University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers Mark Ritenour and Flavia Zambon recently provided an update on a large field trial. The project is called Evaluation of Potential HLB Tolerant Grapefruit Rootstock/Scion Combinations in the Indian River District of Florida. A summary of their update follows:
The project started as a collaborative effort between the UF/IFAS Indian River Research and Education Center, the Indian River Citrus League (IRCL) and the Florida Department of Agriculture and Consumer Services. It was initially funded by the HLB Multiagency Coordination Group. Between August 2020 and April 2022, 42 experimental blocks were planted, each with seven grapefruit scion and six rootstock combinations. Most of these blocks are managed by 16 participant growers. Funding ended last September, but fall measurements were completed.
Most of the blocks were affected by Hurricane Ian, either by dropping their first season’s fruit or by damaged canopies. Most trees have not begun bearing fruit, so data collected focused on canopy volume and trunk diameter. As expected, there were statistical interactions between scion and rootstock.
Results from 2020–21, 2021–22 and 2022–23 growth show US-802 and US-942 as the best rootstocks for developing fuller canopy volume.
The scion hybrids N40-16-11-11 and N40-16-11-7 from the UF/IFAS breeding program had more vigor and canopy volume.
Star Ruby did not perform as hoped, mainly due to a genetic mutation documented for this clone more than 30 years ago. Star Ruby displayed “winter-bleach”-like symptoms during summer 2022. Several growers reported the same symptom. Not much can be done to prevent the problem.
Scion stem diameter three-year averages are more consistent in terms of scion/rootstock combination. Due to the genetic mutation in Star Ruby, its development was stunted regardless of the rootstock it was grafted onto. Jackson is known for its intense vigor and was one of the thickest scions, followed closely by the UF hybrids N40-16-11-11 and N40-16-11-7.
Super Sour #1 rootstock was very slow growing during the tissue-culture phase and at the nursery greenhouse. It was not available for plantings with Flame, Star Ruby and Rio Red scions in 10 of the 42 blocks.
This research is especially critical as the trees enter their fruit-bearing years. Based on the results to date, measurement of Star Ruby will be discontinued, as that variety has performed poorly. In addition, one grower has left the program, but there are still 41 blocks remaining.
This summer, trees will be rated for foliar HLB symptoms, canopy thickness and canopy color. On trees that are bearing fruit this fall, yield and fruit drop will be estimated, and samples will be collected to measure fruit quality.
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