NC STATE UNIVERSITY
College of Agriculture and Life Sciences
Department of Plant Pathology		  

PP728 Pathogen Profile

Pythium ultimum
Lei Cheng
PP728 Soilborne Plant Pathogens
Class project
Spring, 2007



Introduction

Pythium ultimum is a ubiquitous soilborne pathogen which causes damping-off and root rot on plants. Each year, P.  ultimum leads to tremendous economic loss. Originally, the genus Pythium was placed in the Family Saprolegniaceae by Pringsheim in 1858 (Hendrix and Campbell, 1973). Currently, Pythium species are placed in the Family of Pythiaceae, Class of Oomycota. The first Pythium sp. reported in the United States was P. anandrum, which emerged in 1930s. The isolation of P. ultimum was first reported in 1931 by Wager in the Union of South Africa (Wager, 1931). But the occurrence of P. ultimum was not recorded as a problem in the U.S. until 1940s when some researchers found that P. ultimum can infect rhubarb grown in California (Hendrix and Campbell, 1973).


Host range and distribution

P.  ultimum is widely distributed throughout the world and has a wide range of hosts including many important crops. P. ultimum has been found in Australia, Brazil, Canada, China, Japan, Korea, South Africa and many other countries in the world. In the United States, the occurrence of P. ultimum has been reported in most states. Studies have shown that P. ultimum is common in most soils of agronomic crops and forests (Hendrix and Campbell, 1973). For example, P. ultimum is a casual agent of the Pythium blight of turfgrass, which causes serious damage to golf courses (Allen et al. 2004). P. ultimum can also infect other crops such as cabbage, carrot, cucumber, melon, turfgrass, wheat and so on. In general, abundant soil moisture and high soil temperature are the two most important environmental factors that regulate the distribution of P. ultimum. However, Tojo et al., 2001 recently reported that a warm greenhouse could be attributed to the introduction of P. ultimum into the Arctic regions, where the occurrence of P. ultimum has been rarely seen due to very low temperature.

Isolation

Direct isolation of P.  ultimum from soil or plant tissues on common agar media is usually difficult since these media often favor the more competitive saprophytes. Thus, selective media are used to recover P.  ultimum from soil or plant materials. For those developed for determining Pythium spp. in soil, Mircetich and Kraft, 1973 found that selective media MPVM (pimaricin-vancomycin medium) is the best one for most Pythium spp. Mircetich and Kraft, 1973 also concluded that the surface-soil-dilution-plate method (SSDP) is the most suitable procedure for the isolation of Pythium spp. from soil samples. Briefly, soil samples from infested areas will be sieved through a mesh screen with pore size of 0.84 mm, and then a series of 1:50, 1:100 and 1:200 dilutions will be made in 0.3% sterile water agar.  Selective media will first be added to the petri dish allowing the media to solidify. After that, 1 ml of soil suspensions will be added to the surface of selective media.


Identification

Traditionally, the identification of P. ultimum depends on the characteristics of the fungus such as morphology of oogonia, antheridia, (Figure 1) and sporangia (Figure 2). For instance, P. ultimum produces spherical sporangia that germinate directly, which is distinct from lobed sporangia formed by P. aphanidermatum that often produce zoospores (Allen e tal. 2004). Over the last decades, a number of new techniques were developed for detecting Pythium species. For example, restriction fragment length polymorphism (RFLP) analysis has been employed to identify Pythium at species level. The use of enzyme-linked immunosorbent assays (ELISA) for detection of Pythium spp. has also been reported (Yuen et al. 1998). Kageyama et al. (1997) have used a polymerase chain reaction (PCR) technique with species-specific primers to successfully identify P. ultimum from damping-off seedlings of Chinese cabbage, cucumber, and sugar beet.


Figure 1. Oogonium and antheridium of P. ultimum (http://www.apsnet.org/education/IllustratedGlossary/default.htm)



Figure 2. Spherical sporangia of P. ultimum. (Allen et al. 2004, http://www.apsnet.org)

Symptoms

Symptoms of diseased plants are often stunted (Figure 3) or chlorotic, which are very similar to nitrogen deficiency. Moderate infections can cause plants wilt, reduce plant populations, and retard maturation. Severe infections often lead to plants collapse and dead. For example, initial symptoms in turfgrass infected by P. ultimum, are dark green, then severe damage results in spherical or irregularly shaped patches in turfgrass swards (Figure 4) (Allen et al. 2004). Symptoms are much more evident under suitable environmental conditions such as warm weather and high soil moisture. Root tips of diseased plants appear as brown. Round, thick-walled spores could be found in root cells.


Figure 3. Stunted sweet corn caused by P. ultimum (Mathre et al. 1999)

Figure 4. Aggregated patches caused by P. ultimum in turfgrass swards (Allen et al. 2004, http://www.apsnet.org)

Ecology and life cycle

P. ultimum can grow saprophytically and survive as resistant resting structures in the soil and in root residues. When conditions are favorable, the fungi begin to infect the seeds and/or root tips of plants. Vegetative hyphae can directly penetrate plant cells. Mycelial growth and the movement of zoospores can facilitate the spread of P. ultimum to other susceptible plants. P. ultimum can reproduce both sexually and asexually. For asexual reproduction, sack-like sporangia will be formed (Figure 2). Sporangia can directly germinate as hyphae. For sexual reproduction, an oogonium and a club-shaped antheridium (Figure 1) will be produced. When they contact with each other, the nuclei of this two structures will form a zygote, then a thick-wall oospore will be formed. Both sporangia and zoospores are short-lived in soils, while oospores can be survived in the soil for longer periods. For example, sporangia of P. ultimum were found to remain viable for 11 months in the soil (Hendrix and Campbell, 1973), while oospores can survive in the soil for nearly 12 years (Allen et al. 2004).

Links to other sites

Pythium aphanidermatum  (North Carolina State University, Dept. of Plant Pathology)

Pythium blight of turfgrass (Apsnet.org)

Selected References

Allen TW, Martinez A, and Burpee LL. 2004. Pythium blight of turfgrass. http://www.apsnet.org/education/LessonsPlantPath/pythiumblight/default.htm

Hendrix FF and Campbell WA. 1973. Pythiums as plant pathogens. Annu. Rev. Phytopathol. 11, 77-98.

Kageyama K, Ohyama A, and Hyakumachi M. 1997. Detection of Pythium ultimum using polymerase chain reaction with species-specific primers. Plant Dis. 81,1155-1160.

Mathre DE, Cook RJ, and Callan NW. 1999. From discovery to use: Traversing the world of commercializing biocontrol agents for plant disease control. Plant Dis. 83, 972-983.

Mircetich SM and Kraft JM. 1973. Efficiency of various selective media in determining Pythium population in soil. Mycopathologia 50, 151-161.

Tojo M, Hoshino T, Herrero ML, Klemsdal SS, and Tronsmo AM. 2001. Occurrence of Pythium ultimum var. ultimum in a greenhouse on Spitsbergen Island, Svalbard. Euro. J. Plant Path. 107, 761-765.

Wager VA. 1931. Diseases of plants in South Africa due to members of the Pythiaceae. Dept. Agr. So. Afr. Sci. Bull. 105, 1-43.

Yuen GY, Xia JQ, and Sutula CL. 1998. A sensitive ELISA for Pythium ultimum using polyclonal and species-specific  monoclonal antibodies. Plant Dis 82,1029-1032.