Identification and Management of Soybean Seedling Blight Diseases
Plant Pathology / Extension Specialist
Department of Crop Sciences
Several soilborne pathogens have the ability to cause damping-off, seedling blight, and root rot on soybean plants grown in Illinois (Fig. 1). The primary pathogens involved in this “seedling blight complex” are Fusarium species, Phytopthora sojae, Pythium species, and Rhizoctonia solani. Although these pathogens sometimes cause similar symptoms, the conditions that are favorable for disease may be slightly different for each pathogen. In addition, management practices used to manage these diseases may differ. Therefore, it is important to diagnose seedling blight problems correctly so that the proper management actions can be taken to protect soybean fields from stand and yield losses.
Fusarium species. Different species of Fusarium have the ability to cause seedling blight and root rot on soybean. Depending on the species of Fusarium present, infection can take place over a range of different soil conditions. Root rot symptoms caused by Fusarium species appear as dark brown, diffuse lesions, and a general lack of secondary roots present.
Phytophthora sojae. This pathogen causes seedling blight, and a root and stem rot of soybean. The most favorable conditions for infection by this pathogen is when soil is wet and above 65°F. Wet soil conditions are required for this disease to be severe, because the pathogen produces spores (zoospores) that must “swim” through the soil to infect soybean plants. Symptoms of the disease can be observed as wilting plants, root systems with dark lesions that may be rotted, dark stem lesions extending upwards from the soil line, and seedlings that have damped-off.
Pythium species. Different species of Pythium have the ability to affect soybean seedlings. In Illinois, species that are active at soil temperatures <65°F are of the most concern. When wet conditions occur shortly after soybean seeds are planted into cool soils, seed rot and damping-off caused by Pythium are most likely to occur.
Rhizoctonia solani. This pathogen can cause damping-off and root rot under a range of soil conditions. Root and hypoctyl rot caused by this fungus appear as sunken lesions with a reddish-brown appearance. In some cases the lesions may completely girdle the lower stem, eventually causing death.
Management of seedling blights. Two of the primary methods used to manage seedling blights are: i) planting resistant varieties; and ii) plant seeds treated with fungicide seed treatments. Resistant varieties are available for management of Phytopthora root and stem rot, and two types of resistance are available: race specific resistance and field tolerance. Race-specific resistance utilizes one or more Rps genes to provide control of specific races of the pathogen. This provides complete control, but is only effective against the races of the pathogen present in the field. Field tolerance is effective against all races of the pathogen, but does not provide complete control.
Fungicide seed treatments that contain either mefenoxam or metalaxyl can provide some protection against Phytophthora and Pythium. Other fungicide seed treatment active ingredients such as azoxystrobin, fludioxonil, pyraclostrobin, and trifloxystrobin will provide some protection against Fusarium and Rhizoctonia as well as some Pythium species. Seed treatments will not provide season-long control against these pathogens. Research conducted at the University of Illinois over several years has indicated that yield benefits from using fungicide seed treatments are most likely to occur when planting early into cooler soils (Fig. 2).
Fig. 2. Summary of University of Illinois fungicide seed treatment trials on soybean from 2001 to 2008.