Pythium and Phytophthora rot
The causative organs of black rot on roots, rhizomes, stems and leaves of orchids belong to the species of the Oomycetes group. Main species found on Orchids are Phytophthora cactorum Schroet., Pythium ultimum Trow., Pythium debaryanum Hesse and Pythium splendens Braun.
Characteristic features of these fungi are round to lemon-shaped sporangia. This is where the flagellate zoospores are formed that makes the causative organ spread. Sufficient humidity is the decisive factor for multiplication and spreading of the fungi in the plant stand. Constant substrate humidity helps to grow fruiting structures and zoospores. Spraying or irrigation spreads the rot from plant to plant. Most Phytophthora species prefer high temperatures and sufficient humidity, many other fungi, however, grow best in cool and humid conditions. The fungi intrude their host plants with the help of specific enzymes and excrete toxic substances in them. Within a very short time infested plant parts may decompose and there will be extreme soft rot. Whereas Pythium only infests underground plant parts as a parasite, Phytophthora may occur on all plant parts.
