A Diagnostic Guide to Phytopythium helicoides and Phytopythium vexans causing Root and Crown Rot Diseases

Phytopythium was initially grouped with closely related species of the Pythium group, which German botanist Nathanael Pringsheium first described in 1858. Later with the progress in studies of this group, the clade K (Ovatisporagnium) of the genus Pythium was concluded to be more closely related to the genus Phytophthora than Pythium. Ovatisporangium in the Pythium group was then replaced by the term Phytopythium. Hence, the genus Phytopythium is relatively new and is different from Pythium (Bala et al. 2010; Lodhi et al. 2020; Tkaczyk 2020). It exhibits intermediate morphological features between Phytophthora and Pythium but is unique in having internally proliferating papillate sporangia and cylindrical or lobate antheridia (De Cock et al. 2015). Phytopythium helicoides (Abad, De Cock, Bala, Robideau, Lodhi & Levesque) and Phytopythium vexans (Abad, De Cock, Bala, Robideau, Lodhi & Levesque) are two closely related emerging oomycete plant pathogens in the Phytopythium group. Several other names have been provided to P. helicoides in the past such as Pythium helicoides (Drechsler 1930) and Ovatisporangium helicoides (Azizi et al. 2013) whereas P. vexans (Ivors et al. 2008) was called Pythium vexans and Ovatisporangium vexans (Uzuhashi et al. 2010).