Soybean Disease Update: September 21, 2013

Detecting soybean rust at low levels can be extremely difficult in the field. Only a single leaf in the attached image contains any soybean rust even though some of the top of the leaf symptoms look similar. Leaf 7, immediately above the number, is the only soybean rust present.

Soybean rust

Much as in the past, numerous MS counties (45 total as of 9/21/2013) have been determined to contain soybean rust infected plant material; either kudzu (4 counties to date) or soybean (41 counties to date).  However, following the warmer winter and wetter spring we were concerned that soybean rust might have been a greater issue during the 2013 season than it had been in the past.  But, the drought-like conditions that have been encountered over the past month in some parts of the state slowed soybean rust development down considerably.  Even though the disease may not be a widespread concern for soybean farmers in MS, there are still some fields that have soybeans young enough to be affected enough to reduce yield.  At this point in the season, several weeks would be required for inoculum to build up enough to reduce yield especially in fields that are not currently infected.

On Friday, September 13, 2013, hot spots of soybean rust were identified in variety trial plots on the experiment station in Verona, MS.  The plots were at growth stage R6, and therefore out of the woods when it comes to yield loss as a result of soybean rust.  However, an extremely limited number of soybean fields in northeast MS, and more specifically in Lee County in close proximity to the Verona experiment station are somewhere between R5 and R6.  Deciding on a fungicide application at this point in the season is a difficult choice to make.  First and foremost, product selection will depend on a) growth stage of the soybean plants, b) presence of soybean rust in the particular field, and c) overall yield potential of the soybean plants.  Northeast MS has been extremely dry over the past month so in most cases young soybean fields may not have a good yield expectation.  However, the information in the next paragraph may additionally aid in deciding on whether or not to apply a fungicide.  Quite frankly, the number of fields that remain at risk from soybean rust this season is extremely low so a fungicide will likely not be a necessary management decision.

Leaves containing moderate to severe soybean rust infection have been observed in extremely rare cases during the 2013 season.

Aside from the hot spots in Lee County we continue to struggle to find soybean rust in some of the counties.  In some counties, single leaflets are observed with a small cluster of pustules (see photo).  The leaves in the attached image were from a single field in Monroe County, due north of Pine Grove, MS.  The single infected leaf was shaded for most of the day, but was difficult to tell apart from all of the Septoria brown sport present in the field.  In fields where the disease is observed at such low levels, soybean rust won’t have the time to produce enough inoculum to become a yield reducer.  Detecting low levels of the disease early in the season, generally speaking pre-reproductive growth stages, is important to reduce the likelihood of widespread disease losses.  Late in the season the low levels of disease tell us that environmental conditions were not conducive for the disease to develop into a problem during the 2013 season on a large scale acreage.

Frogeye leaf spot

I mention frogeye this late in the season for a few reasons.  First and foremost, frogeye has become a disease of increasing concern over the past two seasons.  Prior to 2012, limited frogeye leaf spot was observed throughout the MS soybean production area.  During 2013, widespread frogeye leaf spot was encountered throughout the state.  I will certainly know more regarding the impact of fungicides on frogeye leaf spot in a susceptible soybean variety at late application timings once plots in Stoneville are completely harvested.  In addition, as soon as the data can be entered and analyzed a list of the reaction of the varieties contained in the MSU soybean variety trials will be uploaded to the Mississippi Crop Situation Blog.

When choosing varieties for the 2014 season try not to grow frogeye susceptible varieties in a continuous soybean system (e.g., susceptible soybean variety in 2012 followed by the same susceptible soybean variety in 2013 followed by the same susceptible soybean variety in 2014).  If the difference between 2012 and 2013 will be evidence for what we could experience next year then I suspect we will have even more frogeye leaf spot scattered throughout the state.  Preliminarily, some of the fungicide efficacy plots conducted during the 2013 season were harvested early this week (9/17/13).  Based on a cursory analysis of the treatments and yields associated with those plots, a fungicide applied to a susceptible soybean variety at R5.5 still provided more than a 9 bushel/acre response in the presence of moderate to severe frogeye.

