Mycol. Res. 99 (5): 574-578 (1995) Printed in Great Britain Physiological adaptation of an Aphanomyces astaci strain isolated from the freshwater crayfish Procambarus clarkii JAVIER DIEGUEZ-URIBEONDO, TIEN-SHENG HUANG, LAGE CERENIUS AND KENNETH SODERHALL'" Deparlmenl of Physiological Bolany, Universily of Uppsala, Villavagen 6, 5-752 36 Uppsala, Sweden 574 Physiological, epidemiological and genetical properties of an Aphanomyces aslaci strain (Pc) isolated from the warm water crayfish, Procambarus clarkii, were compared to other A. aslaci strains isolated from the cold water crayfish Aslacus aslacus, Aslacus leplodaclylus and Pacifaslacus leniusculus. Morphology, virulence at 10 °C and several physiological characters were similar to the rest of A. aslaci strains used for comparison. However, the strain Pc was able to grow faster and release zoospores at higher temperatures. This means that the Pc strain is virulent towards crayfish at higher temperatures than any previously investigated strains. The application of RAPD-PCR demonstrated that this isolate is genetically separated from all other described strains. Aphanomyces astaci Schikora is a parasitic saprolegniaceous fungus specialized to living in the cuticle of freshwater crayfish (Unestam, 1969 a, b). This fungus is responsible for the 'Crayfish plague' which has devastated many European native populations of crayfish. European, Asian and Australian native species have been found to be susceptible (Unestam, 1969 a). Mortalities up to 100% occur in all these susceptible species after attack by A. astaci (Unestam, 1969 a; Unestam, 1972; Alderman et aI., 1987). Three North American species of crayfish have been shown to carry the fungus in melanized spots in their cuticle, Pacifastacus leniusculus (Unestam, 1972; Persson & S6derhiill, 1983), Orconectes limosus (Vey et aI., 1983) and Procambarus clarkii (Dieguez-Uribeondo & S6derhiill, 1993). Morphological, physiological and pathological properties of A. astaci strains isolated from different species of crayfish appear to be similar. Recently, analysis by RAPD-PCR of the genetic diversity in A. astaci isolates from Astacus astacus, Astacusleptodaetylus and Pacifastacusleniusculus has shown that these A. astaci strains can be assigned to three main groups (Huang et al., 1994). Recognition and characterization of subspecific groups within a pathogenic species may be important for under- standing ecology, epidemiology and also for the recognition of virulence factors of a pathogen. Few studies have been carried out on intraspecific variation within the Saprolegniaceae. For instance, Holub et al. (1991), described a wide pathogenic diversity within Aphanomyces euteiches, a plant parasite, and discussed the emergence of biologically unique subspecies of this pathogen. In the fish pathogen • Corresponding author. Saprolegnia parasitica, physiological forms were reported by Kanouse (1932). Willoughby & Copland (1984) suggested that the differences in temperature-growth relationships of Saprolegnia pathogenic to fish isolates might indicate physio- logical adaptations to the fish host. Recently, a physiological adaptation of an Aphanomyces sp. pathogenic to Atlantic menhaden has been described (Hearth & Padgett, 1990). This isolate was shown to be salt resistant with a generally greater salinity tolerance than previously reported for other saprolegniaceous fungi (Hearth & Padgett, 1990). In this paper we report some physiological characters of a strain of A. astaci isolated from the freshwater crayfish P. clarkii, which lives and thrives in subtropical areas of North America, but this species has been spread by humans to several other continents, also to Europe. We show that this strain is genetically different from other A. astaci isolates and that it appears to be physiologically adapted to its host. MA TERIALS AND METHODS Crayfish Freshwater crayfish, Procambarus clarkii were caught in the province of Badajoz, Spain and Astacus astacus were purchased from Kroab AB, Fengersfors, Sweden. All crayfish were kept in aerated tap water at 10 °C and one week before the experiments all crayfish were fed with wheat, potatoes or carrots. Only intermoult animals were used in the experiments. Isolation of Aphanomyces astaci from Procambarus darkii Isolation of the A. astaci strain from Procambarus clarkii was done by following the methods described in Cerenius