Detection of Diplodia corticola spores in Ontario and Québec based on High Throughput Sequencing (HTS) methods.
A total of 252 aerial spore samples from the provinces of Quebec and Ontario were obtained during an extensive research programme focusing on aerobiology of fungal spores. Samples were collected on 95 sticky rods from rotary arm spore collectors and 157 filters from passive rain collectors. DNA from samples was PCR-amplified for fungal ribosomal ITS and sequenced using High Throughput Sequencing (HTS) methods. After bioinformatics analysis of DNA sequences, the presence of Diplodia corticola, an emerging tree pathogen in North America, was observed. In total, 313 DNA sequence reads from aerial spores of D. corticola were found in the Illumina data set, and 199 DNA sequence reads were obtained from the Ion Torrent data set. DNA of D. corticola was found in 16 of the 32 sampled sites, always less than 10 reads per site, with the exception of three sites - Quebec City, Aylmer and Ottawa - where 287, 125 and 73 DNA reads, respectively, were detected. This is not a first report of the presence of D. corticola causing a tree disease in Canada as symptomatic trees have not been identified. Typically, like many other species of the Botryosphaeriaceae, this fungal pathogen is believed to be an opportunistic plant endophyte capable of living asymptomatically for several years before showing up as a pathogen when conducive conditions arise. Its presence as singletons in nearly half the sampled sites in Quebec and Ontario may be the result of long-distance spore dispersal originating from known infected sites in Massachusetts, Maine and other unknown sites in the north-eastern USA. However, its much higher read counts in three sites may indicate the possibility of a few trees being asymptomatically infected and spreading conidia locally.