One of the most important diseases that gardeners need to be aware is Phytophthora (Fy-TOFF-thor-uh). Buying, transporting and planting of nursery stock puts the gardening trade into the triangle of helping to control the spread of this pathogen.
Phytophthora species make up a group of microorganisms that are important plant pathogens. The name Phytophthora derives from Greek and literally means “plant destroyer.” Phytophthora species resemble fungi but are not. These organisms need water to complete their life cycle.
Certain species of Phytophthora cause important diseases in vegetable crops, fruit and nut trees, and forest trees as well as in nursery crops. Phytophthora disease is the number-one disease of nursery crops nationwide.
Phytophthora organisms infects the root system starting in the fine root hairs, gradually moving into larger roots. After plant cells become infected, they lose the ability to take up and transmit water and nutrients. Some Phytophthora species can infect leaves as well as roots.
After the plant cells are killed, the leaves or roots discolor. At first the plant may exhibit subtle above-ground symptoms such as reduced vigor, reduced growth, or ”off” color. The organism continues to infect more and may eventually girdle the plant.
Many plants with Phytophthora root rot do not show aboveground symptoms until summer. As hot, dry weather sets in, the plant does not have enough roots left to keep up with transpiration. Plants may wilt and collapse. Because of the wilting, many people water plants even more than usual, flooding their roots, encouraging the pathogen, and potentially spreading the disease even more.
When buying plant material, inspect the plants to see if they appear to be healthy. Don’t buy plants if they appear to be sitting in muddy areas or standing in water for a long time. Avoid allowing containers to sit in water for long periods or on plastic or muddy woven fabric as Phytophthora can spread through contaminated roots and containers. One of the best practices is to hold your plants on gravel. Avoid reusing plastic nursery pots unless you sterilize the pot prior to reuse. Clean off and disinfect your plant holding area periodically.
Water management is the most important step you can take to control Phytophthora diseases in the landscape. Also, avoid excessive applications of nitrogen fertilizers. The resulting succulent new growth is very susceptible to Phytophthora. Avoid reuse of potting soil medium in landscaping or when up potting plants.
For more information on Phytophthora, check out the on line course through OSU Extension.