Late Season Soybean Diseases

The 2011 growing season has been one of great variability in terms of the weather and its impact on soybean diseases. Early season conditions were cool and wet planting which was then followed by high heat and humidity. Now, as reported in the August 19th blog posting, abnormally dry conditions are being reported in several areas of the state. Thus, a common question as we get closer to harvest is, "Given these variable weather conditions, what is the potential effect of soybean diseases on yield?" Based on recent reports and our observations of different field trials, we highlight below several diseases that are popping up in soybean fields and provide specific links for further information about each disease.

In spite of the heat during flowering in many areas, we have seen white mold occurring. This is not necessarily a surprise as we do find the disease each growing season. Based on our current observations and reports though, it appears that the disease intensity is low in many locations, although variation to the soybean variety is being noted.

Also, over the past week to two weeks, we have seen an increase in Frogeye leaf spot. This disease, caused by the fungus Cercospora sojina, was observed in 2010 across the state and conditions in 2011, especially the hot and humid periods that occurred a few weeks ago, were favorable for disease development. In some of the plots that were recently visited, the severity of the disease appears higher than what we observed in 2010, however, levels still appear to be low.

Similar to some of the conditions we saw in 2008, we have seen fields and soybean plants with symptoms of either Phytophthora root and stem rot or stem canker. These two diseases can easily be confused for one another so it is important to make sure a proper identification.

Last, but not necessarily least, we have had several reports and also seen symptoms of sudden death syndrome (SDS). Reports are still being compiled, but based on our observations, the severity of SDS is probably going to be lower than in 2010. With soybean in the R6 growth stage in many areas, it is also important to make sure that the disease identification is correct for SDS, since foliar symptoms are similar to brown stem rot (BSR). In particular, make sure to examine the whole plant, including stems (looking for internal browning due to BSR) and roots (looking for a root rot and also a bluish hue that is the fungus of SDS). Do not just rely on the foliar symptoms to verify your diagnosis of either disease.

Stress on R6 Soybean

I have logged many miles across Southern WI the past week and have noticed several pockets of soybean that could use some rain. The U.S. Drought Monitor service verifies my wind shield scouting as it places most of southern WI in the abnormally dry category (Image 1). Across southern and central WI the average soybean field I have been in is at the R5.5 to R6 growth stage (full seed). In WI the R6 growth stage on average lasts ~18 days but will range from 9 to 30 days depending upon the weather. Soybean in this stage use about 1/4 to 1/3 inches of water per day. Lack of sufficient water during this growth stage can cause young pods and developing seed to abort reducing the number of seeds per plant (Images 2 &3).

Image 1.

Images 2 & 3. Stress induced seed and pod abortion at R6 soybean.

Soybean plants can reduce the size of their leaf pore openings to reduce the loss of water vapor. This also reduces the intake of carbon dioxide and the manufacturing of photosynthates which slows plant growth. When normal soil moisture returns, normal growth is resumed. This ability to reduce metabolic activity allows plants to tolerate dry spells without dying or harming their ability to resume growth when normal moisture returns.

If stress has severely affected pod set and seed fill, and if livestock feed is needed, soybeans can be harvested as a forage for ensiling. Highest protein and yields are obtained from soybean harvested at the R6 to R7 growth stage. Harvesting soybeans for forage between the R1 and R5 stage will result in a very high quality silage, but dry matter yields will be reduced significantly. Forage quality will be reduced from R5 soybean forward if a conditioning process is used during harvest. Conditioning will cause significant seed shattering.