Interstock Pros and Cons for HLB Management

The pros of using interstocks for HLB management seemed to outweigh the cons, based on a presentation horticulturist Manjul Dutt made at this spring’s Florida Citrus Growers’ Institute. Dutt is a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) assistant professor.

An interstock is a graft of a citrus selection that can be used as a bridge between a tree’s scion and rootstock.

THE PROS

Dutt cited the pros of using interstocks for HLB management as:

• Compatibility: Interstocks can be used to bridge the compatibility gap between the rootstock and scion, allowing for the successful grafting of otherwise incompatible combinations.
• Disease resistance: Interstocks possess resistance or tolerance to specific diseases or pests, providing added protection to the scion. This can contribute to improved overall health and productivity of the tree.
• Vigor regulation: Interstocks can influence the vigor of the scion, helping to balance the growth of the tree. This can be particularly useful in managing vigorous scions or rootstocks, leading to better tree structure and more manageable growth.

THE CONS

According to Dutt, the cons of interstock use are:

• Increased cost: Introducing an interstock adds to the overall cost of tree production due to the additional materials, labor and expertise required for grafting.
• Potential for unintended effects: Introducing a new element into the grafting process introduces the potential for unintended consequences such as changes in tree growth habits, fruit characteristics or susceptibility to certain stresses. These effects may not become apparent until after several years of tree growth.
• Limited availability of suitable interstocks: Finding interstocks that meet the required criteria (like compatibility, disease resistance and vigor regulation) can be challenging.

EXPERIMENTAL DESIGN AND CONCLUSIONS

Dutt said the hypothesis behind the use of interstocks is that HLB-tolerant interstocks would allow growers to use the rootstocks they liked prior to HLB becoming a problem.

Dutt described the experimental design of UF/IFAS research: Interstock candidates were selected based on field performance showing low or no HLB symptoms. Swingle rootstock was cleft grafted with 6-inch interstock sticks obtained from the field trees. The interstock was subsequently cleft grafted with HLB-infected Valencia scion. A year following grafting, trees were planted in the field. There were nine pummelo selections with 10 replications each. Swingle rootstock served as the control.

Conclusions of the study were:

• All interstocks influenced the tree growth rate (either positively or negatively) in the field.
• Most trees were visibly healthy with minimal HLB symptoms even after five years in the field.
• Some of the interstock combinations resulted in higher pound solids.
• HLB-tolerant interstocks can be used to provide enhanced tolerance to susceptible scions.

Dutt said future plans call for development of trees with robust HLB-tolerant interstocks from the UF/IFAS breeding program, including Sugar Belle, Australian lime hybrids, Citrus latipes hybrids and pummelo hybrids.

Learn more about the potential of interstocks from Dutt and other UF/IFAS authors in a 2021 article, Pummelo Interstocks Could Improve Performance of HLB Trees.

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