16 (1.3 m3) (P<0.0001). In Trial 3, Glenn F-56-11 navel orange on C-22 was ~2x larger (2.9 m3) than on Willits (1.2 m3) (P<0.0001). In Trial 4, UF-950 mandarin on US-942 was ~2x larger (2.5 m3) than on WGFT+50-7 (1.2 m3) (P<0.0001). Cultivars grafted on different rootstocks have no differences in tree phenological stage. HLB is spreading in the field and individual trees are getting infected as visual HLB symptoms are seen on some treatments in each trial. Leaf samples were taken in Mar/21 for HLB diagnostic and bacterial titer quantification. Asian citrus psyllids, Diaprepes root weevils, whiteflies, and other insects are less abundant in the field, except for leaf miners, which caused severe damage due to the excessive rainfall and wind gusts that made pesticide application challenging during this quarter. Nonetheless, tree growth has not been significantly affected by these pests. Leaf samples for determining HLB incidence were taken from a pool of trees from each experimental plot in May and Sept/2020, and Feb/2021 and sent to the Southern Gardens lab for analysis; on average, all samples tested negative (no trees with Ct values <32) but there are several positive trees with visible symptoms. Fruit phenology, pests and diseases have been monitored monthly. Canopy thickness, canopy color and HLB incidence have been measured quarterly in all experimental plots. The Ferrarezi Lab organized a very successful drive-thru field day to showcase the results to growers on 10/09/2020 with 49 attendees (limited by covid-19 regulations). An estimated 24,000 acres of citrus were represented at the event (70% of the current grapefruit industry acreage, highlighting the importance of the event and my program engagement with the industry). Attendees came from local and neighboring counties including St. Lucie, Charlotte, and Okeechobee. Another field day took place on 12/10/2020 with 4 large growers and industry leaders. Overall, trees are showing morphological differences among scions/rootstocks. Although HLB is spreading in the field, symptomatic trees are building up vigorous canopies. Longer-term evaluation is required to identify the most promising scions and rootstocks to determine their profitability and capability of meeting grower and market needs.