Pythium seedling disease

​Corn seedlings are frequently attacked by a fungal-like organism of the Pythium genus when the soil is extremely cool and wet. Pythium belongs in a group of organisms called Oomycetes.  These were once considered fungi, but more recent research has shown that they are more related to algae.  Other pathogenic oomycetes include those causing diseases such as downy mildew of several grasses (including crazy top of corn) and Phytopthora root rot of many crops.  Oomycetes were considered fungi because they usually produce filaments and spores plus they absorb nutrients from plants and animals.  They differ from fungi by mostly lacking individual cells within the filaments, resulting in many nuclei within the long filaments.  Most fungal cell walls are composed of chitin whereas the oomycete filament walls are cellulose, a difference that relates to the host resistance system.  Often the first signal of fungal invasion is of the chitin, triggering the production of defense systems.  Detection of an oomycete invasion requires a difference in plant chemistry.
 
Oomycetes frequently produce swimming spores that are released from overwintering spores (oospores) in the soil.  Pythium spores, with flagella, swim to host root cells on which they grow hyphae to invade the host.  Consequently, they cause the biggest problems in fields that are temporarily flooded with heavy rains and cool conditions.  Studies have shown that 55°F is optimum for most Pythium species infecting corn in Midwest USA.  The low temps probably slow down the host plant’s growth rate and inhibit potential competition from soil fungi.
 
As many as 18 species of Pythium have been shown to be pathogenic on corn.  Often, they destroy the outer layer of the new root tissue.  If this occurs before secondary roots become the main source of water uptake, the seedling will wilt. Although seed treatments can offer some protection, genetic diversity within a species often includes resistance to most treatment chemicals.  Pythium genetic diversity also includes ability to attack rotation crops such as soybeans.  Pythium species are easily isolated from soil but completely accounting for all the species or diverse genetics in regard to effective seed treatments or genetic resistance is not easy.
 
Control of the disease is best done with controlling water holding capacity of the soil.