College of Agriculture and Life Sciences
Department of Plant Pathology

PP728 Pathogen Profile

Pathogen profile for: Pythium myriotylum

       By: Sarah Ruark

For: PP728:Soilborne Plant Pathogens, NC State University

Spring, 2007


Pythium spp. are soilborne pathogens and are most important in their ability to infect a wide variety of plants, with infection often resulting in pre- or post-emergence damping off of seedlings, root rot, or wilting.   Pythium myriotylum Drechs. is no exception.  P. myriotylum is often found along with other Pythium spp. in greenhouse production systems where high plant densities and environmental conditions favor disease development. 
Host Range and Distribution:
    P. myriotylum is distributed worldwide in warm regions with a very large host range.  According to Farr et al. (3), the pathogen has been reported in 79 plant hosts in genera such as Antirrhinum, Aponogeton, Arachis, Caladium, Citrullus, Coronilla, Cucumis, Glycine, Ligustrum, Lolium, Peperomia, Phaseolus, Robinia, Solanum, and Triticum.  The pathogen is capable of causing economic losses in tomato, peanut, cocoyam (Xanthosoma sagittifolium), caladium, kangaroo paw (Anigozanthos spp.), and tobacco.
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Isolation: Pythium myriotylum colony
     P. myriotylum can be isolated from plant tissue by surface sterilizing the infected tissue and plating it on selective media such as water agar or media amended with antibiotics (e.g. polymyxin, pimaricin, and penicillin) to limit growth of other organisms.
    The presence of P. myriotylum in soil can be determined by a polymerase chain reaction (PCR)-based assay that is able to detect the pathogen’s oospores.  Oospores are separated from the soil in a sucrose solution and collected on a cellulose nitrate membrane.  The oospores are lysed and a sequence from the ITS1 region of rDNA is amplified by PCR (5).
(Photo courtesy of H.D. Shew)

    P. myriotylum is a member of the  kingdom Straminopila and is not a true fungus due to the presence of cellulose in the cell wall, vegetative diploidy, and flagellated spores (zoospores) (4).  A member of the phylum Oomycota, P. myriotylum is in the order Peronosporales.  Hyphae are aseptate and multinucleate.  The mycelium is diffuse and capable of growing 34 mm in 24 hours at 24ºC  with numerous appressoria [7-11µm in diameter] (2).  Reproduction can occur asexually via terminal or intercalary lobate sporangia (2), resulting in the production of either a new germ tube or a biflagellate zoospore (4). Sexual reproduction requires the formation of terminal or intercalary spherical to subspherical oogonia and curved antheridia to form subspherical or flattened oospores (2).
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    By far, the most common symptom of infection by Pythium spp. (including P. myriotylum) is pre- or post-emergence damping off.  Pre-emergence damping off results in a failure of seed to germinate and it may become soft, brown, and shriveled before disintegrating.  Post-emergence damping off results in the death of seedlings that have already germinated.  Symptoms in these seedlings include water-soaked or discolored areas in the stems at or below the soil line, as well as in the roots.  Death quickly follows.  Older plants are also susceptible to Pythium diseases; however, infected plants are not as likely to die as seedlings.  Symptoms in older plants include root and stem lesions or root rot.  Wilting may also occur.  Diminished growth and yield are commonly associated with Pythium disease in older plants.  (1)
Pod rot of peanut Pythium root rot Pythium wilt Root rot on tobacco
Pythium pod rot on peanut.

Photo by: Clemson University - USDA Cooperative Extension Slide Series.
Pythium root rot on melon.

Photo from:
Pythium wilt on dry bean.

Photo by: Howard F. Schwartz of Colorado State University
© Copyright Kansas State University Research and Extension All Rights Reserved
Pythium root rot on tobacco.

Photo by: Clemson University - USDA Cooperative Extension Slide Series.

Ecology and Life Cycle:
    P. myriotylum causes disease via direct penetration of the host by an appressoria (2).  Cells of the host are broken down through the release of pectinolytic enzymes and the pathogen can grow  through adjacent cells (1).  In the asexual cycle, the mycelium produces sporangia, which germinate to produce vesicles containing zoospores.  These zoospores encyst and germinate to produce a germ tube capable of host penetration or production of another sporangia.  Sexual reproduction occurs via mating of an oogonia and antheridia.  A fertilization tube is produced from the antheridium and enters the oogonium, allowing the nuclei of the two cells to unite.  The resulting zygote remains within the oogonium, which then produces a thick wall and is called an oospore.  Oospores germinate to produce germ tubes capable of perpetuating the asexual cycle (4). 
    Pythium spp. are capable saprophytes, ubiquitous in soils, and most commonly found to infest cultivated soils in the upper regions where host roots are located (1).  Most soils contain several species of Pythium, and the pathogen survives best when soil moisture is high (1).   Zoospores are released during wet conditions, transported in free water and able to swim to new infection sites; therefore, wet conditions are required for disease to occur (1, 4).  Oospores act as survival structures and are capable of overwintering (4).

Links to Other Sites
USDA ARS Systemic Botany and Mycology
Some P. myriotylum gene sequences
Selected References: 
  1. Agrios, G.N. 2005.  Plant Pathology, 5th edition.  Elsevier Academic Press, Burlington, MA. Pp 410-413.

  2. Drechsler, C. 1930.  Some new species of Pythium.  Journal of the Washington Academy of Sciences. 20(16):398-418.

  3. Farr, D.F., Rossman, A.Y., Palm, M.E., & McCray, E.B. (n.d.) Fungal Databases, Systematic Botany & Mycology Laboratory, ARS, USDA. Retrieved April 25, 2007, from

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

  5. Wang, P.H. and Chang, C.W.  2003.  Detection of the low-germination-rate resting oospores of Pythium mytriotylum from soil by PCR.  Letters in Applied Microbiology.  36(3):157. 

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