Identification and characterization of Phytopythium helicoides causing stem rot of Shatangju mandarin seedlings in China

Citrus mandarin (Citrus reticulata) cultivar Shatangju has great economic importance in southern China because of its delicious fruit and medicinal peel. During surveys conducted in 2012 and 2013, stem rot and plant death were observed on 2- to 3-year-old Shatangju grafted seedlings in 21 nurseries located in three regions (Yangjiang City, Sihui City and Deqing County) of Guangdong Province, China. The aims of this study were to characterize the symptoms of seedling stem rot and death; evaluate the incidence and loss; identify the causal agent and evaluate its pathogenicity. The disease was present in all surveyed nurseries but incidence varied with location. A Pythium-like oomycete was consistently obtained from symptomatic plants collected in the nurseries surveyed. On the basis of morphological and cultural characteristics and combined phylogenetic analysis of three DNA sequences including the internal transcribed spacer region (ITS), mitochondrial CoxI and CoxII genes, 21 of the isolates obtained were identified as belonging to the species Phytopythium helicoides. The isolates were pathogenic to the scions but not rootstocks of Shatangju mandarin seedlings cultivated in a controlled greenhouse and symptoms identical to that observed in the field were reproduced. The pathogen was re-isolated consistently from the inoculated plants, confirming Koch's postulates. Seedling stem rot and eventual death caused by P. helicoides represents a serious threat to Shatangju mandarin nursery productivity and longevity in southern China.