Tylenchulus semipenetrans-Induced Slow Decline of Citrus: Biochemical Mechanisms and Management Implications

The slow decline of citrus is a complex syndrome primarily driven by the citrus nematode (Tylenchulus semipenetrans); with unexplained modulating influences from abiotic stressors such as soil salinity and nutrient imbalances. This review synthesises existing knowledge on the physiological and biochemical mechanisms underlying this progressive economic decline; focusing on the interplay between nematode infection; abrupt reduction in root-to-shoot ratio; and consequent osmolyte dynamics; along with management implications. The pruning effects of nematode females at death; result in abrupt diversion of the root-to-shoot ratio from the normal seasonal cycle; which triggers a series of dehydration- and ion-toxicity-avoidance mechanisms; where citrus trees redistribute hyperosmotic osmolytes; which involve chemical and physiological adjustments to restore the stable seasonal root-to-shoot ratio. This review underscores the importance of maintaining a stable seasonal root-to-shoot ratio through integrated management strategies. Avoiding mechanical weeding; while maintaining light shoot pruning; strategic fertilisation with potassium supplementation; improved irrigation management and selection of dual salt-tolerant and nematode-resistant-rootstocks are crucial for sustaining orchard longevity and productivity. By integrating the proposed strategies; citrus growers can improve tree longevity; maintain fruit production; and ensure long-term orchard sustainability. In conclusion; the review highlights the necessity of a holistic; multipronged management approach to mitigate the slow decline of citrus; with future research focusing on molecular pathways that govern the root-to-shoot homeostasis by exploring breeding strategies for citrus rootstocks with enhanced resilience against both biotic and abiotic stressors.