Fusarium avenaceum causes burn spot disease syndrome in noble crayfish (Astacus astacus)

Burn spot disease has been causing epidemics both in the Estonian mainland and in Saaremaa Island in the threatened noble crayfish (Astacus astacus) stocks. To study the cause of the disease, we isolated several Fusarium spp. from Estonian noble crayfish (A. astacus) populations suffering from burn spot disease syndrome. We first identified fungi directly from melanised cuticle by their ITS sequences. Then we isolated Fusarium spp. from melanised spots of crayfish showing burn spot disease symptoms, such as melanisation and shell erosion, from two different crayfish populations and watercourses in Estonia. The isolates were then identified based on ITS and EF1α-gene sequences. Isolates of Fusarium spp. taken from two separate Estonian noble crayfish populations were used in infection studies. Koch postulates confirmed that the studied agent was causing burn spot disease symptoms including shell erosion in the noble crayfish, which were significantly more severe after molts. After the infection period, an identical Fusarium spp. was re-isolated from carapace lesions and was thus shown to be the disease agent causing burn spot disease syndrome and shell erosion in noble crayfish. Based on GenBank database searches, the isolates causing burn spot disease symptoms were identified as Fusarium avenaceum in mainland Estonia and F. solani in Saaremaa crayfish.