Seedling diseases are one reason to use a fungicide treatment on soybean seed. Each planting season, different weather patterns result in different seedling disease problems. This planting season has been smooth in general and seedlings have emerged in many soybean fields. So far, disease risk is lighter than last year.
However, some light occurrence of seedling diseases in central Iowa have been observed this past week, according to Iowa State University Extension plant pathologists X.B. Yang and S.S. Navi. "Damping-off was also found in our research plots," says Yang. "It is now time to check your soybean fields to determine if there are any seedling disease problems."
With weather conditions better than last planting season, "we should see less disease problems, especially in fields treated with fungicides," says Yang. "If you find significant seedling diseases in a field planted with treated soybean seeds, you should reconsider the seed treatment you used. Knowing what disease causes the problem is critical to correcting the problem when you plant beans next time in that field."
Keep an eye out for three key seedling diseases of soybeans
This year, soybean seedling diseases caused by three fungi are likely to be found, Pythium, Rhizoctonia and Phytophthora.
"For fields that were planted early in cool soil, Pythium damping-off is the most likely to be found, as we did last week," says Yang. "In most of years, this is the first disease found in a growing season because the fungus prefers cold soil temperatures. Dead seedlings may be visible on the ground with infected plants killed before the first true leaf stage. Plants often have a rotted appearance. Leaves of infected seedlings are initially gray-green and then turn brown. A few days later, the plants die. Diseased plants are easily pulled from the soil because of rotted roots."
The symptoms of Phytophthora is similar to Pythium and can be mistaken for Pythium damping-off, he points out. Phytophthora is more likely to infect soybean plants in later-planted soybeans because the fungus prefers warm soil temperatures and high soil moisture conditions.
Rhizoctonia may be found in later-planted soybeans this year
Another disease that may be found in later-planted soybean this year is Rhizoctonia damping-off, says Yang. Caused by Rhizoctonia fungus, this disease likes soil temperatures warmer than that for Pythium. Soybean seedling disease caused by Rhizoctonia exhibit symptoms different from those caused by Phytophthora.
Unlike Phytophthora damping-off, stem discoloration by Rhizoctonia is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry. Typical symptoms are localized brown-to-reddish brown lesions on the hypocotyl. Root rot is visible on severely infected plants.
Seedling disease risk differs from field to field. River bottom fields are more likely to have Pythium and Rhizoctonia, sandy soil is more likely to have Rhizoctonia, and Phytophthora is more likely to occur in heavy soils. Within a field, some spots are more likely to have seedling disease than other areas. "To quickly spot the disease problems, you can check areas or fields that are most likely to have disease problems," advises Yang. "Seedling diseases usually occur first in low spots with higher soil moisture, in areas with poor drainage or in compacted areas."
Damping off of soybean seedling caused by Pythium (Photo credit: SS Navi)