Greenhouse Disease Management
Rhizoctonia Root Rot
Rhizoctonia root rot is common under a range of environmental conditions. Rhizoctonia spp. usually attack plants at the soil line, causing root loss and constriction of the stem, which results in girdling and death of the tops. But this pathogen can attack leaves as well, and is especially severe when plants are grown close together and kept moist. If conditions are optimal, entire stock beds or flats can be lost to Rhizoctonia in a few days. The pathogen is soilborne and lives in the soil or potting medium and causes both pre- and post-emergence “damping off” of many seed-produced crops. Rhizoctonia root rot has a wide host range, including virtually all herbaceous greenhouse crops such as dianthus, impatiens, coreopsis, ageratum, chrysanthemum, and petunia.
Disease Cycle
Rhizoctonia is a common soil inhabitant and can produce small sclerotia that can persist for several years in the soil. Rhizoctonia does not produce spores but is moved from place to place along with particles of soil moved by water or wind. Sclerotia in soil germinate, and the resulting mycelium colonizes plant surfaces where nutrients are available.
Symptoms
Symptoms of Rhizoctonia root rot are similar to those caused by other root pathogens, but Rhizoctonia lesions on lower stems and roots are often drier and lighter in color than other root rots (See Figure 28.9). This disease is the most common cause of damping off of young seedlings. Unlike Pythium and Phytophthora, dry soil is more favorable for disease development. For this reason, Rhizoctonia is more active in the upper portion of the soil.
Cultural Management Strategies
Soil-borne pathogens such as Rhizoctonia spp. can escape notice, causing serious losses before they are identified. Therefore, prevention is the key to controlling Rhizoctonia diseases. Use healthy plants, new or sterilized pots, and potting media, and avoid placing plants directly on the ground whenever possible.
Chemical Management Strategies
Fungicides remain an important part of greenhouse plant production. Determining the application site for a fungicide control is crucial; as mentioned earlier, Rhizoctonia spp. occur in the potting medium and can cause stem and leaf diseases. Terraclor (PNCB), Terraguard (triflumizole), Cleary’s 3336/OHP 6672 (t-methyl), and Medallion (fludioxonil) applied as a drench have been important tools in preventing Rhizoctonia and halting its spread.
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