The use of foliar fertilizers and products that induce systemic acquired resistance (SAR) to enhance tree health in citrus groves plagued by HLB has been front and center in the fight against the disease for years now. What was at first viewed as an alternative approach has become mainstream, and most growers have increased both their ground and foliar nutrition programs to pump up trees in the face of HLB.
The approach also has been a focal point of controversy. At first because it had the potential to lead growers away from strict inoculum removal, and later, allegations that not enough funding and resources were being dedicated to learning more about how HLB-infected trees were reacting to nutritional therapy.
Recently, Dr. Harold Browning, COO of the Citrus Research and Development Foundation (CRDF), authored a white paper providing a history of the research that has been dedicated to nutritional supplementation as a management tool in the fight against HLB.
“The citrus industry has been engaged in application of supplemental nutrition since early in the history of HLB,” says Browning. “With the increased interest in and discussion of this topic, CRDF felt it would be useful to summarize the investments that have been and are being made in understanding how nutrition can contribute to citrus productivity. It was felt that the ongoing discussion within the industry could benefit from a compilation of what research is being supported.”
The National Academy of Sciences (NAS) provided strategic guidance on how research should approach HLB in 2009. The NAS report, in some ways, laid the foundation on how the industry would move forward studying the disease.
The NAS did not make nutritional therapy a research priority, but made reference to it as a part of cultural practices or new approaches to growing citrus in the presence of HLB. The group also suggested evaluation of SAR materials’ role in disrupting the bacterial infection of HLB.
NAS recommended test plots for evaluation of new scouting and therapy approaches to HLB. It called for support of small-scale studies on the feasibility of “alternative” horticultural approaches suited to endemic HLB, and longer-term, development and testing of bactericides, therapeutics, and SARs.
CRDF’s paper seeks to quantify research projects that have been funded by CRDF that address the role of nutritional programs and SARs in responding to HLB. Browning added that other research (UF/IFAS, USDA, etc.) complements the work of CRDF. In addition, grower-sponsored experiments are spread across the state, along with fertilizer company-sponsored research.
In total, CRDF has funded 21 projects in the area of supplemental nutrition for a total of $4,787,456. This figure is compared to a total number of projects in excess of 300 since inception of the CRDF, covering all research priority areas. The total financial investment to date is more than $65 million.
“The feedback from the distribution of this white paper indicates many people appreciate having a better sense of what is being done to address nutrition,” says Browning. “They can then decide for themselves whether adequate investment is being made and can help in deciding what additional research might be needed. Discussion always benefits from infusion of information.”
Browning notes that some of the projects being funded have broad objectives looking at a range of nutritional treatments. It is hoped these should provide guidance on how growers can best use the tools. “These seem to be of greatest interest to growers who are trying to make important decisions in the face of HLB, and when completed, should guide growers how to incorporate appropriate nutrition into their production practices,” he says.
Challenge Of Research
According to Browning, the wide array of nutrition programs being used and variability in how trees react to therapy has provided some challenges in the study of supplemental nutrition.
“The area of citrus nutrition in general, and its interaction with HLB, is made difficult by the fact that effects of nutrients are not immediate and may take several seasons to show themselves fully,” he says. “This is true when deficiencies occur, as well as when adjustments are made to overcome nutrient imbalances. In addition, nutrition interacts with soil, rainfall, and other factors, which adds variability to the scenario.
“The fact that nutritional health is defined by the interactions of a large number of macro and micronutrients also complicates establishing simple cause and effect relationships. When you add to this complexity the highly variable and protracted development of disease symptoms when trees are infected, it becomes increasingly difficult to predict how these interactions will occur and over what time period. Thus, research to evaluate the role of individual and collective effects of various nutritional treatments is subject to tremendous variability and only through replication in multiple locations and over time will the details be revealed.”
Response To Critics
There has been criticism in some corners that not enough funding has been dedicated to nutritional therapy and its impact on HLB. Browning acknowledges the frustration, but adds every issue has two sides.
“This criticism is probably true for all areas of research, that there are those who feel not enough is being done,” says Browning. “Actually, there are two sides to this dilemma — those that feel more work needs to be done and those who feel less is needed. The critical point is whether the research is adequately addressing the most important questions and making reasonable progress. It is not necessarily a matter of how many projects or how many dollars are being spent. The goal of the research is to answer important questions with solid data.”
Browning added, like all other areas of research, the CRDF continues to accept ideas on this topic and has relied on the wisdom of the research committee and the board, mostly growers, to determine the right balance when it comes to future funding. “It is hard to predict how much additional research will be supported, because in the end, it is determined by the number of projects proposed and the quality of the planned experimentation that is submitted,” he says.