Molecular identification of oomycete species affecting aquaculture in Bangladesh

Fish mycotic disease outbreaks occur due to infections with oomycete pathogens such as Saprolegnia spp. and Aphanomyces invadans, and cause large-scale fish production losses. Despite its negative impact on aquaculture, little is known about the diversity of oomycete pathogens. The aim of this study was to identify the diversity of pathogenic oomycetes causing infections in major aquaculture zones of Mymensingh and Jessore regions in Bangladesh. A total of 449 water and infected fish samples were collected from 28 fish farms in both regions of which 29 samples were able to grow out with mycelia on the Potato Dextrose Agar (PDA)/Glucose Peptone Yeast Agar (GPYA) plates. Sequence database searches using the rRNA Internal Transcribed Spacer (ITS) region revealed that 15 belonged to Pythium spp., 12 were Aphanomyces invadans and two corresponded to Saprolegnia parasitica. Five isolates of Pythium spp. were identified to the species level: one was closely related to Pythium catenulatum, four to Pythium rhizo-oryzae, rest were identified up to genus. The Pythium spp. were only isolated from water samples whereas A. invadans and S. parasitica were found in fish lesions. Phylogenetic analysis revealed that a single A. invadans clone exists in the sampled area. The results obtained confirm the existence of pathogenic oomycetes in Bangladesh fish farms and this will pave future research on diversity, prevention and control measures.