Nathan Kleczewski, Extension Specialist – Plant Pathology; email@example.com
There is a typical rogue’s gallery of diseases currently in soybean fields. We are seeing the Septoria brown spot in many fields. This is a residue-borne disease that tends to hang out in the lower canopy. In cases where more than 25% of the canopy is defoliated by R3 you may see some yield loss, but there is little information on how often this occurs in the Mid-Atlantic, particularly in irrigated and double crop soybeans.
A disease that can be confused with Septoria leaf spot is bacterial blight. This also is a residue-borne disease that can cause lesions on foliage. Lesions may occur around the leaf edges and tend to follow the veins. As lesions age they can fall from the leaf, giving the canopy a tattered appearance. Unlike Septoria brown spot, bacterial blight can also be found in the upper canopy. This isn’t typically a yield –limiting disease and control is not warranted.
A soybean plant with early symptoms of bacterial blight. In later stages the foliage may develop a tattered appearance.
In some fields that are further along I have noticed downy mildew. This can be residue or seed-borne and can also spread over large distances on air currents. Again, this is not considered to be a yield-limiting disease, but last year levels across several states were significantly greater than usual. Fungicides for downy mildew control aren’t typically recommended.
The upper surface of a soybean leaf with downy mildew. On the underside of the leaf grey/white fuzzy masses are often observed. Lesions can expand and turn grey over time.
You also may see two different causes of lesions with red boarders and cream/white-colored centers: herbicide damage or Frogeye leaf spot. The use of herbicides with PPO modes of action (e.g. Reflex) can sometimes produce whitish lesions with red boarders. Affected leaves may also be crinkled. New tissues will be free of symptoms. In addition, the entire field is likely to be even in terms of symptomology. With Frogeye leaf spot you can see similar lesions, but leaves will not be puckered. If the leaf is flipped over you should be able to see a black/brown structure and if you are lucky some fuzzy, silvery growth. Lesions might be present on newly developed tissue and the disease may be less uniform throughout the field. If frogeye leaf spot is present at significant levels at R1 then a spray may help reduce severity. This disease seldom builds to levels that necessitate treatment, but if it is present in a field, that field should be carefully scouted.
Soybeans with herbicide damage to lower tissues. Notice that the new tissues are asymptomatic and the puckering of affected leaves.
A soybean field with Frogeye leaf spot. Notice the lack of foliar puckering and the presence of lesions on the upper, young tissues. The presence of fungal structures and tissues may be observed on the undersides of lesions, particularly under humid conditions.
We also are looking for symptoms of soybean vein necrosis virus this year in a project funded by the Delaware Soybean Board. Symptoms may be appearing in some fields in the near future. This is a fairly new disease and currently we are determining how widespread it is and what factors may be associated with increased levels of the disease in fields. If you do see symptoms characteristic of this virus feel free to contact me and I’d be more than happy to collect samples for confirmation.
A soybean with symptoms characteristic of early soybean vein necrosis virus (SVNV).