PP728 Pathogen Profile
PP728 Soilborne Plant Pathogens
Pythium ultimum is a ubiquitous soilborne pathogen which causes
and root rot on plants. Each year, P. ultimum leads to tremendous
loss. Originally, the genus Pythium was placed in the Family
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
The isolation of P. ultimum was first reported in 1931 by Wager in the
of South Africa (Wager, 1931). But the occurrence of P. ultimum was not
as a problem in the U.S. until 1940s when some researchers found that
ultimum can infect rhubarb grown in California (Hendrix and Campbell,
Host range and distribution
P. ultimum is widely distributed throughout the world and has a
range of hosts including many important crops. P. ultimum has been
in Australia, Brazil, Canada, China, Japan, Korea, South Africa and
other countries in the world. In the United States, the occurrence of
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
1973). For example, P. ultimum is a casual agent of the Pythium blight
turfgrass, which causes serious damage to golf courses (Allen et al.
P. ultimum can also infect other crops such as cabbage, carrot,
melon, turfgrass, wheat and so on. In general, abundant soil moisture
high soil temperature are the two most important environmental factors
regulate the distribution of P. ultimum. However, Tojo et al., 2001
reported that a warm greenhouse could be attributed to the introduction
P. ultimum into the Arctic regions, where the occurrence of P. ultimum has
been rarely seen due to very low temperature.
Direct isolation of P. ultimum from soil or plant tissues on
agar media is usually difficult since these media often favor the more
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
medium) is the best one for most Pythium spp. Mircetich and Kraft, 1973
concluded that the surface-soil-dilution-plate method (SSDP) is the
suitable procedure for the isolation of Pythium spp. from soil samples.
soil samples from infested areas will be sieved through a mesh screen
pore size of 0.84 mm, and then a series of 1:50, 1:100 and 1:200
will be made in 0.3% sterile water agar. Selective media will
be added to the petri dish allowing the media to solidify. After that,
ml of soil suspensions will be added to the surface of selective media.
Traditionally, the identification of P. ultimum depends on the
of the fungus such as morphology of oogonia, antheridia, (Figure 1) and
(Figure 2). For instance, P. ultimum produces spherical sporangia that
directly, which is distinct from lobed sporangia formed by P.
aphanidermatum that often produce zoospores (Allen e tal. 2004). Over the last
a number of new techniques were developed for detecting Pythium species.
For example, restriction fragment length polymorphism (RFLP) analysis
been employed to identify Pythium at species level. The use of
immunosorbent assays (ELISA) for detection of Pythium spp. has also
reported (Yuen et al. 1998). Kageyama et al. (1997) have used a
chain reaction (PCR) technique with species-specific primers to
identify P. ultimum from damping-off seedlings of Chinese cabbage,
and sugar beet.
Figure 1. Oogonium and
antheridium of P. ultimum
2. Spherical sporangia of P. ultimum.
et al. 2004, http://www.apsnet.org)
Symptoms of diseased plants are often stunted (Figure 3) or chlorotic,
are very similar to nitrogen deficiency. Moderate infections can cause
wilt, reduce plant populations, and retard maturation. Severe
often lead to plants collapse and dead. For example, initial symptoms
turfgrass infected by P. ultimum, are dark green, then severe damage
in spherical or irregularly shaped patches in turfgrass swards (Figure
(Allen et al. 2004). Symptoms are much more evident under suitable
conditions such as warm weather and high soil moisture. Root tips of
plants appear as brown. Round, thick-walled spores could be found in
Figure 3. Stunted
caused by P. ultimum (Mathre et al. 1999)
Figure 4. Aggregated
caused by P. ultimum in turfgrass swards (Allen
et al. 2004,
Ecology and life cycle
P. ultimum can grow saprophytically and survive as resistant resting
in the soil and in root residues. When conditions are favorable, the
begin to infect the seeds and/or root tips of plants. Vegetative hyphae
directly penetrate plant cells. Mycelial growth and the movement of
can facilitate the spread of P. ultimum to other susceptible plants. P.
ultimum can reproduce both sexually and asexually. For asexual reproduction,
sporangia will be formed (Figure 2). Sporangia can directly germinate
hyphae. For sexual reproduction, an oogonium and a club-shaped
(Figure 1) will be produced. When they contact with each other, the
of this two structures will form a zygote, then a thick-wall oospore
be formed. Both sporangia and zoospores are short-lived in soils, while
can be survived in the soil for longer periods. For example, sporangia
P. ultimum were found to remain viable for 11 months in the soil
and Campbell, 1973), while oospores can survive in the soil for nearly
years (Allen et al. 2004).
Links to other sites
aphanidermatum (North Carolina State University, Dept. of
blight of turfgrass (Apsnet.org)
Allen TW, Martinez A, and Burpee LL. 2004. Pythium blight of turfgrass.
Hendrix FF and Campbell WA. 1973. Pythiums as plant pathogens. Annu.
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
Mathre DE, Cook RJ, and Callan NW. 1999. From discovery to use:
the world of commercializing biocontrol agents for plant disease
Plant Dis. 83, 972-983.
Mircetich SM and Kraft JM. 1973. Efficiency of various selective media
determining Pythium population in soil. Mycopathologia 50, 151-161.
Tojo M, Hoshino T, Herrero ML, Klemsdal SS, and Tronsmo AM. 2001.
of Pythium ultimum var. ultimum in a greenhouse on Spitsbergen Island,
Euro. J. Plant Path. 107, 761-765.
Wager VA. 1931. Diseases of plants in South Africa due to members of
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.