By Connie Strunk
SDSU Extension Plant Pathology Field Specialist
Seed and seedling diseases can decrease plant populations and result in replanting and production losses. Wet and cool soils favor most pathogens that cause damping off in corn and soybean. Slow growth, compacted soils, and heavy clay soils increase chances of seedling fungal infection. The first areas to inspect for seedling diseases are wet spots and low laying areas of the field.
Damping off can develop both pre-and post-emergence (Figure 1). Pre-emergence damping off occurs when seedlings get infected before they emerge from soil. Fungal pathogens can infect the seed as it germinates or shortly after germination takes place, killing the seedling before it emerges from the soil. Post-emergence damping off occurs when seedlings have already emerged from the soil and then get infected by the fungi.
Figure 1. Rhizoctonia can damage seeds at pre-emergence (extreme left) and seedlings post-emergence (center to right). Photo by Sara Berg.
Seedling Diseases on Soybean
The major soybean seedling diseases are caused by Phytophthora, Pythium, Fusarium, and Rhizoctonia fungal pathogens. So far this growing season, Rhizoctonia and Pythium are being observed on soybean. Pythium tends to attack plants under cool, wet soil conditions. Rhizoctonia is often a problem in warmer soils (70-80°F).
Rhizoctonia is frequently seen in late planted soybean fields. Rhizoctonia infection appears as a reddish-brown colored lesion on the hypocotyl and lower stem of the plant which is not observed above the soil line. Plants may not appear damaged or there may be a girdled stem where the plants either become stunted or die.
Pythium symptoms are characterized by soft, brownish-colored, rotting tissue. The hypocotyl on the seedlings is typically narrow and is often pinched off.
Other pathogens which also attack soybeans include Phytophthora (often a problem in warmer soils, 70-80° F) and Fusarium (prefers warm and dry soils or cool and wet soils). Phytophtora infected plants typically have a brown discoloration extending from the soil line up the stem of the soybean plant. Fusarium often causes light to dark brown lesions on the roots causing the root system to appear shrunken.
Seedling Diseases on Corn
The most common seedling diseases on corn are caused by Pythium and Fusarium species. These fungal pathogens infect the mesocotyl causing it to rot. We observed Pythium in South Dakota corn fields already this season.
Infected corn seedlings appear stunted, may be yellow or purple in color, and are scattered around the field. These symptoms are similar to those caused by insect feeding, herbicide injury, and/or environmental stress. To determine if a fungal infection is responsible, inspect the mesocotyl for soft, rotted, reddish-brown tissues.
Scouting for Seedling Diseases
Scouting for seedling diseases early may help the grower to assess prevalence of diseases present in the field and also assess the effectiveness of the seed treatment utilized. Record keeping of fields with diseases will help with the decision making process when it concerns cultivar selection, effectiveness of seed treatment, and time of planting. Scouting and record keeping could save growers unwarranted future fungicide seed treatments in cases of negligible or no disease pressure.
When scouting, look for areas with poor emergence and stunted plants. Dig up a few plants and examine the tissues at and below the soil line for browning or rotting tissue.
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