Comparison and correlation of Corynespora cassiicola populations from kiwifruit and other hosts based on morphology, phylogeny and pathogenicity

Corynespora leaf spot, which is caused by Corynespora cassiicola (Berk. & M. A. Curtis) C.T. Wei (C. cassiicola), has been globally reported in many plant species. ‘Hongyang’ was reported as highly sensitive kiwifruit cultivar to C. cassiicola. This cultivar is an important germplasm resource in the Actinidiaceae family and is widely cultivated throughout China. Even though C. cassiicola has been identified as the pathogen associated with kiwifruits in China, the C. cassiicola population from kiwifruit has not been characterized based on morphology, phylogeny and pathogenicity. In this study, 133 and 48 representative C. cassiicola isolates from kiwifruit and 11 other hosts, respectively, recovered from symptomatic leaves were classified into eight morphological subgroups based on host origins. Using three loci (rDNA ITS, caa5 and act1), phylogenetic tree showed that C. cassiicola isolates in Sichuan Province were grouped into three clades. All kiwifruit isolates were genetically identical to the rubber isolates from different countries. While the most isolates from other hosts in this study were genetically identical to the cucumber, soybean and cowpea isolates in China, Brazil and the United States. And two strawberry isolates clustered with isolates from tomato and other hosts in China, Brazil and the United States. Furthermore, we confirmed host shift of C. cassiicola among different plant species in this study. Although 51 isolates from kiwifruit and different hosts were pathogenic to kiwifruit, blueberry, cucumber, and soybean, virulence levels of the pathogen were diverse for four hosts. Kiwifruit isolates exhibited host specificity with regards to the original host in degree. In addition, those isolates revealed a correlation between morphology and pathogenicity. The results suggest that C. cassiicola in Sichuan Province were derived from three different phylogenetic lineages. Promotion of the susceptible ‘Hongyang’ cultivar led to the emergence of a regnant C. cassiicola population from kiwifruit. In conclusion, rapid development of the crop which is sensitive to C. cassiicola in agricultural system leading to the emergence of regnant C. cassiicola population. In some dominant populations (eg. C. cassiicola population from kiwifruit in this study), host origin was found to be a key factor influencing the morphologic, genetic and pathogenic characterization of C. cassiicola.