Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
This week fields were scouted throughout Delaware. Overall, we encountered few issues. The most common disease was Septoria brown spot, a disease that is not typically a yield limiting issue in soybean production. Infection of leaves by Septoria glycines results in the production of small irregular black/purple spots on the lower foliage, giving the leaf the appearance of being splashed with paint (Figure 1). Spots can grow over time eventually resulting in yellow or brown leaves that drop prematurely. Septoria brown spot builds up over time in soybean stubble or residue. Spores produced on residue are spread short distances up the plant by rain splash. The pathogen requires high levels of humidity to produce spores, thus it is often limited to the lower parts of the canopy. Symptoms may occur earlier or be more severe on some varieties. Other factors that may contribute to disease development include minimum-tillage and continuous soybean production. Septoria brown spot can be easily mistaken for bacterial diseases on soybean such as bacterial blight. To distinguish the two, look at the affected foliage. Septoria brown spot affects old leaved lower in the canopy, whereas bacterial blight affects young or newly developing foliage.
Figure 1. Septoria brown spot on the lower leaves of a soybean plant.
We also saw some fields with symptoms of virus infection and are awaiting confirmation of viral ID. The symptoms we observed included puckered leaves, mosiac, and twisted or distorted growth. We identified Soybean Vein Necrosis Virus (SVNV) again in young soybeans (Figure 2). This virus is spread by thrips and we are still learning about the effects on soybeans. Our current data indicate that it can reduce plant growth and may impact seed quality to some degree. Remember that viruses alter growth regulators in the plant and affect plant physiology in many ways, so leaf drop or degree of foliar necrosis is not necessarily predicative of yield or quality impacts.
Figure 2. Image of early symptoms of Soybean Vein Necrosis Virus