2019-12_16-007 – Field evaluation of the selected variants of Ruby Red grapefruit volunteer seedlings for greater HLB resistance/tolerance

Field evaluation of the selected variants of Ruby Red grapefruit volunteer seedlings for greater HLB resistance/tolerance

Report Date: 01/15/2020
Project: 16-007   Year: 2019
Category: Plant Improvement
Author: Yongping Duan
Sponsor: Citrus Research and Development Foundation, Inc.

The objectives of this proposal are: 1) conduct a field trial using the selected grapefruit seedlings to ensure the productivity of the trees in Florida where HLB is endemic; and 2) evaluate the quality of the fruit produced. Achievement of these goals will produce a more resistant/tolerant variety that could be available in the near future since its use would not require regulatory approval.

Based on two year’s graft-inoculation assays in greenhouse with two HLB bacterial isolates and the performance of individual seedlings in the field, four lines of the seedlings (with greater HLB resistance/tolerance) were selected for further propagation on three different rootstock (commercial sour orange, newly selected USDA-sour orange and 942). The fruit quality (Brix, sucrose, glucose and fructose, soluble solids, pH, % TA and total ascorbic acid) of the four selected seedlings showed no significant difference from their maternal trees.

The first group of the propagates on three different rootstock from the selections of Scott Grove’s seedling variants were grown at our research farm, Picos Farm, where the plants are under extreme high HLB disease pressure with very aggressive HLB pathogens. These new plantings (July, 2017; Nov, 2017; and May, 2018) showed different disease index, the longer the planting was, the higher the disease index, which was also highly correlated with the titers of Ca. Liberibacter asiaticus (Las) in infected plants. It is worth noting that the new HLB isolate from Picos Farm caused severe HLB disease on most of grapefruit selections of seedlings and bud sports in our latest, graft-based greenhouse evaluation. Those selections were either resistant or tolerant to the previous HLB isolates we maintained in greenhouse. Prelimilnary data showed some of the selections are better than the others with either lower disease index or better canopy growth. Some of the selections showed much lower infection rate (less than 20%) than the control (40%) and poor performers (40-50%) after planting in Picos research farm for 26 months. Among the Scott’s seedling selections, one of the 4 selections displayed its outperformance than the others with the lowest disease rate of ca. 13.0% and better growth canopy. All the plants were verified by Las-specific qPCR assays, and there were no significant difference among infected genotypes in term of Las bacterial titers. It is worth noting that some of the plants propagated via cutting (without a rootstock) showed some promising outcome with less disease index. However, there was no significant difference observed among the three rootstocks in the trials. Some of the genotypes planted in Nov. 2017 (about 10%) bears fruits, and the fruits from these genotype selections were collected, and they are going to be evaluated for juice quality via taste panels and instruments. In summary, we have yet oberved at least two selections (one from bud sport) that are significantly better than the others (including control grapefruit plants).

The second group of the propagates on the three different root stocks mentioned above (Ca. 750 plants) were planted in Scott Groves in two lots, with this experiment fully planted by mid-September 2019. All of the propagates have been tested for the presence of Las via qPCR. All the 700 plus plants in Scott grove grow as expectedand and will be further evaluated.

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