We compared the seasonal occurrence of Leptomitus lacteus (Roth) C. Agardh (Oomycetes, Leptomitales) in freshwater samples from autumn 2003 to summer 2004. For this purpose, we analysed 42 different water samples of running waters from Hesse and Thuringia and additionally of stagnant waters and of a spring from Hesse. For isolating L. lacteus, the baiting technique was used. The simultaneous limnochemical characterisation of the examined waters resulted in information on the trophic status of the lakes and the water quality classification of some rivers. L. lacteus occurred in all three types of waters examined. Based on the number of samples in which it occurred, L. lacteus seems to prefer rivers rather than stagnant waters. L. lacteus could be isolated during the whole year. Nevertheless, most isolates were obtained in winter. Hence, a preference for growth and propagation in winter is postulated.

In this study, we examined the occurrence of species of aquatic Oomycetes of Saprolegniales und Leptomitales in Lake Aue and the river Fulda in Kassel (Hesse, Germany) with special consideration of fish pathogenic species. We monthly collected water samples of both water bodies from July 2003 to June 2004. The baiting method was used to gain an insight into the specific seasonal occurrence of aquatic Oomycetes. Some species show a seasonal periodicity. Nine of 11 species of Saprolegniales isolated from the two water bodies, and Leptomitus lacteus (Roth) C. Agardh, one of the two species of Leptomitales, were already documented on fish according to references. Additionally, 7 of 10 species could according to references already be detected on fish species present in the two water bodies. Simultaneous measurements of limnological-chemical parameters were done during the sampling of water for the isolation of aquatic Oomycetes. The results of some measurements, concerning the river Fulda, were additionally compared with the predetermined guideline values and limit values of water bodies of fish of the "Cyprinid region".