Leaf- and canopy-level chlorotic symptom variations are associated with differential Candidatus Liberibacter asiaticus infection patterns in Ponkan mandarin under field conditions

Ponkan mandarin ( Citrus reticulata cv. Ponkan) is an important fruit crop planted across different countries. One of its most devastating diseases is citrus huanglongbing (HLB), which is associated with infection by the bacterium Candidatus Liberibacter asiaticus ( C Las). Although studies on HLB in Ponkan mandarin have been conducted, symptom heterogeneity and its association with C Las titer under field conditions remain underexplored. We examined the association between symptom variation and C Las infection status in a Ponkan mandarin grove by categorizing symptoms at both the leaf and canopy (tree) level. Chlorotic leaves were sampled and classified into different categories, including leaves exhibiting interveinal chlorosis (Cat. A), those showing vein yellowing (Cat. B), and blotchy mottled leaves (Cat. C). Polymerase chain reaction (PCR) and quantitative PCR (qPCR) assays targeting C Las revealed that Cat. A leaves, which resembled nutrient-deficient leaves, had higher C Las detection rates and a larger proportion of samples with high C Las titers. Canopy-level assessment of symptom diversity within each tree also revealed that plants bearing Cat. A leaves were more likely to carry C Las and have high pathogen loads. The likelihood was also greater in trees with higher leaf symptom diversity. Additional prophage typing of the detected C Las strains revealed that leaf symptom variations were independent of prophage type difference. Nutrient analyses revealed reduced magnesium, manganese, and zinc contents in Cat. A leaves, indicating that leaves prone to C Las infection also exhibited nutrient deficiencies. These findings highlight the complexity of potential cultivar-specific associations between C Las and citrus under field conditions.