Melanised exoskeletal lesions in the narrow-clawed crayfish (Pontastacus leptodactylus) are associated with an oomycete-dominated microbial assemblage without detectable Fusarium: evidence from a Belarusian reservoir.

Burn spot disease, a form of shell disease in crayfish, has been linked to Fusarium avenaceum (in Astacus astacus and Pontastacus leptodactylus from Estonia and Turkey) and Fusarium solani (in a distinct form on Saaremaa Island, Estonia). Severe melanised lesions in P. leptodactylus from Belarus's Lyubanskoye Reservoir resemble the Saaremaa syndrome. This study aimed to characterise the culturable microorganisms in these lesions and to test for Fusarium species. We performed the first culture-based molecular characterisation of lesion-associated fungi and oomycetes in this host, using ITS rDNA sequencing, phylogenetic analysis, Fusarium-specific PCR and genotype-specific real-time PCR. From 15 severely affected individuals, 19 isolates were obtained: 14 oomycetes and five fungi. Oomycetes included Aphanomyces astaci, Saprolegnia parasitica, Saprolegnia australis, two unidentified Saprolegniaceae lineages, Pythium flevoense, and unidentified Pythium spp. Fungi comprised Epicoccum nigrum, Plectosphaerella sp., Penicillium sp., Cladosporium sp., and Catenaria sp., the last being a first record from crayfish. The A. astaci isolate belonged to genotype group B. No Fusarium isolates were recovered in culture, and no Fusarium DNA was detected by universal ITS, genus-level nested TEF1 or species-specific assays applied to lesion-derived DNA. Severe melanised lesions in this population harbour a taxonomically diverse, oomycete-dominated microbial assemblage. Because non-lesioned control tissue, histopathology and experimental infections were not included, the recovered taxa are reported as lesion-associated organisms, and their individual and combined roles in lesion initiation and progression remain to be tested.