Pythium infestation on Phalaenopsis usually occurs on seedlings and young plants, less frequently on older ones. Newly thinned out seedlings die quickly within a short time, the roots turn brown and rot. Often the root cortex is destroyed whereas the stele is still more or less intact. Different animal organisms feed on the roots as a consequence of or as initial cause of the disease. Older plants of Cattleya, Paphiopedilum, Oncidium or Odontoglossum show brown to black infested areas on the leaf base, the pseudobulbs and the rhizomes. The infested plant parts rot away and finally the plants die. The disease usually occurs in the darker seasons as a result of inadequate cultivation. Phytophthora almost exclusively infests older plants. These causative organisms that occur frequently on orchids mainly attack the stem base, but can also infest the leaves. Quickly enlarging brown or black rot spots occur on the infested parts which finally make the plant die when there is extensive soft rot. Phytophthora is a causative organism that has ideal infection conditions in high humidity and high temperatures, i.e. in the summer.
