Combining PGRs and OTC Boosts Returns

By Prudhvi Vulchi, Amit Levy and Tripti Vashisth

A three-year commercial grove study evaluated plant growth regulators (PGRs) and oxytetracycline (OTC) in HLB-affected Valencia trees. The results suggest that these practices help HLB-affected trees by two distinct mechanisms. PGRs mainly supported tree growth and productivity by reducing callose accumulation and improving growth-promoting hormone levels in leaves. OTC reduced Candidatus Liberibacter asiaticus (CLas), the bacterial pathogen that causes HLB.

RESEARCH RESULTS

PGR-treated trees produced higher yields than untreated trees, with the clearest improvements occurring during the first two years. In trees without OTC, PGR programs increased yield by 28% to 44% in the first year and 152% to 188% in the second year. PGR treatments also increased fruit retention and canopy growth. In addition, they reduced callose accumulation by about 60%. These results suggest that PGRs may improve productivity by helping the tree function better, rather than by reducing CLas.

OTC reduced CLas levels, although this effect was no longer detected at time of harvest. A strong yield response to OTC was seen in the third year, when the application method was changed to two half-dose injections placed on opposite sides of the trunk. Because the injection method changed, the study cannot determine whether this yield response was due to repeated OTC use, better distribution from the two-sided injection or both. Please refer to Ute Albrecht for OTC application guidance.

Over the three years of the study, trees receiving both OTC and PGRs had the highest yields and net returns. Adding PGRs to OTC increased net returns by $245 to $363 per acre compared with OTC alone or by $862 to $980 per acre compared with the untreated control. These estimates account for OTC and PGR application costs and assume that PGRs were tank-mixed with other planned sprays.

WHAT THIS MEANS FOR GROWERS

• PGR programs generally improved yield.
• PGRs reduced callose, supported canopy growth and affected growth-promoting hormone levels.
• PGRs did not reduce CLas, suggesting their benefits came mainly from supporting tree physiology.
• OTC temporarily reduced CLas.
• The clearest OTC yield improvement occurred in the year when two-sided injection was used, but the reason for this response cannot be confirmed from this study.
• Combining OTC and PGRs produced the highest numerical yield and net return. Using the two practices together may offer a more complete approach to managing HLB-affected trees.

Prudhvi Vulchi is a former master’s student, Amit Levy is an associate professor, and Tripti Vashisth is associate center director and professor, all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.