Pythium aphanidermatum is a cosmopolitan 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, 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.

Pythium is an Oomycete in the order Peronosprales. The hyphae are hyaline and the mycelium has no cross walls. To differentiate P. aphanidermatum from other Pythium species requires examination under a microscope of the sporangia, oogonia and antheridia. Sporangia are the asexual spores and in the case of P. aphanidermatum, they are lobate (inflated). The apluerotic oogonium, (oospore doesn't fill the oogonium) and the intercallary (rarely terminal) attachment of the antheridia further distinguishes P. aphanidermatum from other Pythium species.
 
 

Best results for isolation of P. aphanidermatum from plant tissue is by rinsing the tissue in tap water, rinsing in sterile water, blotting dry on sterile paper towels and then plating on a selective medium. Some of the most commonly used media are P₅ARP and Pythium aphanidermatum medium. The antibiotics in these media limit bacterial contamination and pimaricin will prevent growth of Phytophthora species.

When isolating from soil, soil dilution plating is the most efficient method. Air-dried soil samples are added to 10-ml water blanks, vortexed and a small volume is pipetted onto selective medium plates. A sterile, bent glass rod is used to disperse the sample over the entire plate. The plates should be stored upside down and read in 24 hours.

Pythium aphanidermatum occurs world wide, particularly in warm regions and greenhouses. The fungus prefers temperatures between 27 and 34 C and wet conditions (water potential of 0 to –0.01 bars). It has a very wide host range, including many annuals and bedding plants. It causes economic losses on beets, pepper, chrysanthemum, cucurbits, cotton, and grasses.

Pythium survives in the soil as oospores, hyphae and sporangia. The fungus can survive unfavorable soil moisture and temperatures for several years as oospores. Oospores may form a germ tube directly to infect the plant or they may form sporangia. The sporangia produce zoospores, which are the motile form of the fungus. The zoospores swim around briefly before encysting and forming a germ tube, which can cause infection. Sporangia that have developed on plant tissue can germinate in a similar manner as the oospores, by either germinating directly or by forming zoospores. The pathogen is dispersed when infected debris is transported to uninfested areas and when the soil moisture is enough to allow the zoospores to swim freely.
 

Pythium aphanidermatum infects seeds, juvenile tissue, lower stems, fruit rot and roots. The symptoms and extent of damage caused depend on the region infected.

Damping off

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

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.
 

Root Rot

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 (six inch), 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.
 
 
 
 
 
 
 

Cottony Blight

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.
 
 
 
 

Cultural Practices

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.
 

Fungicides

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.
 

Acknowledgments

I would like to thank Gloria Abad for supplying slides and information for this web page.  Thanks also to Jillanne Burns for providing information on chemical controls.
 

References

1.  Agrios, G.N. 1997. Plant Pathology 4^th ed.  Academic Press, San Diego, CA. pp. 266-270.

2.  Bruehl, G.W. 1987. Soilborne Plant Pathogens. Macmillan Publishing Company, London. p. 326

3.  Chase,  A.R. 1987. Compendium of Ornamental Foliage Plant Diseases.  APS Press. St.Paul, MN. p.42

4.  Chase, A.R. 1997. Foliage Plant Diseases Diagnosis and Control.  APS Press. St. Paul, MN.

5.  Martin, F.M. Pythium, in Methods for Research on Soilborne Phytopathogenic Fungi, edited by L.L Singleton,  J.D. Mihail and C.M. Rush. 1992. APS Press. St. Paul, MN. pp. 39-49.

Turf Diseases (Texas A& M University, Dept of Microbiology and Plant Pathology)
 Pythium Blight on Turf Grasses (University of Nebraska,Cooperative Extension)
 Root Rots of Dry Beans (Colorado State University, Cooperative Extension)