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Pathogen Biology

Members of the genus Pythium belong to a group of fungal-like organisms broadly known as the "water molds." Pythium spp. form hyphae that are devoid of septa and contain cellulose in their cell walls. Pythium spp. are not considered to be "true fungi," and recently molecular evidence has indicated a relationship with the red algae in the Kingdom Stramenopila. Species of Pythium are placed within the phylum Oomycota and are commonly referred to as the oomycetes.

Two species of Pythium are most notably associated with Pythium blight: P. aphanidermatum and P. ultimum. Some researchers list as many as 15 species of Pythium as causative agents of this disease. The most severe disease symptoms are caused by P. aphanidermatum . The relative abundance of P. aphanidermatum and P. ultimum from samples submitted to plant disease diagnostic clinics is not readily known. For the most part, when submitted disease samples are diagnosed as Pythium blight, the species of Pythium is not normally reported.

Asexual reproduction. Pythium spp. produce sack-like structures called sporangia which vary in shape from filamentous to spherical. Pythium aphanidermatum forms lobed sporangia (Figure 17) while P. ultimum forms spherical sporangia (Figure 18). When mature, a sporangium can germinate and produce a hypha or give rise to a vesicle. Asexual, motile spores called zoospores are formed in the vesicle and released when the vesicle wall ruptures. Zoospores use flagella to "swim" in free water in soil or on plant surfaces (see disease cycle). Zoospores encyst on potential infection sites, and then the cyst germinates to form a hyphal germ tube that infects host tissues. In addition to sporangia and zoospores, mycelium serves as inoculum by growing through the foliar canopy of turfgrasses and penetrating leaves directly.

Figure 17

Figure 18

Sexual reproduction. In addition to asexual reproduction, most Pythium spp. reproduce sexually. Sexual reproductive structures include an oogonium and a club-shaped antheridium (see disease cycle). The two structures can form from a common strand of hyphae, or from two different hyphal strands. When the two structures come in contact, a fertilization tube enters the oogonium from the antheridium, and the antheridial nucleus is transferred to the egg within the oogonium (Figure 19). The antheridial and oogonial nuclei unite and form a zygote. The wall of the egg cell then thickens creating an oospore which contains the zygote. The oospore (Figures 20-21) can survive periods of drying and remain viable for up to 12 years.

Figure 19	Figure 20
Figure 21