Corn and Soybean Seedling Diseases

• Seedling diseases can be caused by several common soilborne pathogens.
• Multiple factors contribute to pathogen infection in seedlings, such as soil moisture and temperature.

Diseases and their causal pathogens can sometimes be difficult to identify. Proper identification is often the first step in determining the best future management practice. University and third-party labs can often isolate and identify specific diseases, which may help in evolving future management. The following information can help identify specific diseases based on common symptoms.

Fusarium

• Multiple species are responsible. Most common are graminearum, oxysporum, culmorum and moniliforme.
• Fusarium virguliforme is the causal fungi for sudden death syndrome in soybeans.
• Mostly soilborne but can sometimes be seedborne.
• Most common in dry soils but can survive under moist conditions.
• Causes decay of the growing point stem tissues.
• Identified by tan to reddish brown lesion on the root or mesocotyl, the first internode of the stem (Figure 1).
• Corn mesocotyl often has shriveled appearance under more severe infection.
• Attacks root surface area by reducing root hairs and small fibrous roots.
• Reduces ability of roots to take up nutrients and moisture.
• Fusarium species infection can often develop into late season crown and root rots.

Rhizoctonia

• Can cause both preemergence and postemergence damping-off.
• Prefers warmer, drier soil.
• Distinctive reddish-brown sunken cankers form on roots, crown and brace roots (Figure 2).
• Causes brown lesions on corn mesocotyl and roots that can eventually girdle and rot off roots.
• Above-ground symptoms, other than stunting, are not often noticed.
• Typically infects seedlings when they are under stress conditions, such as drought, higher temperatures, herbicide damage, compaction and insect feeding.
• Has many hosts, including weeds.
• May also cause crown rot in older plants.

Phytophthora — Phytophthora sojae (Soybeans Only)

• Water-mold fungus, Phytophthora sojae, survives as (fertilized) oospores.
• Moves in soil water as (asexual) zoospores.

• Spores germinate and infect roots in warm, wet soils. Infection can occur at any stage of plant development but is most common in seedlings and young plants.
• Infected plants appear soft and bruised and have rotted secondary roots. Leaves are yellow and later turn brown, followed by wilting and death (Figure 3).
• Plants that die or appear stunted later in the season show dark stem discoloration. Dying leaves stay attached to the plant.
• There are more than 40 races of Phytophthora identified in soils.
• There are 15 RPS (resistance to Phytophthora sojae) genes identified, of which 8 are commonly used to develop resistant soybean cultivars.
• Due to RPS genes specificity to designated races, infection can still be observed depending on the race present and RPS genes used, although the same RPS genes can protect against other races.

Pythium

• Water-mold fungus/brown algae that survives as oospores.
• Moves in soil water as zoospores, requiring wet soils to produce infection.
• Most common in cool, wet soil conditions.
• Some species have adapted to develop under warmer, wet conditions.
• More than 14 species of Pythium have been identified to affect corn and soybeans.
• No known genetic resistance.
• Can infect young seedlings prior to and after emergence, often causing seed to rot or emerged plants to damp-off or die (Figure 4).
• Symptoms occur scattered across entire fields and may make identification difficult.

General Seedling Disease Management

• Utilize a robust fungicide-containing seed treatment that has multiple modes of action against diseases most problematic to the field (Figure 5).
• Create a field environment that reduces the likelihood of disease:
  • Bury residue with tillage
  • Rotate to non-host crops if possible
  • Improve soil drainage
  • Minimize field compaction
• Delay planting until soil temperatures are nearing 50°F and 48-hour forecast is favorable.
• Avoid planting into wet soil conditions.