Soybean Disease Update: July 18, 2015

Up until the last week the soybean disease situation had been quiet. However, as soybean fields have matured, the diseases encountered have increased. Generally speaking, as soybean plants develop the diseases encountered will increase throughout the season. Monitoring soybean fields during important reproductive growth stages will help reduce the likelihood of yield loss.

Cercospora leaf blight can develop a leathery appearance as well as the more common bronzing or purpling of the leaf tissue.

Cercospora blight

Once plants reach the R5 growth stages, Cercospora blight becomes more visible. Leaves that appear bronzed or purple in color in addition to purple lesions on the petioles and main stem are the most characteristic symptoms associated with the disease. In addition, leaves will develop and feel leathery to the touch as well as curl and sometimes flip over. In severe cases the leaves can develop a “frosted” appearance whereby the leaf material appears burned around the blight lesions, especially on tips and around leaf edges. In addition, main stems and pods can become purple to almost black in color in the most susceptible varieties. High temperatures and several days of sun can stress plants. The fungus that causes the disease also produces a photosensitive toxin. Therefore, when plants are exposed to several days of intense sunlight, the toxin will manifest itself in leaf material and produce the bronze to purple coloration.

In the past, fungicides have likely prevented some yield loss as a result of this disease. But, with that in mind, fungicide applications should not be triggered once the disease is observed because they will not likely stop the disease progression. Recent information from Louisiana suggests the Cercospora blight fungus is likely resistant to the strobilurin chemistries as well as thiophanate-methyl (Topsin). More recent information from Louisiana also suggests that applications of iron may reduce the disease. However, limited research has been conducted with iron and no research has been conducted with this particular product outside of Louisiana.

Frogeye leaf spot in a susceptible soybean variety at the R3 growth stage.

Frogeye leaf spot

Frogeye calls have increased over the past week. In most situations the soybean plants have reached R2 through R4 and frogeye continues to be observed in moderately susceptible to susceptible varieties. Even though the environment could be considered non-conducive for the development of the disease, extremely susceptible varieties continue to become infected. Fungicides provide some benefit by reducing the yield losses attributed to the disease. However, fungicides will not make the lesions that have already formed go away and in most cases when the fungicide loses the residual activity additional lesions will develop. Even though the temperatures over the past two weeks have been extreme, I have regularly observed frogeye lesions in susceptible varieties. If making an automatic application to a frogeye-susceptible variety at the R3/R4 growth stage use a product with at least two modes of action. However, if a frogeye-resistant variety was planted, making an automatic application with a stand-alone strobilurin is still acceptable since frogye leaf spot won’t be an issue.

For information on how some popular fungicides performed in fungicide trials during 2014 refer to:

http://www.mississippi-crops.com/2015/07/05/2014-frogeye-leaf-spot-and-cercospora-blight-foliar-fungicide-trials-trial-data/

Herbicide injury

Herbicide injury and fungicide phytotoxicity

I honestly cannot count the calls I have received this year regarding herbicide injury following an application of a residual herbicide. In general, depending on the particular herbicide applied and specific growth stage at the time of application, symptoms can be present in the middle canopy or the top of the canopy. If you know the timing or date of the application count nodes to determine if the application coincides with the symptoms present as well as general location within the canopy. In general, diseases do not present themselves across an entire field, especially not at one particular point in the canopy.

Be mindful that some fungicide products can produce a slight injury (phytotoxicity) to the leaf surface post-application. Even though there doesn’t seem to have been as much occurrence of phytotoxicity this season, following the hot and humid days that have occurred over the past two weeks we will likely see some phytotoxicity associated with fungicide applications made during that time period. Trials are currently underway to determine if a yield loss is associated with fungicide phytotoxicity. Data analysis following the last two seasons suggests that the commercially available products, even though some of them have the ability to produce from a mild to a moderate phytotoxic response, do not reduce yield as a result of the injury.

Symptoms commonly associated with the “mystery disease” in the upper plant canopy can easily be confused with SDS or several other soybean diseases.

Mystery disease

Clearly the last two cool, wet springs have had something to do with the increase in this particular disease. Over the past two weeks I have observed several fields to contain severely infected plants. In most cases, the worst field situations are in fields of continuous soybean on light soil classes. However, as the temperatures have increased and farmers have continued to rely on supplemental irrigation the disease can be observed in heavier soil classes. Keep in mind that the symptoms associated with this particular disease can easily be confused with Sudden death syndrome (SDS).

The symptoms associated with this particular disease can easily be confused with Sudden death syndrome (SDS). However, SDS will generally be observed in fields containing light, silty soils. In addition, SDS is typically observed in patches, rather than on 1 out of 10 plants or 1 out of 20 plants. The two easiest ways to tell the two diseases apart from one another are:

1) Soil class

2) Presence of a tap root when attempting to roughly jerk the plant from the soil

If the tap root breaks off below the soil surface and there is a dark, blackened growth on the root in the area where the taproot broke off from the plant, then you are dealing with the mystery disease and not SDS.

Septoria brown spot as appearing in the lower canopy of most soybean fields during the 2015 season.

Septoria brown spot

An increasing number of fields have started to drop leaves prematurely as a result of Septoria brown spot. Brown spot is generally only a concern if the disease progresses into the upper canopy and begins to defoliate the upper canopy. Once plants lap the middles and reach mid-R5 reproductive growth stages the leaves shed in the lower canopy are not as important. Leaves in the lower canopy at growth stages near R6 do not contribute much to overall yield since they are shaded from direct sunlight. In addition, due to the stressful environment encountered over the past two weeks, mildly stressed soybean plants will begin to shed leaves in the lower canopy to compensate for stress. Watch fields with excessive amounts of brown spot and be mindful that in the past the disease has appeared to be more prevalent during hot, dry years.

Soybean rust

To date, no soybean rust has been observed in MS.  Sentinel plots continue to be monitored on a weekly basis throughout the state.  As has been stated in the past, hot and dry conditions are not conducive for the development of soybean rust.  Stay tuned to the Crop Situation Blog should something change.