Methods of treating contamination, particularly fungal contamination, in cultures of Haematococcus pluvialis with hydrogen peroxide and salt are described herein. The method comprises dosing the culture comprising a concentration of salt with a concentration of hydrogen peroxide based on the stage of the cells in the culturing process and at a frequency to increase the likelihood of the cells surviving until the process of accumulating carotenoids, such as astaxanthin, is complete.