Abstract
Animals and plants are increasingly threatened by emerging fungal and oomycete diseases. Amongst oomycetes, Saprolegnia species cause population declines in aquatic animals, especially fish and amphibians, resulting in significant perturbation in biodiversity, ecological balance and food security. Due to the prohibition of several chemical control agents, novel sustainable measures are required to control Saprolegnia infections in aquaculture. Previously, fungal community analysis by terminal restriction fragment length polymorphism (T-RFLP) revealed that the Ascomycota, specifically the genus Microdochium, was an abundant fungal phylum associated with salmon eggs from a commercial fish farm. Here, phylogenetic analyses showed that most fungal isolates obtained from salmon eggs were closely related to Microdochium lycopodinum/Microdochium phragmitis and Trichoderma viride species. Phylogenetic and quantitative PCR analyses showed both a quantitative and qualitative difference in Trichoderma population between diseased and healthy salmon eggs, which was not the case for the Microdochium population. In vitro antagonistic activity of the fungi against Saprolegnia diclina was isolate-dependent; for most Trichoderma isolates, the typical mycoparasitic coiling around and/or formation of papilla-like structures on S. diclina hyphae were observed. These results suggest that among the fungal community associated with salmon eggs, Trichoderma species may play a role in Saprolegnia suppression in aquaculture.
Highlights
Saprolegniosis, caused by Saprolegnia species, results in tremendous losses in wild and cultured fish species including salmonids such as salmon and trout, and non-salmonids such as tilapia, catfish, carp, and eel [1]
Based on the phylogenetic analyses of internal transcribed spacer (ITS) sequences, all the nine Microdochium isolates are closely related to M. lycopodinum and M. phragmitis, and no distinct separation is observed between isolates from diseased or healthy salmon eggs (Figure 2)
Our study is the first to assess the diversity among Trichoderma and Microdochium isolates from aquaculture samples
Summary
Saprolegniosis, caused by Saprolegnia species, results in tremendous losses in wild and cultured fish species including salmonids such as salmon and trout, and non-salmonids such as tilapia, catfish, carp, and eel [1]. No vaccine is currently available to control this disease [1,5] Bacterial genera such as Bacillus, Enterococcus and Lactobacillus have been shown to reduce specific diseases in aquaculture and several of these beneficial bacteria are being commercialized [6,7,8,9,10]. Several beneficial fungi and/or their bioactive compounds are applied to control diseases These fungal species are isolated either randomly or systematically [19,20,21,22,23,24,25]. Their abundance in diseased and healthy salmon egg batches and their activity against Saprolegnia diclina were investigated here
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