Abstract: The causes of mop-top disease of Chinese fir (Cunninghamia lanceolata (lamb. ) Hook. ) are fungi (Pestalotiopsis guepinii (Desm. ) Stey. , and then Fusarium oxysporum Schlech). The optimum temperatures and pH range for myceliae growth of P. guepinii and F. oxysparum are 25 to 28 C and 6-7 respectively , and the optimum temperature and pH for conidia germination are similar to the myceliae growth. The optimum relative humidity over 95% were found to be favourable to the spore germination of P. guepinii and F. oxysporum respectively.