Pythium

I am getting more and more calls and emails asking "what is wrong with my corn? or "how bad is the pythium in your area", Ed? Some are asking what is working and what is not. It is good we review what it is, first.

Many seedling blights can infect germinating plants at this time of the year. One of the most common and earliest groups of fungi that attack corn and soybeans belongs to the genera Pythium. Fungi in the genera Pythium are called "water molds" because they thrive in soils that are wet. In addition, these fungi are earliest and very common because the various species are active over a wide range of temperatures and moisture regimes. In fact, Pythium spp. are often grouped by the temperature regimes that induce optimum infection. Cooler soils (50 to 60°F) favor three species (P. debaryanum, P. torulosum, and P. ultimum) that are more common in northern areas, particularly in early-planted fields. Several other species, including P. aphanidermatum, have higher optimum temperatures (86 to 97°F) for infection, but they can also be present in the field at temperatures as low as 60°F. These species of Pythium cause problems in more southern areas and in late-planted crops.

Pythium fungi overwinter in the soil and in plant debris as oospores. Moisture is necessary for oospore germination and provides a medium for movement "swimming" of the germinated motile spores, called zoospores, which infect the plant root system. Three to four hours of wet conditions can be sufficient for initiating zoospore production. Exudates from seeds and roots also induce fungal spore germination, hyphal growth, and penetration. Damaged seed encourages increased fungal attack because damaged seed leaches root exudates into the soil, attracting fungi, and the wounds provide entry for pathogen penetration.

Although Pythium may cause minimal damage to germinating corn, this fungus can infect a substantial portion of the developing root system including the mesocotyl. Infection of the mesocotyl can result in loss of the primary root system, causing the developing seedling to die, unless adequate secondary roots have developed. Corn plants during the first few weeks after emergence may grow more slowly and appear less healthy when only their primary roots are infected with Pythium. Root tips or the entire root system of the corn plant can become infected with Pythium, appearing brown and becoming soft-rotted and water-soaked. Often the outer tissue of the root is infected and may peel off, revealing a white stele. On severely infected plants, symptoms may include root system discoloration along with yellowing and stunting of the aboveground plant.

I am no plant pathologist but I do understand this much. We treat seed to prevent these disease outbreaks. Usually they work but sometimes they fail. Like any other organism, pythium develop resistance to our best plans. All species seem to be resistant to every seed treatment we have in this severe a situation. Everything failed to some extent but the best stands have some bean guard on them with SabrEx. Captan in the bean guard helps and if SabrEx can colonate(50 degrees or so), we get season long control of pythium, fusariums, rhizoctonias and other soil borne diseases. Oxygenation of soils is critical so drainage and soil fertility balance becomes important.

We will probably be talking about this problem throughout the growing season.

Ed