Scouting tips for seedling disease
I’ve survived another Midwest winter! And a new crop season is upon us. Folks are out planting despite the cold soil temperatures, and according to the Iowa Mesonet, soils will not be warming over the next few days either. Cold soil temperatures (<55F) delay or slow germination. Moreover, germinating seeds leak nutrients that attract seedling pathogens such as Pythium and Fusarium species and consequently increase the risk of seedling disease.
Symptoms of seedling disease
Seedling disease may affect scattered plants in the field or may occur in patches. Seedling disease typically includes (i) seed rot, and (ii) root rot (Figure 1). Seed rot often occurs before emergence and may be referred to as pre-emergent damping-off. Seedlings with root rot emerge, but they may die after emergence (post-emergent damping off). If affected seedlings do survive, they are less vigorous, that is, they may be stunted, yellowed and one or more growth stages behind neighboring plants (Figure 2). Soybean plants will compensate for less vigorous neighbors and consequently yield may not be affected. Less vigorous corn plants however, usually become “weeds’ that either produce small ears or no ear at all, leading to reductions in yield.
Figure 1. Root rot is a characteristic symptom of seedling disease.
Figure 2. Stunted, less vigorous plants may be suffering from seedling disease.
Scouting for seedling disease
Assessing corn and soybean stand counts soon after emergence and again a couple of weeks after is the best way to scout for seedling disease. If there are skips in the row, dig up the seeds and check for seed rot. Post-emergent damped off plants may be visible on the soil surface. While assessing stand counts, take note of the growth stage of the plants you are counting. If a plant is behind in terms of growth stage, carefully dig up the plant and examine the roots. Infected roots will be discolored brown and rotted. Remember to examine the mesocotyl of corn seedlings for rot as well (Figure 3). It is difficult to differentiate between seedling disease caused by Pythium and Fusarium species in the field. Symptomatic seedlings may be sent to the Iowa State University Plant Disease and Insect Clinic for diagnosis. Remember to send seedlings with a range of symptoms as well as some healthy seedlings.
Figure 3. Corn seedling with rotted radicle, seminal and mesocotyl tissues characteristic of seedling disease.
Will a seed treatment protect seed?
Yes, seed treatments protect germinating seeds against infection; however, their efficacy may vary against different species. Visit the Crop Protection Network for more information of seed treatment efficacy for soybean. In addition, seed treatments are only effective for approximately 14 days. Thus, in cold soils, when germination and emergence are slowed, a seed treatment may “run out” and leave a seedling vulnerable to infection.
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- Professor of Plant Pathology and Microbiology
- Extension Field Crops Pathologist