Pythium irregulare

Melanie Katawczik
PP728 Soilborne Plant Pathogens, NCSU
Fall, 2008



 Pythium irregulare is a soilborne pathogen found world wide on hundreds of different plant hosts. Like most members of the Pythium species, it causes damping off (i.e. blight) and root rot of plants located in both greenhouses and the field especially under moist conditions that favor the spread of this pathogen through zoospores. While P. irregulare can be isolated by itself from a host, it is often found in conjunction with another Pythium species.

Host Range and Distribution

 P. irregulare had been reported on all major continents minus Antarctica and identified on over 200 host species (Farr et al. 2008 and van der Plaats-Niterink, 1981). In the United States it has been found across the country on pineapple, cereals, grasses, celery, pepper, tobacco, pecan trees, citrus trees, strawberries, lentils, corn, soybean, cucumber, onion, carrot and a number of floricultural crops like Gerbera daisies and geraniums. P. irregulare prefers conditions of high moisture for root colonization and 50% saturation capacity of soil for infection (Hendrix and Campbell 1973) and is commonly found in greenhouses as the pathogen is spreads through movement in water often from contaminated irrigation sources.  P. irregulare infects at lower temperatures then other Pythium species like P. aphanidermatum, P. myriotylum or P. volutum although this can also be affected by host (Hendrix and Campbell 1973).


 P. irregulare can be isolated from plant tissue by surface sterilizing the tissue, rinsing in sterile water and plating on either water agar or PARP, a semi selective media with a base of cornmeal agar and amended with antifungal and antibacterial agents (pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene) (Martin 1992).  Soil isolation can either be done through two methods: baiting or dilution methods. Baiting techniques use live tissue placed in or on the soil, incubating them and then transferring the tissue to selective media (Martin 1992). Soil dilution plating involves weighing out a certain amount of air dried soil, completely suspending in a known water amount and plating out different diluted volumes on selective media such as Mircetich’s MPVM (Mircetich and Kraft 1973).    


Oogonium with irregular wall projection.
Photo courtesy Plant Health Progress. 

P. irregulare is an Oomycete of the family Pythiaceae and with use of a compound light microscope it can be differentiated from other Pythium species by identification of structural morphology (van der Plaats-Niterink 1981). Hyphae are typically 5 um in diameter and can have limoniform swellings. Sporangia are asexual structures and in P. irregulare are spherical and can be terminal or intercalary and produce zoospores which are about 7-10 µm when encysted. Antherida and oogonia are the sexual structures. The antheridia are monoclinous or hypogynous, are stalked one to three per oogonium and can be branched. Oogonia are spherical, smooth walled or may have a number of irregular projections (1-5) and can be intercalary or terminal. Oospores are the end result of an oogina fertilized by antheridia and are mostly aplerotic.

There is great variation in structures for not only P. irregulare but other Pythium species as well, which can be influenced by environment and plant host. Therefore molecular techniques such as isozyme analysis, ITS-RFLP, and AFLP are now used in taxonomic studies (Matsumoto et al. 2000 and Garzon et al. 2005).
Limoniform hyphal swellings. Aplerotic oospore. Intercalary oogonium. Terminal sporangia.



Like most Pythium species pathogens, P. irregulare can cause pre- and post-emergence damping off (i.e. blight) of seed and seedlings and root rot of older plants (Hendrix and Campbell 1973, Koike et al 2007). Both pre- and post- emergence can occur in the greenhouse, often in transplant trays from contaminated potting mix or irrigation water. Root rot typically occurs in the field from P. irregulare populations already in the soil.

Pre-emergence damping off attacks seeds before they germinate or after germination but before they reach the soil surface. It can be hard to observe as the seeds and seedlings will quickly turn brown and soft before decomposing.  Post-emergence damping off occurs on seedlings after they break the soil surface and P. irregulare attacks near the soil line. The roots become brown, may appear water soaked and the tips necrotic. Water soaked areas of the hypocotyl observed above the soil may develop which eventually dry to a brown lesion, foliage may turn chlorotic or a greenish-grey and/or wilt. Rotting may occur followed by plant collapse otherwise they may become stunted.

