Pythium

​Corn is often planted in temperate zones as early as possible to take full advantage of the warmth of spring and summer.  The spring weather is not always predictable, and temperatures affect the growth rate and metabolism of the seedlings.  A battle begins between potential pathogens and corn plants.
 
The field environment of corn germination includes many organisms.  One group active in early spring are the Oomycetes.  These organisms were once classified as fungi but now their distinctiveness has most specialists agreeing that they are more closely related to brown algae.  Fungi have chitin cell walls whereas Oomycetes have cellulose walls.  Oomycetes have swimming spores, zoospores, whereas this is not a feature of most true fungi.  This is the feature that makes Oomycetes genera such as Pythium so significant to corn seedling survival.
 
Pythium species reproduce with swimming sperm cells fertilizing egg cells, while in infected live or dead plant tissue.  These then form a thick-walled oogonium that persists during stress, including winter temperatures.  When in water, and spring temperatures in the 50’s, sporangia growing from the oogonia release the swimming zoospores.  Attracted to sugars released by primary roots and the mesocotyl of corn seedlings.  In some cases, the oogonia produce filaments (hyphae) that infect the roots also.  Infection of these tissues can cause the seedlings to die, cutting off water to the emerging leaves.  If the seedlings survive this early infection of the primary root and mesocotyl, secondary roots emerging from the crown area bypass the infection and outgrow the damage.  Low temperature and oxygen deficiency because of water-soaked heavy soil contribute to the seedling vulnerability to damage.  Seeds with previous membrane damage resulting in slow early seedling growth are often the most vulnerable, perhaps because they are slower to produce the more resistant secondary roots.
 
There is evidence that the same Pythium species infecting corn also infect soybeans and several grasses as well. Pythium species do exist in a competitive environment with other microorganisms capable of inhibiting Pythium success.  Apparently low oxygen, cool environment of water-soaked heavy soils favor the Pythium species.  Seed treatments on corn (and soybeans) are often aimed at not allowing the seedling infection.  Races of Pythium are known to overcome some of the treatments.  It is unfortunate that genetic variability works for all organisms!