Season wrap up on soybean diseases, part I
It has been one of the most challenging years on record for getting this crop in the ground and getting it harvested and now we are trying to make sense of all the research data. In the meantime, let’s recap some things that actually did not happen and some that did.
a. Soybean Rust.
Really have not had to say much about this pathogen this year. Inoculum levels were very, very low in the spring thanks to a very hard winter last year in the southern U.S. It was hot and dry early and it took a long time for this disease to get started. Researchers in the South who search for soybean rust, were talking about the Mississippi river spilling over its banks into fields that were totally suffering from drought.
More evidence of a very strange year. As it ended up, soybean rust was only found in three states, Florida, Louisiana and Georgia, very late in the season. The Georgia findings are quite interesting as rust skipped the whole middle of the state, and was found in a northern county – most likely due to the spore movement of an earlier hurricane. Monitoring for this will begin again in the southern states in March and April to assess, how much will survive the winter.
b. Those plants with strange looking pods.
Figure 1. Leaf malformations from virus infection. Several samples came in this year with plants that had crinkled leaves, and pods that were turned up. All of these, to date, came back positive for bean pod mottle virus based on tissue assays with ELISA. ELISA is a lab test that uses antibodies to match the specific virus. Researchers first identified this virus in Ohio in 1999. Severe infections can have leaf malformations (figure 1), reduced plant height, reduced yield, but also some streaking of the hilum. This streaking of the hilum impacts food grade beans more severely as they are docked or no longer suitable.
This virus is spread most commonly by leaf feeding insects, specifically the Bean leaf beetle, which was quite common this year in some parts of the state. If a beetle is carrying the virus, once it feeds on a plant, that plant becomes infected. Ohio State University scientists have inoculated plants at 3 different growth stages and all became infected to some degree. A very simple, yet effective, management strategy is to plant food grade soybean after those that will be used for grain. The overwintering adults will feed on the soybean for grain, and will either be finished with their cycle by the time the food grade beans emerge.
There is some discussion in the north central region if BPMV is the sole cause of the “green stem syndrome”. This is where totally green plants are scattered throughout the field and never mature. There are physiological issues with soybean plants, if a plant never forms pods then it will also not mature and stay green. There are most likely several other causes that can contribute to this phenomenon, and we would expect that they would be different in the different parts of the soybean belt.