Keep an eye out for sudden death syndrome in soybeans & notify me of suspected cases

This article was written by Angie Peltier.

The 2024 growing season couldn't have been any more ideal for sudden death syndrome (SDS) symptoms to appear in soybeans during reproductive growth stages, provided your field was infested with Fusarium virguliforme, the fungus that causes SDS. If you encounter soybeans in northwest Minnesota, please send me an email (apeltier@umn.edu) or give me a call {(218) 281-8692} as I would like to come and check out your field and collect isolates for further study.

Wet weather this spring lead some to 'mud in' their soybeans, likely lead to both side wall and general compaction and wet weather after planting all favor infection by F. virguliforme. Infection can take place just as soon as the radicle breaks through the seed coat. 

While SDS can cause root rot, it is most often much later in the growing season that the most conspicuous symptoms of SDS become evident. While the fungus remains below ground, plenty of rain during the reproductive growth stages can pull phytotoxins that it produces up and into leaves by the normal process of evapotranspiration. Foliar symptoms begin with yellow 'flecking' between leaf veins (Figure 1), progressing chlorosis or yellowing between leaf veins and then death of tissue between leaf veins (Figures 2 & 3). Severe infections can result in leaflet loss and premature plant death which can significantly limit soybean yield potential. 

Figure 1. Yellow 'flecking' between the leaf veins is often the first symptom of SDS observed.

Figure 2. Interveinal (between veins) chlorosis (yellowing) and necrosis (death) characteristic of SDS in soybean.

Figure 3. Interveinal necrosis that precedes leaflet drop due to SDS in soybean. 

The fungus that causes SDS can cause discoloration of the internal taproot tissue (Figure 4) and when soil has been moist can sometimes be seen growing on the outside of the taproot as blue/white fungal growth (Figure 5). 

Figure 4. Brown discoloration of taproots of plants suffering from SDS can also sometimes be observed.

Figure 5. Blue fungal growth of Fusarium virguliforme, the fungus that causes SDS can sometimes be seen on taproots when soil conditions have been moist.

Relationship between SDS & SCN.

Symptoms of SDS will not be evenly distributed across the field because the pathogen is not evenly distributed across the field, but rather will be in pockets. SDS symptoms tend to be more severe when a field is also infested with soybean cyst nematode (SCN), a pathogen that is also concentrated in pockets throughout a field. 

It is too late this growing season to see SCN cysts on roots to determine whether a field is infested, but it is definitely not too late to collect a soil sample to determine SCN egg counts. SCN management recommendations are based on SCN egg counts and so it is worthwhile to collect soil samples for both a first SCN detection or to determine what your field's SCN egg count is in fall 2024 (as population densities tend to rise, regardless of varietal resistance each time a susceptible crop like soybeans is grown). 

Management of SDS.

As F. virguliforme produces thick-walled spores for survival and can have many host crops (some on which it can cause symptoms and others on which it doesn't) on which it can survive and reproduce, including wheat, corn, alfalfa, canola, dry beans, pea, crop rotation is unlikely to be an effective management practice. Tillage has also not proven effective. 

The primary means of control are planting varieties with partial resistance. However, several years ago when SDS was found in North Dakota in a county that borders Canada, I communicated with representatives from two of the largest seed companies in the nation. I was concerned after moving from Illinois, where SDS resistance scores were always included in seed catalogues to NW MN where there were no such resistance scores. The seed companies at that time had not yet begun breeding for this multi-gene trait. Let's hope that they have begun to make headway with breeding resistance into our early maturing varieties!

The other main management tactic is to use fungicidal seed treatments with efficacy against SDS. There are two trade names of seed treatments that are very effective, although they tend to be pricey as they are still on patent: ILeVO and Saltro.