Effect of temperature and bacteria on sporulation of Phytophthora alni in river water.

: In order to evaluate the role of river water on the spread of the alder disease caused by Phytophthora alni, water samples were collected at different periods of the year 2004 in two rivers displaying contrasting biological quality indices. Sporangia were produced from isolates of P. alni belonging to the three subspecies, at the river temperature (between 8 and 15 degrees C according to the sampling period). The sporulation efficiency was evaluated according to a scale of 0-9, based on the amount of sporangia produced on mycelium plugs immersed in the water for two days. Sporulation was also evaluated in river water sterilised by filtration. The amount of sporangia increased with the water temperature for both rivers, regardless of the biological quality. No sporangium was produced at the lowest temperature (8 degrees C). Sterilisation of the water drastically reduced the sporangia-stimulating effect for most of the P. alni isolates.