Dominance of Calonectria queenslandica and geographic variation in species composition causing Eucalyptus leaf blight in Guangxi, southern China.

Calonectria leaf blight is one of the most destructive diseases affecting Eucalyptus plantations worldwide. Although Guangxi holds the largest area of Eucalyptus plantations in China, the species diversity and geographic distribution of Calonectria associated with this disease remain poorly understood. To address this gap, a systematic field survey was carried out across 19 Eucalyptus plantations in Guangxi in 2025, planted with E. urophylla × E. grandis or other commercial hybrids. A total of 497 Calonectria isolates were obtained from symptomatic plant tissues, and eight species belonging to three species complexes were identified based on multi-locus phylogenetic analyses. These included C. queenslandica (83.3%), C. pseudoreteaudii (9.7%), C. reteaudii (4.6%) in the C. reteaudii species complex, C. aciculata (0.2%), C. eucalypti (0.8%), C. honghensis (0.2%) in the C. colhounii species complex, C. hongkongensis (0.8%), C. ilicicola (0.4%) in the C. kyotensis species complex. Calonectria species composition varied among the three studied clusters, with seven species detected in the West, three in the South and two in the East. Calonectria queenslandica was present and dominant in all three clusters, indicating that it is the main causal agent of Calonectria leaf blight outbreaks across the province. MaxEnt modeling identified annual mean temperature (Bio1, contribution 66.8%) as the most influential bioclimatic variable associated with the predicted distribution of C. queenslandica, with high suitability habitats concentrated in central-southern Guangxi, central-eastern Guangdong, and southern Fujian. Precipitation of warmest quarter contributed 12.2% but had zero permutation importance, indicating no independent effect, though rainfall and humidity may still influence disease development. Pathogenicity tests confirmed that all species were pathogenic on E. urophylla × E. tereticornis trees, with significant variation in aggressiveness both between and within species. This study provides a comprehensive assessment of Calonectria diversity and distribution in Guangxi Eucalyptus plantations. The findings extend the known geographic ranges of Calonectria and offer a scientific basis for developing sustainable management strategies for the disease.