"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"