No. 12/June 12, 1998
Seedling Blights Continue
With a return to cooler temperatures, combined with frequent rain storms, seedling blight fungi will continue to be a problem for both corn and soybeans. Pythium, a common seedling blight fungus, continues to reduce stands and cause poor early season growth in corn and soybeans.
The Pythium fungus is capable of rotting roots, the mesocotyl on corn, and the hypocotyl on soybeans. Normally this pathogen is not a problem past about June 1 because it is favored by cool soils. However, in years when cooler weather prevails past the first of June, we continue to see Pythium problems in both corn and soybeans. This year, Pythium is continuing to damage replanted corn as well as soybeans. Some growers are replanting corn for the third time and still having mesocotyl rotting from Pythium.
In corn, the Pythium fungus will colonize either the primary (or seminal) or the mesocotyl. Root rotting is characterized by areas of brown discoloration and a soft rotting. The tissues may appear to be water-soaked, and the outer tissue layers of the root may simply rot off, leaving only the central root core. Mesocotyl rotting is similar, with the fungus producing a brown soft rot along the mesocotyl. On very young corn plants, rotting of the mesocotyl canproduce a rootless plant that falls over easily because the plant lacks resources to produce permanent roots when the mesocotyl is destroyed. If the young plant has produced adequate permanent roots to support the plant independent of the seminal roots and the seed, then plants usually recover from Pythium infection. However, when infection occurs just as the crown is developing or the shoot is emerging, then the mesocotyl is needed for nutrient uptake by the young plant and infection at this time will commonly mean the loss of that plant. Even if plants survive, they often do not perform as expected and produce a very small ear to no ear at all.
Pythium is a difficult fungus to manage. There are no resistant soybean varieties or corn hybrids currently on the market. Unlike Phytophthora on soybeans, Pythium has a very broad host range, and several species are active in corn and bean fields. Fungicide seed treatments with Apron, Allegiance, or Apron XL are usually adequate to protect young corn plants from this fungus. But, in those years with long, cool, wet springs, the prolonged delays in emergence may decrease the effectiveness of these fungicides. There is no problem with the fungicides, but rather the problem is that these materials are designed to provide an early season "boost" to plants under average conditions. When there are very long periods of adverse weather, the fungicides may not remain effective for the 10 to 20 days needed for emergence.
H. Walker Kirby (firstname.lastname@example.org), Extension Plant Pathology, (217)333-8414