Xueren Cao’s research while affiliated with Chinese Academy of Tropical Agricultural Sciences and other places

Phytoplasmas are a group of plant prokaryotic pathogens distributed worldwide. To comprehensively reveal the diversity of the pathogens and the diseases they cause on Hainan, a tropical island with abundant biodiversity in China, a survey of phytoplasmal diseases was performed from 2009 to 2022. Herein, molecular identification and genetic analysis were conducted based on the conserved genes of phytoplasmas. The results indicated that phytoplasmas could be detected in 138 samples from 18 host plants among 215 samples suspected to be infected by the pathogens. The phytoplasma strains from 27 diseased samples of 4 host plants belonged to the 16SrI group and the strains from 111 samples of 14 hosts belonged to the 16SrII group. Among them, 12 plants, including important tropical cash crops such as Phoenix dactylifera, cassava, sugarcane, and Piper nigrum, were first identified as hosts of phytoplasmas on Hainan Island. Based on BLAST and iPhyClassifier analyses, seven novel 16Sr subgroups were proposed to describe the relevant phytoplasma strains, comprising the 16SrI-AP, 16SrI-AQ, and 16SrI-AR subgroups within the 16SrI group and the 16SrII-Y, 16SrII-Z, 16SrII-AB, and 16SrII-AC subgroups within the 16SrII group. Genetic variation and phylogenetic analysis indicated that the phytoplasma strains identified in this study and those reported previously on Hainan Island mainly belong to 4 16Sr groups (including I, II, V, and XXXII) and could infect 44 host plants, among which the 16SrI and 16SrII groups were the prevalent 16Sr groups associated with 43 host plant species. The diversity of host plants infected by the phytoplasmas made it difficult to monitor and control their related diseases. Therefore, strengthening inspection and quarantine during the introduction and transit of the related phytoplasmal host crops would effectively curb the spread and prevalence of the phytoplasmas and their related lethal diseases.

Litchi (Litchi chinensis) is an evergreen fruit tree grown in subtropical and tropical countries. China accounts for 71.5% of the total litchi cultivated area in the world. Anthracnose disease caused by Colletotrichum species is one of the most important diseases of litchi in China. In this study, the causal pathogens of litchi anthracnose in Hainan, China, were determined using phylogenetic and morphological analyses. The results identified eight Colletotrichum species from four species complexes, including a proposed new species. These were C. karsti from the C. boninense species complex; C. gigasporum and the proposed new species C. danzhouense from the C. gigasporum species complex; C. arecicola, C. fructicola species complex; C. arecicola, C. fructicola and C. siamense from the C. gloeosporioides species complex; and C. musicola and C. plurivorum from the C. orchidearum species complex. Pathogenicity tests showed that all eight species could infect litchi leaves using a wound inoculation method, although the pathogenicity was different in different species. To the best of our knowledge, the present study is the first report that identifies C. arecicola, C. danzhouense, C. gigasporum and C. musicola as etiological agents of litchi anthracnose.

In order to manage agricultural pathogens, it is crucial to understand the population structure underlying epidemics. Rubber tree powdery mildew, caused by Erysiphe quercicola, is a serious threat to rubber plantations worldwide especially in subtropical environments including all rubber tree growing regions in China. However, the population structure of the pathogen is uncertain. In this study, 16 polymorphic microsatellite markers were used to genotype powdery mildew samples from the main rubber tree growing regions including Yunnan (YN), Hainan (HN), western Guangdong (WG) and eastern Guangdong (EG). YN had higher genotypic diversity (Simpson's indices), genotypic evenness, Nei's gene diversity, allelic richness and private allelic richness than the other regions. Cluster analysis, DAPC analyses, pairwise divergence and shared MLGs analyses all showed that the YN differed significantly from the other regions. The genetic differentiation was small among the other three (HN, WG and EG) regions. Analysis of molecular variance indicated that the variability among regions accounted for 22.37% of the total variability. Genetic differentiation was significantly positively correlated (Rxy = 0.772, P = 0.001) with geographic distance. Linkage equilibrium analysis suggested possible occurrence of sexual recombination although asexual reproduction predominates in E. quercicola. The results suggested that although significant genetic differentiation of E. quercicola occurred between YN and the other regions, pathogen populations from the other three regions lacked genetic differentiation.

Rubber tree powdery mildew, caused by the obligate pathogen Erysiphe quercicola , is a major threat to rubber tree plantations worldwide. To investigate the temporal changes in the population structure of E . quercicola , diseased samples were collected at three disease epidemic stages (overwinter, early epidemic and late epidemic) from three counties in Hainan, China. Population structure was assessed using 16 polymorphic microsatellite markers. Both permutational multivariate analysis of variance (PERMANOVA) and analysis of molecular variance (AMOVA) indicated that there were significant differences among the population structure of E . quercicola at different epidemic stages. Furthermore, the late epidemic populations had higher genetic diversity than the other two stages. Discriminant analysis of principal components (DAPC) and STRUCTURE analysis showed that E . quercicola samples grouped into four clusters. One of the clusters included only samples from the late epidemic stage, while the other three clusters included samples from all three sample stages. Further analysis confirmed that there were significant ( p = 0.001) genetic differences among these four clusters. As the teleomorph stage of E . quercicola has not yet been reported on rubber tree, these findings suggest that the epidemic of rubber tree powdery mildew is not only established by local inoculum sources, but also may be by inoculum drifting from other plantations or through host jumps from other hosts.

Powdery mildew, caused by Oidium heveae, is a major threat to rubber plantations worldwide. Population studies are scarce for this pathogen due to the lack of polymorphic molecular markers. In this study, sixteen polymorphic microsatellite loci were identified using DNA extracted from single lesions based on the whole-genome sequences of the pathogen. Primers of these loci were applied to 138 O. heveae samples from five counties in Hainan, China. The number of alleles per locus ranged from 2 to 11 and the gene diversity varied from 0.014 to 0.728. In total, 119 multilocus genotypes (MLGs) were observed for the 138 samples. However, only one sample was significantly distinct from the other samples base on DAPC analysis. Further study indicated that there were three subclusters for the other 137 samples, but no evidence for regional genotypic subdivision was identified. The microsatellite markers developed are very useful to study the genetic structure and the dispersal route of O. heveae, especially as it can use DNA extracted directly from infected leaves.