Extreme Cercospora blight can be observed following several days of rain, cooler temperatures, and windy weather.

Cercospora blight

Don’t be surprised if following the rainfall we received over the past two days, as well as the cooler temperatures associated with that particular storm front that fields that didn’t appear to have reached R6 prior to the rain take on a brown cast to them.  In the past few years, following several days of rain and cooler temperatures, Cercospora blight has appeared in what seems to be an “overnight” situation.  The disease has been present in the field at low levels and the changing environment produces some other physiological response in the plant that makes the disease appear in a short period of time.  Cercospora blight is present in every soybean variety in the MSU variety trial program.  Some varieties appear to be more tolerant to the disease than others but Cercospora blight affects every soybean variety planted in MS.  Even though fungicides are labeled for management of Cercospora blight the efficacy of fungicides on the fungus and resulting disease appears to be extremely low.

Root rots

As the season winds down, numerous root rot diseases can regularly be observed in soybean fields.  More often than not, root rots are misdiagnosed.  Root rots that occur in light soil classes (e.g., sandy loam, silt loam) can generally be grouped into either SDS, charcoal root rot, or in cases where nematodes are present misdiagnosed as the previously mentioned diseases.  One of the more common root rot diseases in MS is charcoal root rot.  Charcoal rot can produce foliar symptoms similar to other diseases; however, leaves will be stuck to plants that are dead and severely infected by charcoal root rot.  In addition, the small black pepper grains in the plant tissue (microsclerotia) will be evident in plants with charcoal rot. But, the foliar symptoms of charcoal rot can appear similar to other diseases such as Phytophthora root rot.  However, Phythophthora is a disease that will generally occur in soils that contain a greater concentration of clay since the fungus that causes the disease prefers high water content.  Proper disease diagnosis prior to making a variety selection next season is important.  Dry areas of a field, such as pivot corners, aren’t likely affected by Phytophthora root rot and are more likely the result of charcoal root rot.  Presently, commercially available varieties with tolerance to charcoal root rot aren’t present.  Managing soybean fields to reduce stressful conditions is the best way to reduce the impact from charcoal root rot

Severe root-knot nematode infestations can be easily observed by galling of roots. However, reniform or soybean cyst nematode infestations generally require a nematode lab to confirm the presence of the nematode.

Nematode issues

End of season nematode sampling is a good practice to consider in situations where either continuous soybean has been the normal cropping system on light-textured soils or in fields with a history of continuous cotton production.  Collecting soil samples at the end of the season means that nematode numbers are likely at their highest point and will provide important information going into next season’s crop.  If soybean plants are still in the field and you see a problem area, collecting a soil sample from the part of the field that looks to be affected and comparing to a part of the field that doesn’t have the same symptom is the best practice.  When collecting a soil sample I typically use a bucket, and pull 10 to 15 soil cores from the affected area, placing them in the bucket, mixing them completely and putting them in a ziplock bag.  Remember to keep the samples cool and out of the sunlight prior to sending to the nematode lab for analysis.

In the Mississippi soybean production system, reniform, root-knot, and soybean cyst nematode can cause large yield reductions in continuous soybean systems.  Knowing the particular nematode present can help in making a varietal decision for next season.  Commercially available soybean varieties with resistance to root-knot and soybean cyst nematode are available.  However, if reniform nematode is an issue then crop rotation to a non-host, such as corn, for more than one season is the best option.  Keep in mind, that different nematode laboratories use different scales for reporting the number of nematodes present.  The MSU Plant Disease and Diagnostic Laboratory reports nematodes from a pint of soil (=473 cc).  So, if looking at many of the thresholds available online that are reported in either 100 cc of soil or 250 cc of soil, some math will be necessary to convert to one of the scales available.  Nematode thresholds by crop can be located at the bottom of the page at: http://msucares.com/lab/.