Wide Distribution and Intraspecies Diversity in the Pathogenicity of Calonectria in Soil from Eucalyptus Plantations in Southern Guangxi of China

Abstract

Eucalyptus spp. are extensively cultivated in southern China because of their adaptability and versatile timber production. Calonectria leaf blight caused by Calonectria species is considered a major threat to Eucalyptus trees planted in China. The GuangXi Zhuang Autonomous Region is the provincial region with the largest distribution of Eucalyptus plantations in China. The present study aimed to expound the species diversity and pathogenicity of Calonectria isolates obtained from the soil of Eucalyptus plantations in GuangXi. A total of 188 Calonectria isolates were recovered from the soil located close to Eucalyptus trees, and the isolates were identified based on the DNA sequence comparisons of the four partial regions of the translation elongation factor 1-alpha (tef1), β-tubulin (tub2), calmodulin (cmdA), and histone H3 (his3) genes. The isolates were identified as Calonectria aconidialis (74.5%), C. hongkongensis (21.3%), C. pseudoreteaudii (2.1%), C. kyotensis (1.6%), and C. chinensis (0.5%). The inoculation results indicated that 40 isolates representing five Calonectria species were pathogenic to the three Eucalyptus genotypes. Two inoculated experiments consistently showed that the longest lesions were produced by the isolates of C. aconidialis. Some isolates of C. aconidialis, C. hongkongensis, and C. kyotensis produced significantly longer lesions than the positive controls, but not the isolates of C. pseudoreteaudii or C. chinensis. These results indicated that Calonectria isolated from the soil may pose a threat to Eucalyptus plantations. Some Calonectria isolates of the same species differed significantly in their virulence in the tested Eucalyptus genotypes. The resistance of different Eucalyptus genotypes to Calonectria isolates within the same species was inconsistent. The inoculation results in this study suggested that many Calonectria isolates in each species had different levels of pathogenicity, and many Eucalyptus genotypes need to be tested to select disease-resistant Eucalyptus genetic materials in the future. The results of the present study enhance our knowledge of species diversity and the potential damage caused by Calonectria in the soil of Eucalyptus plantations. Our results also provide new insights into the breeding of disease-resistant Eucalyptus genotypes for controlling Calonectria leaf blight in China in the future.

Keywords: Calonectria leaf blight; Eucalyptus disease; fungal pathogen; pathogenicity; species diversity.

Figure 1

Map of GuangXi Zhuang Autonomous Region showing sampling sites in this study. The seven sampling sites are indicated as letters A to G.

Figure 2

Phylogenetic tree of Calonectria species based on maximum likelihood (ML) analyses of a combined DNA dataset of tef1, tub2, cmdA, and his3 gene sequences. Bootstrap support value ≥ 70% for ML and posterior probabilities values ≥ 0.95 for Bayesian inference (BI) analyses are presented above the branches as follows: ML/BI. Bootstrap values < 70% or probabilities values < 0.95 are marked with “*”, and absent analysis values are marked with “-” Ex-type isolates are marked with “T”. Isolates sequenced in this study are highlighted in bold and shown in color. Two isolates of Curvicladiella cignea (CBS 109167 and CBS 109168) were used as outgroups.

Figure 3

The isolate number and percentage of each Calonectria species in the GuangXi Zhuang Autonomous Region. Different species are indicated by numbers with different colors.

Figure 4

The pathogenicity results of experiment one. Column chart indicating the average lesion length (mm) on leaves resulting from inoculation trials of three Eucalyptus hybrid genotypes inoculated with 40 isolates of five Calonectria species and positive and negative controls. Horizontal bars represent the standard error of the means. Different numbers on the right of the bars indicate treatment means that were significantly different (p = 0.05). The “***” represents no lesions produced by the negative controls.

Figure 5

The pathogenicity results of experiment two. Column chart indicating the average lesion length (mm) on leaves resulting from inoculation trials of three Eucalyptus hybrid genotypes inoculated with 40 isolates of five Calonectria species and positive and negative controls. Horizontal bars represent the standard error of the means. Different numbers on the right of the bars indicate treatment means that were significantly different (p = 0.05). The “***” represents no lesions produced by the negative controls.