Current weather conditions are conducive for seedling diseases to develop. Several parts of the state have had over 5 inches of rain in the last seven days. Also soil temperatures for the eastern part of the state are below average (50 to 61° F as of May 26 2013 at the 4’’ soil depth). Wet and cool soils favor most pathogens that cause damping off in corn and soybean. Also, 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 (Fig. 1 ).
Fig. 1. Corn field with a wet spot in Kingsbury County on May 23 2013. Corn seedlings in water-logged areas are more prone to seedling infection.
Damping off can be both pre-and post-emergence. Pre-emergence damping off is when seedlings get infected before they emerge from soil. Fungal pathogens infect the seed as it germinates or shortly after germination, causing death of the seedling before it can emerge from the soil. Post-emergence damping off occurs when seedlings have already emerged out of soil and get infected by the fungi.
The most common seedling diseases on corn are caused by Pythium and Fusarium species. These fungal pathogens infect the mesocotyl causing it to rot (Fig.2.) The mesocotyl is an important channel for water and nutrients to move from the endosperm to the developing seedling before the main roots take over.
Fig.2. Brown, collapsed mesocotyl caused by seedling pathogens on corn. The plant on the right has healthy (white color) mesocotyl.
Photo by: Alison Robertson, Iowa State University.
Infected corn seedling appear stunted, yellowing or purplish, and are scattered around the field. These symptoms, however, may be similar to those caused by insect feeding, herbicide injury, and/or environmental stress. Inspect the mesocotyl for soft, rotted, brown-reddish tissues, which indicate fungal infection.
Major soybean seedling diseases are caused by Phytophthora, Pythium, Fusarium, and Rhizoctonia fungal pathogens. Infected soybean seedlings are soft and rotted and have soil adhering to the tap root (Fig. 3). Phytophthora root rot (PRR) is the most prevalent and most damaging of the root rot diseases in South Dakota. Up to 50 races of PRR have been reported in North America.
Fig. 3. Stand reduction caused by damping off.
Photo by: X. B. Yang, Iowa State University.
Scouting for seedling diseases at this time may help the grower to assess prevalence of diseases present in the field and also assess the effectiveness of seed treatment (if it was done). Keeping records of field diseases will aid making decisions concerning cultivar selection, effectiveness of seed treatment, and time of planting. Scouting and keeping records of disease incidence could save growers unwarranted fungicide seed treatments in future in cases of low 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 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 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 Extension article on various products available for seed treatment. If replanting is to be done, fungicide treatment is recommended.
- Use tillage methods that promote good drainage. Saturated/poorly drained soils increase the risk of seedling infection. Avoid deep planting and working the soil when it is too wet.
- 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, fungicide seed treatment may be considered.
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