Species Diversity, Mating Strategy and Pathogenicity of Calonectria Species from Diseased Leaves and Soils in the Eucalyptus Plantation in Southern China

Abstract

Many Calonectria species are causal agents of diseases on several forestry, agricultural and horticultural crops. Calonectria leaf blight is one of the most important diseases associated with Eucalyptus plantations and nurseries in Asia and South America. Recently, symptoms of leaf rot and leaf blight caused by Calonectria species were observed in a one-year-old Eucalyptus experimental plantation in GuangXi Province, southern China. To better understand the species diversity, mating strategy and pathogenicity of Calonectria species isolated from diseased tissues and soils, diseased leaves and soils under the trees from ten Eucalyptus urophylla hybrid genotypes were collected. Three hundred and sixty-eight Calonectria isolates were obtained from diseased Eucalyptus leaves and soils under these trees, and 245 representative isolates were selected based on the sampling substrates and Eucalyptus genotypes and identified by DNA sequence analyses based on the translation elongation factor 1-alpha (tef1), β-tubulin (tub2), calmodulin (cmdA) and histone H3 (his3) gene regions, as well as a combination of morphological characteristics. These isolates were identified as Calonectria hongkongensis (50.2%), C. pseudoreteaudii (47.4%), C. aconidialis (1.6%), C. reteaudii (0.4%) and C. auriculiformis (0.4%). This is the first report of C. reteaudii and C. auriculiformis occurrence in China. Calonectria pseudoreteaudii was isolated from both Eucalyptus diseased leaves and soils; the other four species were only obtained from soils. MAT1-1-1 and MAT1-2-1 gene amplification and mating type assignment results showed that C. pseudoreteaudii is heterothallic and an asexual cycle represents the primary reproductive mode, C. reteaudii and C. auriculiformis are likely to be heterothallic and C. hongkongensis and C. aconidialis are homothallic. Based on the genetic diversity comparisons for C. pseudoreteaudii isolates from diseased leaves and soils, we hypothesize that C. pseudoreteaudii in soils was spread from diseased leaves. Both the mycelia plug and conidia suspension inoculations indicated that all five Calonectria species were pathogenic to the two Eucalyptus genotypes tested and the tolerance of the two genotypes differed. It is necessary to understand the ecological niche and epidemiological characteristics of these Calonectria species and to select disease resistant Eucalyptus genotypes in southern China in the future.

Species Diversity and Distribution Characteristics of Calonectria in Five Soil Layers in a Eucalyptus Plantation

“…Many species of Calonectria are important pathogens of agricultural, horticultural, and forestry crops and these species occur in approximately 335 plant species in nearly 100 plant families [1]. Species of Calonectria have been isolated from soils, fruits, leaves, stems, and roots [1,4,[7][8][9][10][11][12][13][14]. The fungi are best known as foliar, shoot, and root pathogens [1,2,4,5], and they are commonly associated with disease symptoms, including seedling damping-off, seedling rot, cutting rot, leaf spots, leaf blight, shoot blight, crown cankers, stem lesions, collar and root rots, and tuber rot [1,[14][15][16][17][18][19][20][21][22][23].…”

“…Calonectria ilicicola is a soil-borne fungal pathogen of worldwide importance that causes black rot disease in peanut and red crown rot in soybean [21,[24][25][26][27][28]. Recently, we isolated five Calonectria species, namely C. aconidialis, C. auriculiformis, C. hongkongensis, C. pseudoreteaudii, and C. reteaudii, from soils in a plantation of Eucalyptus trees [14]. Inoculation results showed that all five species caused leaf spot, leaf blight, and seedling rot to the tested Eucalyptus genotypes within three days [14].…”

“…Recently, we isolated five Calonectria species, namely C. aconidialis, C. auriculiformis, C. hongkongensis, C. pseudoreteaudii, and C. reteaudii, from soils in a plantation of Eucalyptus trees [14]. Inoculation results showed that all five species caused leaf spot, leaf blight, and seedling rot to the tested Eucalyptus genotypes within three days [14].…”

“…Previous research results indicated a high level of species diversity of Calonectria in southern China, especially in soils [9,11,13,14,23]. Currently, a total of 125 Calonectria species have been described using DNA sequence-based phylogenetic analyses and morphological comparisons [5,13,[29][30][31][32][33][34][35].…”

“…Currently, a total of 125 Calonectria species have been described using DNA sequence-based phylogenetic analyses and morphological comparisons [5,13,[29][30][31][32][33][34][35]. A total of 25 species of Calonectria have been identified and described in China based on DNA sequence data [5,9,11,13,14,36]. Of these species, 17 have been isolated from soils, with 11 from soils under plantation Eucalyptus trees [5,9,11,14].…”