Phytophthora, is a microscopic soil borne organism that causes root rot on a wide variety of plants, including orchids. Infection eventually leads to the death of the plant. Phytophthora requires moist soil conditions and warm temperatures to be active, the very same conditions most orchids require.

The zoospores are good swimmers and attach and infect roots, normally behind the root tip. After the infection, phytophthora grows through the root destroying the tissue. Because phytophthora attacks the roots, the orchids unable to absorb enough water and nutrients from the soil, die. Given the right conditions, zoospore numbers can build up quite rapidly and kill the plant faster. The spores can survive extended periods of unfavorable conditions when conditions improve they germinate and start a life cycle.

Prevention is the best cure.

Orchid growers can prevent root rot from spreading by: 1. Throwing away contaminated plants (pot + soil + plant). 2. Always use pre-treated and disease free potting components. 3. Sterilized pre-used pots by soaking in a solution of a disinfectant/detergent solution. 4. Clean and sterilize all implements used; cutting knives, secateurs etc.). 5. Keep the working area clean, dry and free of dead plant material and refuse.

Fungicides containing potassium phosphonate are registered for control of this disease in certain situations.