# Field resistance of orange fruit to citrus black spot and citrus canker in D‐limonene synthase downregulated trees

## TL;DR

Genetically modifying citrus trees to reduce D-limonene increases resistance to two major citrus diseases in field conditions.

## Contribution

First field validation of terpene modulation as a strategy to enhance citrus resistance to black spot and canker.

## Key findings

- GM 'Navelina' oranges showed 45% reduction in citrus black spot severity compared to non-GM controls.
- GM 'Pineapple' oranges showed over 68% reduction in citrus canker incidence compared to non-GM controls.
- Lower D-limonene and higher monoterpene alcohol levels correlated with increased disease resistance.

## Abstract

Citrus black spot (CBS), caused by Phyllosticta citricarpa, and citrus canker (CC), caused by Xanthomonas citri subsp. citri, are among the most important diseases affecting citrus production worldwide, causing premature fruit drop, reduced market value, and trade restrictions. Previous studies demonstrated that reducing D‐limonene levels through genetic engineering and increasing monoterpene alcohol accumulation enhances resistance to these pathogens in controlled conditions. However, the effectiveness of this strategy had not been validated under field conditions. This study investigated field resistance to P. citricarpa and X. citri in fruit from genetically modified (GM) ‘Navelina’ and ‘Pineapple’ sweet orange trees (Citrus × sinensis L. Osb.) with downregulated CitMTSE1 and enhanced monoterpene alcohol content.

In both assessed seasons, results revealed a reduction of 45% in CBS severity on fruit of the GM ‘Navelina’ in comparison with non‐GM ‘Navelina’ orange control. CBS symptom reductions were strongly correlated with lower D‐limonene concentrations and higher monoterpene alcohol contents in fruit peel of GM fruit. The GM ‘Pineapple’ orange lines showed more than 68% of reduction in incidence of CC affected fruit across the two seasons compared to non‐GM ‘Pineapple’ orange, as it showed reduced D‐limonene content and higher monoterpene alcohol concentrations accumulated in the fruit peel.

The terpene modulation approach showed a potential strategy to increase field resistance against P. citricarpa and X. citri in citrus fruit. Further studies may investigate ways to enhance resistance rates by combining different strategies, thereby enabling the integration of genetic resistance into the management of these important citrus diseases. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Genetically modified (GM) sweet‐oranges with downregulated CitMTSE1 gene showed reduced citrus black spot severity and citrus canker incidence, confirming terpene modulation as a promising strategy for sustainable field citrus disease resistance.