Soybeans are one of Canada’s major pulse crops. As new shorter-season varieties are developed, the soybean-growing area is increasing.
All the more reason to be vigilant for new and emerging threats like Sudden Death Syndrome.
Sudden Death Syndrome (or SDS) in soybeans is a relatively new fungal disease, first discovered in Arkansas in 1971. In the past four decades it has spread throughout the American Midwest and up into southwestern Ontario. Already it is one of the leading causes of soybean loss in the United States. It can also occur in conjunction with the soybean cyst nematode, a tiny parasite that also lives in soil. When both occur together the effect can be devastating, in some cases leading to near total crop loss. SDS has not been reported in the Canadian Prairies though warmer winters and migration via intermediate host plants may enable it to take root and become a problem here.
The symptoms of SDS
The disease is caused by Fusarium verguliforme, a persistent fungus. Its early symptoms are not detectable until the plant has begun growing above the soil. The first signs are mottled and mosaiced leaves, followed by the yellowing of the leaf veins and the death of the whole leaf. In cases where the fungus is growing strongly the leaflet will drop off completely, leaving only the leaf stalk attached.
These symptoms, while distinctive, can readily be mistaken for brown stem rot or chemical burn, two common afflictions of soybeans. In order to tell whether you have SDS, you’ll need to examine the lower stem and tap roots. In SDS infected plants, the lower stem will exhibit a tan to light brown discoloration (compared to a healthy plant) while the pith will remain white or cream colored. In brown stem rot the pith will be darkened. In chemical burn and brown stem rot the leaflets tend to remain attached.
One very distinctive symptom of SDS, though one that is hard to catch, is located on the taproot’s surface near the soil line. If you dig up a plant affected by the fungus when the soil is moist there may be small, light blue patches on the taproot — these are the fungal colonies of the Fusarium verguliforme.
The fungus is difficult to diagnose definitively, as it is hard to culture in a laboratory from diseases plant samples as colonies grow only on the roots and stem to a few inches above the soil. These colonies are the key reason SDS can spread so far as they are moved about in a field by irrigation and rain.
SDS is a major problem in the fields where it is present. The worst part is that there are only limited management options: though there are varieties of soybeans less sensitive to SDS, the resistance is only partial. Fungicides applied as seed treatments have only limited effects, and those applied to growing plants have no effect on the disease because the fungal infection is restricted to the roots. The spores of the fungus survive on crop residue and in the soil. Worst of all, it can survive wide soil temperature fluctuations (including subzero temperatures) as well as through droughts.
The good news
Now that all sounds pretty bad, and it is. However that doesn’t mean nothing can done about an infection in your field once it gets started. If you have SDS you’ll want to plant later in the season, as fusarium is most active in the cool, wet soil of the early planting season. Tilling to improve water permeability may also restrict the fungi’s growth, as it thrives in wet, saturated, poorly drained soil. Crop rotation can be helpful: , rotate through corn and wheat before returning to soybeans. Removing infected residue and leftover plant parts after harvesting will reduce the infections ability to overwinter and reducing incidences in the future.
Though not yet present in Manitoba, Saskatchewan, or Alberta it is important to stay vigilant for this disease. If you suspect SDS is present in your fields, report it immediately to your local Ministry of Agriculture office.