|A cucumber showing symptoms of Pythium blight|
| Pythium root and stem rot
Pythium blight (Pythium aphanidermatum) is a plant pathogen with a wide host range. It is an aggressive species of Pythium, causing damping off, root and stem rots, and blights of grasses and fruit. It is of economic concern on most annuals, Cucurbits, Beets, Pepper and grasses. It is considered one of the water molds because it survives and grows best in wet soils. Warm temperatures favor the pathogen, making it an issue in most greenhouses. The fungus prefers temperatures of 27-34°C (81-93°F).
Pythium aphanidermatum infects seeds, juvenile tissue, lower stems, fruit rot and roots. The symptoms and extent of damage caused depend on the region infected.
Plants are most vulnerable to infection by P. aphanidermatum during the germination and juvenile stages. The initial symptoms may be poor or uneven germination (pre-emergence damping off). Seedlings that do germinate are susceptible to post-emergence damping-off. An infected seedling will appear water-soaked and the plant will collapse. Entire plantings can be completely destroyed at this stage.
Stem rot infects the lower stem of many annual and bedding plants, especially during the juvenile stage. Symptoms begin as a water soaked region near the soil line. The plant tissue becomes slimy. If the lesion expands to encircle the stem, the plant will collapse. If the pathogen is limited by the plant, the lesion will eventually dry out and the plant begins to recover. The lesion eventually becomes sunken and is brown in color.
Pythium root rot causes wilting, loss of vigor, stunting, chlorosis and leaf drop. Root growth is inhibited and roots are blackened, mushy and rotten. Symptoms begin at the root tips. The rotten part of the roots may slough off exposing the inner root core. Underground storage structures are also at risk of infection by P. aphanidermatum. Even mature tissue can be infected and destroyed. Beets and other fleshy plant organs are susceptible to rot in the field and during storage.
Pythium blight of turf grasses
Pythium blight is an aggressive disease of turf grasses. The grasses usually die and are slow to recover. The blight begins as small (15cm/6in), circular, reddish brown spots that begin to coalesce. The grass leaves take on a dark green, water-soaked appearance and cottony mycelium may be present on the blades of grass. 
Above ground, mature plant tissue is susceptible to infection to infection by P. aphanidermatum, particularly when it is in contact with the soil. It is most commonly seen on cucumbers and other cucurbits. The blight begins as water soaked regions, later enlarging and developing cottony mycelium on the rotting fruit. 
Proper sanitation can greatly reduce the risk of infection by P. aphanidermatum. Use of clean pots, media and plant material can help prevent the infection of plants by the fungus. Proper air circulation between plants can make conditions less favorable for disease development. Poor drainage is a contributing factor to the dispersal of Pythium, as with other water mold fungi. A key to controlling disease development is to keep soil well drained and to avoid over irrigating. A well-drained soil not only limits the dispersal of zoospores, but also prevents plants from becoming predisposed to root rot fungi. Growing the plants at optimum conditions will limit the disease, since vigorous plants are less likely to be infected and more likely to recover.
Soil drenches of many fungicides are effective in controlling P. aphanidermatum. Subdue Maxx, Fore and Alliette are registered for use on lawns. Several fungicides are available for control of P. aphanidermatum on nursery crops. The appropriate treatment depends on the specific plant. Subdue is registered for use on begonia, chrysanthemum Iris and pansy. Ridomil Gold is registered for use on beans, beets, broccoli and cucumbers. For all crops, lower rates can be used for preventative control and higher rates for curative control.
- Parker, K.C. Pythium aphanidermatum. NC State University. Retrieved: 2010-08-25.