Pythium ultimum was recorded to be the first time to cause citrus root rot in Cambodia. The pathogen causes a serious damage almost everywhere planted to citrus in Battambang province. The pathogen infects the plants starting from seedlings which show yellow leaves, die back, stem rot, root rot and die. Mainly, the citrus trees express slowly decline from the second year and slowly die which starting from six to seven years old, even using the chemical fungicides. In laboratory test, the detached leave method has shown that only three days after inoculation of pathogen, the citrus leaves turning completely dark brown showing aggressive pathogen and proved for pathogenicity. In field trials, the chemical and biological fungicides namely Chaetomium and Trichoderma biological products were peroidically applied to four year old citrus trees in one year. All products were spayed above plants and to rhizosphere soil every month (metalaxyl-10g/20L of water in combination with chemical fertilizers, Chaetomium20g/20 L of water and Trichoderma-20g/20L of water in combination with biocompost). Result showed that all treated citrus trees recovered significantly within 3-4 months of applications. The new flashes of leaves and root were emerged and citrus tress recovered. It is proved that the biological products of Chaetomium and Trichoderma gave significantly disease control as equal as the chemical fungicide (metalyxyl) when compared to the non-treated control
