In Iowa, every planting season has different seedling diseases. In the last two planting seasons, there were fewer spring rains compared with this year, and seedling diseases were not a problem. Because of the frequent spring rains this year, we have less early-planted soybean and more fields planted in mid-May or later. Reports and samples of seedling diseases also are later this year. On May 19, Mark Carlton, Iowa State University field specialist in crops (southern Iowa), reported observing diseased soybean seedlings infected by Rhizoctonia for the first time this year.
When soybean is planted later in spring, it is more likely to be infected by fungi that prefer warm soil temperatures, provided soil moisture is high. During field scouting for stand establishment, pay attention to seedling diseases that occur in warmer soils. If damping-off occurs, Phytophthora and Rhizoctonia are more likely to be the cause than Pythium and Fusarium. The former two fungi often occur in warmer soils (70-80°F), and the latter two attack soybean when soil temperatures are cooler (<60°F). Warm soil temperatures are not suitable for Pythium damping-off, a disease commonly found in early-planted soybean.
Keep in mind that although Phytophthora prefers warmer soil temperatures, it does not mean late planting this spring increases Phytophthora risk drastically. My comparison is mainly made among different pathogenic fungi for their relative importance within a planting season and to compare the previous two seasons that had less rain with this season.
Although warmer soils favor seedling diseases caused by Rhizoctonia and Phytophthora, soybean seedling disease caused by Rhizoctonia exhibits different symptoms from those 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. Phytophthora prefers heavy soil, whereas Rhizoctonia prefers light or sandy soil.
Symptoms of Phytophthora-infected plants may be visible on the ground, with infected plants killed. Leaves of infected seedlings are initially gray-green and then turn brown. A few days later, the plants may die. Diseased plants are easily pulled from the soil because of rotted roots. Phytophthora damping-off has similar symptoms to those of Pythium damping-off. However, Pythium-damping off is unlikely to be a problem this year. If soil is wet, Phytophthora damping-off may continue to develop on the soybean stem, resulting in chocolate brown discoloration from the soil line up, a unique symptom of this disease
This article originally appeared on pages 78-79 of the IC-490(10) -- May 26, 2003 issue.