Genetic Improvements Offer Best Long-term Solutions to Citrus Greening Disease
by Ben McLean III, Uncle Matt’s Organic, Head of Research
2019 has been a productive year of research for Uncle Matt’s Organic as we work to find solutions to Citrus Greening Disease, a bacterial disease spread by the Asian Citrus Psyllid. Our ongoing efforts to eradicate this disease from the industry include partnering with public researchers to discover technical solutions, working with private landowners to replant and manage new varieties, and collaborating with non-profit organizations to help communicate and publish positive results.
What solutions look most promising today? We have a triad of promising research that is demonstrating promise for not only supporting our existing organic groves but creating confidence for new groves to be planted.
New rootstocks, new varieties: Promising genetic solutions
New varieties of both rootstocks and scions developed through traditional breeding programs at the University of Florida — and the USDA – have shown disease resistance to Greening. These promising genetic solutions will be available for use by organic growers, as they constitute a “non-GMO” solution derived from traditional plant breeding.
For those not familiar with citrus production, the rootstock is the base material upon which the desired cultivar or variety is grafted onto. In comparison to seedlings, rootstocks confer several benefits, including earlier maturity, disease resistance and enhanced productivity. University of Florida researchers, most notably Dr. Jude Grosser, have developed several promising resistant rootstocks, named UF R1, R4, R5, and R17. USDA researchers, including Dr. Kim Bowman, have also developed several promising selections including US 897 and 812. These rootstocks are made from crosses of other citrus rootstocks that have shown disease tolerance, then, selected from larger plantings as desirable traits are expressed.
Scions, which are the desired cultivars grafted onto the rootstock, are also being developed by traditional breeding methods by researchers at the University of Florida. One of the most promising is the Mandarin hybrid “Sugar Belle”, a tangerine-tangelo cross which shows much promise as a fresh fruit variety that withstands the onslaught of the Greening disease, and, could serve as a future variety for organic growers to replant. Efforts are being made to obtain further grant funding, in conjunction with Florida Organic Growers (FOG) and The Organic Center (TOC,) to fund further planting sites of this promising variety on resistant rootstocks, in the hope of demonstrating that organic citrus production can not only survive, but thrive, with these improved genetic selections.
At Uncle Matt’s Organic, we have assisted one local grower by replanting almost 17 acres of these new tolerant rootstock/scion combinations, with approximately 12 acres planted in Sugar Belle on the USDA’s 812 rootstock, and 5 acres of another promising hybrid known as “Tango”, which is planted on USDA’s 897 rootstock. It is our hope that this young grove and others like it can be a template for future organic plantings.
Nutrient balance: Supporting the tree’s natural defenses
In addition to improved genetics for disease resistance, we are finding that genetic improvement must be coupled with good nutrition. Nutrient balance affects the tree’s ability to fight disease. Dr. Grosser has proven that adequate nutritional levels of boron, manganese, magnesium and calcium help to reduce infection. Dr. Kelly Morgan of UF’s Immokalee REC, has shown that elevated levels of Zinc, Manganese and Boron assist the tree in defense and recovery. At Uncle Matt’s, we continue to explore these nutritional inputs and their role in fighting Greening and restoring health and productivity to infected trees, as well as preventing disease in healthy trees.
Uncle Matt’s, in a successful partnership with The Organic Center (TOC), has participated in a study funded by the UNFI Foundation. The results of the study were published in the January 20 issue of the scientific journal International Journal of Horticulture, Agriculture and Food Science. This study was the result of a joint effort by the University of Florida and The Organic Center and used “available literature, unpublished research data and grower observations” on inputs and methods for managing citrus greening in organic groves. The Grower Guide, and the published paper, can both be downloaded here. This was an important project because it provides guidance to organic growers on organic compliant methods to combat Citrus Greening.
Further, The Organic Center’s Grower Guide gives the organic citrus producers a roadmap for future production decisions. We have shared and collaborated these results with some of our suppliers and partners in California, Mexico, and Texas, assisting these growers in gaining confidence to replant existing groves and even plant new citrus groves.
“Organic citrus producers have suffered terrible losses from citrus greening, and they need to be aware of organic solutions to ward off this disease”, said Dr. Jessica Shade, Director of Science Programs for The Organic Center. “Our goal in releasing this Guide is to help organic citrus growers fight this deadly disease without resorting to dangerous chemicals, genetic engineering or other methods not in compliance with organic standards.”
Survivor trees: What their genetics tell us about combating Greening
As we identify “escape” or “survivor” trees, we are learning that these trees contain beneficial microbes that may unlock biological answers for a cure. Newly discovered populations of microbes found in HLB-survivor trees, that appear healthy in otherwise heavily diseased groves, may harbor potentially beneficial microbes for citrus trees.
These findings could lead to probiotic inoculations for citrus trees that would confer an acceptable level of HLB-tolerance to help trees fight off the disease. In addition, natural products produced by these microbes may have the potential for use as antimicrobial formulations against the HLB pathogen. This area of work is being led by Dr. Caroline Roper, University of California, Riverside. Another exciting find was produced this spring by Jessica Dominguez of Florida International University, who discovered that several bacterial isolates taken from Uncle Matt’s groves showed activity in laboratory settings against bacterial pathogens similar to the bacteria that causes Citrus Greening. We continue to assist the ongoing efforts of both researchers to gather further samples and identify other escape trees.
In summary, much progress has been made in the ongoing battle to manage and cure this devastating Citrus Greening disease. What we do currently know: 1) the planting of resistant genetics, 2) the implementation of a solid nutritional program containing certain micronutrients, and, 3) the use of an integrated pest management plan to manage the Asian Citrus Psyllid with biocontrol and botanical oils, are roadmaps to success. Productivity and profitability can be attained when these factors are addressed in the planting of a new organic citrus grove. We at Uncle Matt’s look forward to continuing the research work and aiding the communication and ongoing conversation about future organic citrus production.