Pythium irregulare

Pythium irregulare
Scientific classification
(unranked):	SAR
Superphylum:	Heterokonta
Class:	Oomycetes
Order:	Pythiales
Family:	Pythiaceae
Genus:	Pythium
Species:	P. irregulare
Binomial name
Pythium irregulare Buisman, (1927)

Pythium irregulare is a soil borne oomycete plant pathogen. Oomycetes, also known as "water molds", are fungal-like protists. They are fungal-like because of their similar life cycles, but differ in that the resting stage is diploid, they have coenocytic hyphae, a larger genome, cellulose in their cell walls instead of chitin, and contain zoospores (asexual motile spores) and oospores (sexual resting spores).

Pythium irregulare is an oomycete that causes pre- and post-emergence damping off, as well as root rot. Pre-emergence damping off occurs when P. irregulare infects seeds before they emerge, causing them to rot and turn brown, thus preventing successful growth. Alternatively, post-emergence damping off occurs when the oomycete infects just after the seed has germinated. This usually causes infection in the roots and stem which appears as water soaking and necrosis. Depending on the severity, plants may collapse or be severely stunted. In plants that are older and more established, P. irregulare causes root rot. This will initially cause necrotic lesions, which leads to chlorosis, reduced yield, poor growth, and stunting due to inadequate water and nutrient acquisition by the roots. Additionally, P. irregulare is often found coinfecting with other Pythium species. All three of these diseases caused by P. irregulare can be caused by other pathogens as well, so a disease diagnosis is not necessarily indicative of P. irregulare

In order to identify Pythium irregulare it is necessary to isolate the organism and observe it microscopically. First, it is important to identify that the microbe is an oomycete by looking for characteristics that are specific to oomycetes, such as coenocytic hyphae, zoospores, and oospores. After that, one can identify the microbe as being in the genera Pythium by observing disease symptoms, host range, as well as the presence of a vesicle, where zoospores form, which is attached to the sporangia. In contrast, most other oomycetes do not have a vesicle and the zoospores form in the sporangia. Finally, once the genera has been identified, it is helpful to use a dichotomous key to identify the species, such as the one at http://plantpath.psu.edu/pythium/module-3/plaats-niterink-key-pythium. Some of the key identifiers for P. irregulare include oogonia with irregular shaped, cylindrical projections, sporangia that occur singly, sporangia that are not filamentous, and oogonia smaller than 30 μm. There are also many genomic tests that can be done to determine species based on specific DNA markers. It is also important to note that many diagnosticians do not identify to the species level because it can be difficult to find all necessary microscopic structures and many management techniques can be applied to all Pythium species.

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Wikipedia