This week, we have had a lot of moisture and with more rain in the forecast soils are going to be saturated or near saturation. Rain over the weekend left some fields flooded (Figure 1). Both corn and soybeans are at a growth stage that is vulnerable to root rot development, especially with increased soil moisture. Diseases favored by high moisture conditions, such as the water molds, are also likely to develop. Water molds produce spores that swim in water and infect plants.
Figure 1. (Above) Flooded soybean field in Brookings County on June 2.
Photo by: Emmanuel Byamukama
High soil moisture in corn may lead to root rots caused by the Pythium species. However, this pathogen infects corn under cooler soil conditions, so there may be a low risk for this disease to develop. Producers should check their fields for root rots. Infected corn seedlings appear stunted, yellowing or purplish, and are scattered around the field. Inspect the mesocotyl for soft, rotted, brown-reddish tissues, which indicate fungal infection (Figure 2).
Figure 2. (Above) Brown, collapsed mesocotyl caused by seedling pathogens on corn. The plant on the right has healthy (white) mesocotyl.
Photo by: Alison Robertson, Iowa State University
Corn standing in water for more than 24 hours has increased chances of Physoderma brown spot and later on Physoderma stalk rot and crazy top disease. The pathogens that cause these diseases survive on corn residue. Spores of these pathogens swim in water and infect the plant at the leaf whorl. Physoderma infected leaves usually have limited impact on yield but stalk infection may lead to stalk lodging later in the season (Figure 3). Crazy top infected plants have a classic symptom of proliferation of the tassel (Figure 4) and sometimes excessive tillering. These symptoms are seen around tasseling but infection happens when corn is still young.
Figure 3. (Above) Corn plants lodged due to Physoderma stalk rot last year. The lodged plants were in a wet spot area.
Figure 4. Crazy top symptom. Notice the proliferation of the tassel. The distortion of the tassel is a result of the pathogen interfering with plant growth hormones.
Photo by: Emmanuel Byamukama
Under the current weather conditions seed and root rot may be more pronounced in soybean than in corn. With most of the soybeans just emerging, damping off development is likely. 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 the 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.
Excessive moisture in young soybeans can result in root rot. The major soybean seedling pathogens favored by high soil moisture are Phytophthora and Pythium, spp. Pythium will infect the soybean’s young roots when soil temperatures are low. The current soil temperatures may not favor Pythium infection but they do favor Phytophthora infection. Phytophthora root rot (PRR) is the most prevalent and most damaging of the root rot diseases in South Dakota. One of the effective ways to control PRR is host resistance. Seed companies do provide ratings for PRR resistance but this pathogen has many races. Soybean cultivars with stacked resistance genes have durable resistance.
Flood water is also going to move pathogens from one point to another. Run-off water carries plant debris and soil and spreads pathogens like the soybean cyst nematode (SCN). Both eggs and cysts can be moved by run-off water and saturated soils provide a moisture film for the juveniles to swim and infect roots.
Scouting for seedling diseases may help producers assess the prevalence of diseases present in the field and also assess the effectiveness of any seed treatment. Keeping records of field diseases will aid in 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 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 tissue at and below the soil line for browning or rotting.
Managing high soil moisture associated diseases
There is very little that one can do to control water mold diseases once infection has taken place. However, when they are noticed, the following measures should be considered to prevent or reduce their chance of developing in the next growing season.
- Select high yielding, resistant or tolerant cultivars. For example, soybean cultivars have good 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. 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 soybeans will be next crop, fungicide seed treatment may be considered.