Planting in metallized reflective mulch did not significantly benefit development of new hybrid seedlings

Authors: Stover, Eddie; Mayo, Stephen; Driggers, Randall; ADAIR, ROBERT - Florida Research Center For Agricultural Sustainability (FLARES)

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2021
Publication Date: 1/10/2022
Citation: Stover, E.W., Mayo, S., Driggers, R.E., Adair, R.C. 2022. Planting in metallized reflective mulch did not significantly benefit development of new hybrid seedlings. HortScience. 57:2. https://doi.org/10.21273/HORTSCI16248-21.
DOI: https://doi.org/10.21273/HORTSCI16248-21

Interpretive Summary: The USDA citrus scion breeding program is urgently working on developing HLB-tolerant cultivars with excellent fruit quality and productivity when HLB-affected. The slow process of assessing new citrus hybrids is a major impediment to delivery of these much needed cultivars. We generate thousands of hybrids each year, germinate the seedlings, grow them for two years in the greenhouse, plant them at high density in a field where the disease citrus greening is abundant, grow them for five to ten years, and make selections based on tree performance and fruit quality of these citrus greening-affected trees. Based on promising reports of accelerated citrus growth when grown in a metallized reflective mulch (MRM, a long roll of plastic laid down over the soil with drip irrigation underneath, into which trees are planted) system, we tested the hypothesis that such a system may accelerate growth and selection of new hybrid seedlings. An experiment was established at the USDA farm in Ft. Pierce, FL in Aug. 2018. Four blocks were established with MRN applied to one row in each of four two-row raised beds, with conventional soil planting (CSP) on the opposite rows on the same beds. After recommended site preparation, sixty-inch rolls of MRM were laid down on the four assigned rows and soil was applied to hold down edges The MRM rows received drip irrigation and nutrients through continuous fertigation (nutrients dissolved in the irrigation water), while CSP was microsprinkler irrigated and mainly received soil application of dry/granular fertilizers. New hybrid seedlings, for each of four divergent crosses, were planted in groups on each of the eight experimental rows. Trees were evaluated for growth and health in Feb. 2020 and April 2021. Tree canopy volume was significantly greater with MRM in 2020 (27%) but not 2021, and MRM tree height was greater in 2021 (7%). Mortality was significantly greater with MRM in both 2020 and 2021(in 2021, 32% vs. 17% under CSP), and MRM trees were scored as having more yellow leaves. MRM received substantially less irrigation water overall (5 gallons/tree/week vs. 28), since water use is more efficient with drip irrigation under plastic mulch, but both MRM and CSP received supplemental water as needed and were judged optimally irrigated by the farm manager. Weed control was less effective on MRM as we were advised that our principal weed control would dull the reflective coating. Due to staff limitations, plant debris and soil were not routinely cleared from MRM diminishing any benefit from the reflective surface, and greater maintenance might have shown more sustained evidence of MRM growth benefits.

Technical Abstract: The USDA citrus scion breeding program is urgently working on developing huanglongbing (pathogen Candidatus Liberibacter asiaticus) tolerant cultivars with excellent fruit quality and productivity when HLB-affected. The slow process of assessing new citrus hybrids is a major impediment to delivery of these much needed cultivars. We generate thousands of hybrids each year, germinate the seedlings, grow them for two years in the greenhouse, plant them at high density in a field where the disease citrus greening is abundant, grow them for five to ten years, and make selections based on tree performance and fruit quality of these citrus greening-affected trees. Based on promising reports of accelerated citrus growth when grown in a metallized reflective mulch (MRM) system, we tested the hypothesis that the MRM system may accelerate growth and selection of new hybrid seedlings compared to conventional soil culture (CSC).In the MRM system, tree rows are covered with a layer of metallized plastic film and drip irrigation is installed beneath the plastic. In two years of analysis, tree canopy volume was significantly greater with MRM in 2020 (27% greater than CSC) but not 2021, and MRM tree height was greater in 2021 (7% greater than CSC). Mortality was significantly greater with MRM in both 2020 and 2021(in 2021, 32% vs. 17% under CSC), and MRM trees were scored as having more chlorotic leaves. Due to staff limitations, plant debris and soil were not routinely cleared from MRM diminishing any benefit from the reflective surface, and greater maintenance might have shown more sustained evidence of MRM growth benefits. With current resource availability the MRM system does not appear to accelerate assessment of hybrid seedling trees.