Evidence of High Genetic Diversity and Differences in the Population Diversity of the Eucalyptus Leaf Blight Pathogen Calonectria pseudoreteaudii from Diseased Leaves and Soil in a Plantation in Guangxi, China

Abstract

Calonectria pseudoreteaudii is an important causal agent of Eucalyptus leaf blight in southern China. This pathogen causes Eucalyptus tree disease across numerous regions in southern China. In addition to diseased leaves, C. pseudoreteaudii has occasionally been isolated from soil in Eucalyptus plantations. The aim of this study was to clarify whether C. pseudoreteaudii causing Eucalyptus leaf blight in China is mainly clonally reproduced and to determine the potential spreading mechanism of C. pseudoreteaudii between diseased leaves and soil. To this end, 10 polymorphic microsatellite markers were analyzed to detect the genetic diversity of 97 C. pseudoreteaudii isolates from diseased leaves and soil in a Eucalyptus plantation in Guangxi Zhuang Autonomous Region, southern China. The analysis showed that the genetic diversity of the isolates from both the diseased leaves and soil was high. However, the gene and genotype diversity of the C. pseudoreteaudii isolates from diseased leaves were higher than those of the isolates from the soil. Moreover, all genotypes detected in the isolates from the soil were also found in the isolates from the diseased leaves. Structural analyses did not show clear population structures related to the population substrates of the diseased leaves or soil, and molecular variance analyses indicated that no significant genetic differentiation existed between the diseased leaf and soil populations. These results suggest that C. pseudoreteaudii in soil spreads from diseased leaves, and that an asexual cycle is the primary reproductive mode in both diseased leaf and soil populations. This is the first study on the genetic diversity and population structure of C. pseudoreteaudii. The high genetic diversity and spread pathways of this pathogen may pose challenges in controlling the disease. C. pseudoreteaudii from both diseased leaves and soils in Eucalyptus plantations needs to be carefully monitored for disease control and management.

Keywords: Calonectria leaf blight; forest disease; fungal pathogen; genetic diversity; plantation disease; population biology.

Figure 1

Multilocus genotypes (MLGs) generated from two C. pseudoreteaudii populations from diseased leaves and soil. Twenty-three genotypes were detected in the full dataset. The number of individuals residing in each genotype is indicated in each bar graph.

Figure 2

Minimum spanning network (MSN) constructed with Bruvo’s genetic distance. Each node represents a single multilocus genotype (MLG), and the node size is directly proportional to the sample size. The thickness and shading of the lines represent the genetic distance between the two genotypes (a thicker line denotes a smaller genetic distance).

Figure 3

Structure analyses of C. pseudoreteaudii isolates from diseased leaf and soil populations. (A) Each individual is displayed as a bar, which is divided into K colors, where K is the possible number of clusters. (B) The optimal number of genetic clusters (ΔK) = 2.