By Kim D. Bowman
The breeding of new citrus rootstocks is not an endeavor for the impatient. Two of the most popular rootstocks in Florida, US-942 and US-812, each were included in field evaluation for more than 20 years before the U.S. Department of Agriculture (USDA) released them for commercial use. In these cases, the time invested in thorough testing paid off in what became their eventual commercial value.
The time from when a cross is made until release of a new citrus rootstock from breeding programs around the world averages more than 25 years. Part of that time is the delay until the new hybrid can itself produce uniform seed which can be used to propagate large numbers of trees for field testing and then for commercial use. An even larger part of the time needed is for suitable field testing of the new hybrid rootstock under field conditions with a grafted scion.
Given the current urgency to find and commercially use better new rootstocks to battle HLB in Florida, the USDA rootstock program has implemented a SuperSour Strategy to create a broad array of hybrids likely to reconstitute the outstanding characteristics of sour orange, be tolerant to HLB, and also drastically accelerate the field testing of those hybrids in the HLB-endemic environment.
I began crosses aimed at the SuperSour Strategy in 2008, and the first SuperSour trials were planted into the field in 2011–12. At present, I have 16 replicated SuperSour field trials in Florida, including more than 350 different SuperSour rootstocks from a broad array of parental combinations. The trials range from the Indian River, to the Central Ridge, to the Southwest region near Immokalee. All make use of sweet orange scion to provide a good comparison of results among the different trials and locations. As a follow-up to outstanding performance with sweet orange, the best new rootstocks are subsequently included in trials with other commercial scions.
Results from the SuperSour field trials are growing rapidly, with some of the new hybrids in each trial showing performance that is superior to the standard rootstocks, including sour orange. One SuperSour trial in Fort Pierce was summarized in my presentation at the Citrus Show in January and demonstrates the promise of this approach. After four years of yield and fruit quality data from that trial, several of the new SuperSour hybrids appear outstanding, including US-1688, US-1709 and US-2338. These are on a short list for potential release in the coming year.
Comparison of results from other trials is an important part of deciding which of these new rootstocks are likely to provide the most consistent outstanding performance. Obviously, the best new rootstocks are those which demonstrate superior field performance over many years and also in many different locations. The continuation of evaluation from the multiple trials long-term is key to identifying the best new rootstocks that will provide for the reinvigoration of the citrus industry in the Indian River region and throughout Florida.
I am grateful for the broad support from citrus growers and the Citrus Research and Development Foundation and look forward to the exciting fruition of this work over the next few years.
Kim D. Bowman is a research geneticist and lead scientist at the USDA Agricultural Research Service in Fort Pierce, Florida.
Source: Indian River Citrus League, River Ramblings, May 2022
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