By Connie Strunk and Emmanuel Byamukama
Last week most of the state received much needed moisture. Unfortunately, our current weather conditions (cool and wet) are conducive for seedling disease development.
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 the wet spots and low laying areas of the field.
Diseases to Watch For
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 can emerge 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. Healthy soybean seedling is on extreme left. Damping-off can occur pre-emergence (extreme right) and post-emergence (center).
The most common seedling diseases on corn are caused by Pythium and Fusarium species. These fungal pathogens infect the mesocotyl causing it to rot. Infected corn seedlings appear stunted, may appear yellow or purple in color, and are scattered around the field. These symptoms may be similar to those caused by insect feeding, herbicide injury, and/or environmental stress. To determine if a fungal infection has occurred, inspect the mesocotyl for soft, rotted, brown-reddish tissues, which indicate a fungal infection.
With soybean planting just starting around the state and with the cool, wet soils we expect to see soybeans also attacked by Pythium. The hypocotyl on seedlings infected by Pythium is typically narrow and is often pinched off.
Other pathogens which may attack soybeans are Phytophthora and Rhizoctonia (which are often a problem in warmer soils, 70-80° F). Phytophtora infected soybean plants typically have a brown discoloration extending from the soil line up the stem. Rhizoctonia is often seen in late planted soybean fields. With Rhizoctonia infection you will often see a reddish-brown colored lesion on the hypocotyl and lower stem of the plant which is not observed above the soil line.
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. Record keeping in fields with diseases will help with the decision making process when it concerns cultivar selection, effectiveness of seed treatment, and time of planting. 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 of the tissue.
Managing Seedling Diseases
Seedling diseases reduce plant stand and affect seedling vigor leading to reduced yield. Management of seedling diseases should be based on the field history and severity of disease(s).
- Select high yielding, resistant or tolerant cultivars. For example, soybean cultivars have disease ratings for Phytophthora root rot (PRR). Keep records of cultivars grown in order to track PRR races that may be present in the field.
- Fungicide seed treatments are effective against common seed- and soil-borne pathogens. See the SDSU Pest Management Guides for seed treatment information. If replanting must be done, utilize a fungicide seed treatment.
- Use tillage methods that promote good drainage. Saturated/poorly drained soils increase the risk of seedling infection.
- Practice crop rotation. Crop rotation helps break disease cycles and accumulation of inoculum. Pythium spp., however, can infect both soybean and corn. If significant stand reduction is noted this season, and soybean is the next crop, a fungicide seed treatment may be considered.
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