A general survey was carried out on the environmental conditions and the nutritional requirements for the growth of Pythium strains which seem to be the most prevalent pathogen involved in laver red rot in Ariake Sea. Five test strains grew in sea water concentrations of 0 to 100%, the growth maxima being between 40 and 100%. The optimal range of pH and temperature for growth seemed to be pH 7.0-8.5 and 15-25°C, respectively. When 4 strains were tested for nutritional requirements, vitamins were found to be not essential. However, yeast extract, liver extract and bovine serum stimulated their growth. Glucose, sucrose and starch were favorable sources of carbon. Amino acids such as alanine, serine, arginine, aspartate, glutamate and asparagines gave better growth than inorganic nitrogen compounds. By testing two strains, it was demonstrated that mannan (konjak), xylan (straw), casein and gelatin were attacked while agar, porphyran, mannan (seaweed) and xylan (seaweed) were not at all or only negligibly attacked.
Comparing the physiological characteristics as described above, the strains from Ariake Sea farm were, for the most part, similar to Pythium porphyrae from Miyagi Prefecture of Tohoku, but differed from P.porphyrae in the growth ranges of pH and temperature which resembled rather those of Pythium from Fukushima Prefecture of Tohoku.