Geraniums with root rot caused by P. irregulare.  
Photo courtesy Michael Evans, University of Arkansas
Chrysanthemum stunting and wilt from Pythium.
Photo courtesy Department of Plant Pathology Archive, North Carolina State University,
Foliar wilt and chlorosis from Pythium infected tobacco seedlings.
Photo courtesy of Walter A. Gutierrez, USDA APHIS
Browning and sloughing off of outer tissue of chrysanthemum roots caused by Pythium infection.
Photo courtesy Paul Bachi, University of Kentucky Research & Education Center,
Pythium root rot of tobacco seedlings.
Photo courtesy Walter Gutierrez, USDA APHIS
Post emergence damping off of seedlings.
Photo courtesy Clemson University - USDA Cooperative Extension Slide Series,


Ecology and Life Cycle 

Photo courtesy Walter Gutierrez, USDA APHIS.
Like many other Pythium species, P. irregulare has sexual and asxual stages which both end in infection by hyphal germ tubes (van West et al. 2003). P. irregulare can overwinter in soil as an oospore, the result of sexual reproduction between the antheridium and the oogonium. Release of seed or root exudates cause the oospore to germinate. It can either directly infect the root through means of a germ tube or indirectly through forming a sporangium. The sporangium release zoospores which are motile in water and encyst then infect by formation of a germ tube. This is why crops in low lying areas with bad drainage or plants in a greenhouse with a contaminated irrigation system are more at risk for infection. The mycelium can grow inter or intracellularlly and when the plant dies, sporangia can form and the infection cycle can repeat with either direct germination of the sporangia or formation of zoospores. Or it could also survive saprophytically on the dead plant tissue in hyphal form. Eventually oogonium may develop, which are fertilized and mature into a dormant oospore, waiting for the next root or seed exudates to indicate a host.

Oogonium and antheridium. Germ tube. Zoospore release from vesicle. Photo courtesy Walter Gutierrez, USDA APHIS.
Oospores in a root.
Photo courtesy Walter Gutierrez, USDA APHIS.

 Links to Other Sites

USDA ARS Fungal Databases

Oomycete Genomics Database

Ill ustrated glossary of plant pathogen terms

Pythium Root Rots in Greenhouse Crops, PSU


Selected References

1.                  Farr, D. F., Rossman, A. Y., Pa lm, M. E., and McCray, E. B. 2004. Online. Fungal Databases, Systematic Botany & Mycology Laboratory, ARS, USDA.

2.                  Garzón, C.D., Geiser, D.M., and Moorman, G.W. 2005. Diagnosis and population analysis of Pythium species using AFLP fingerprinting. Pl. Dis. 89: 81-89.

3.                  Hendrix F.F. and Campbell W.A. 1973. Pythiums as plant pathogens. Annu. Rev. Phytopathol. 11, 77-98.

4.                  Koike, S.T., Gladders, P., and Paulus, A.O.  2007. Vegetable Diseases: A Color Handbook. Academic Press. Boston, MA. Pg 87-88, 226-228, 370-371.

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. Pg. 39-49.

6.                  Matsumoto, C., Kageyama, K., Suga, H., and Hyakumachi, M. 2000. Intraspecific DNA polymorphisms of Pythium irregulare. Mycol. Res. 104: 1333-1341.

7.                  Mircetich S.M. and Kraft J.M. 1973. Efficiency of various selective media in determining Pythium population in soil. Mycopathologia 50: 151-161.

8.                  van der Plaats-Niterink AJ. 1981. Monograph of the genus Pythium. Studies in Mycology. vol 21.  Centraalbureau voor Schimmelcultures, Baarn. Pg 1-27 and 85-87.

9.                  van West, P., Appiah, A.A., and Gow, N.A.R. 2003. Advances in research on oomycete root pathogens. Physiological and Molecular Plant Pathology. 62: 99–113.