Foliar Soybean Disease Update: July 30, 2016

With the increased and frequent rain showers over the past 7 to 10 days, the number of lower to mid-canopy soybean diseases have increased. At present frogeye leaf spot, Septoria brown spot and target spot continue to be observed on a somewhat widespread distribution.  Frogeye leaf spot continues to be observed at low levels, even in susceptible varieties.  Soybean rust continues to be observed on kudzu in southern MS, with one single observation on kudzu (on one leaf) in Rankin County.

Frogeye leaf spot

Frogeye leaf spot continues to be observed at low levels in some fields with susceptible soybean varieties. However, following the rainfall over the past week to 10 days in some areas I would expect frogeye leaf spot to increase in incidence as well as severity in some locations.  Check the variety planted in each field and scout susceptible varieties more thoroughly to determine disease incidence and severity.  Keep in mind that frogeye is generally present on younger leaves in the upper most canopy, but can also be observed in the mid-canopy.  Lesions are generally small, with maroon margins and a grey to tan center.  Fungal reproductive structures are formed on the bottom of the leaf and can be observed with the naked eye, although they are easier to observe with the aid of a hand lens.  Frogeye leaf spot susceptible soybean varieties, exhibiting disease, can benefit from a well-timed fungicide application.

See: https://www.mississippi-crops.com/2016/06/19/2015-frogeye-leaf-spot-fungicide-evaluations/ for information regarding the response of a frogeye-susceptible soybean variety to multiple fungicides.

Aerial blight of soybean. Photo taken at 11 am with little to no dew present.

Aerial web blight

On Tuesday and Wednesday (July 26 and 27) I looked at soybean fields in south MS (south of Jackson in several counties). One of the main diseases observed was aerial web blight.  Aerial web blight can cause serious yield losses if not treated in a timely fashion.  Look for matted, water-soaked leaves early in the morning.  Later in the morning, around 11 am, or when the dew has burned off, the disease can be more difficult to diagnose.  However, leaves that appear to have been eaten by an insect, with large missing centers and extremely brown margins can be the result of aerial blight.  Soybean fields with extensive aerial blight benefit from a fungicide application using a product that contains a strobilurin active ingredient.  Applications should be made by ground, in no less than 15 gallons of water, and using as high a pressure as possible (60 psi) to assure good coverage.  Remember, the fungus that causes aerial blight is soilborne and moves up the plant.

Soybean rust

Over the past week, additional kudzu patches have been identified containing soybean rust through southern MS. However, even though we now have 22 counties with confirmed soybean rust on kudzu.  In addition, the first observation of soybean rust on soybean was made in Rankin County, MS on Friday (July 29, 2016).  The particular field was at the R4/R5 growth stage and only a single leaf was observed with a low number of pustules.  The farmer was consulted and it was not suggested that the field be sprayed.  I suspect with the increasing humidity and frequent rain showers as well as cloudy conditions throughout much of the state the number of observations of soybean rust infected plant material will increase.  As of this point in time, many of the early-planted soybean fields are no longer at risk to yield losses as a result of soybean rust.  Soybean sentinel plots as well as commercial soybean fields will continue to be scouted for the presence of foliar diseases in addition to soybean rust.  Prior to this July, the greatest number of soybean rust infected counties was observed was five.

Septoria brown spot

Over the past week I have observed several fields with heavy Septoria brown spot in the lower canopy. Normally, brown spot is a low to middle canopy inhabitant.  Rarely will the disease move above the middle canopy unless the field is extremely stressed or the environmental conditions remain conducive for disease development for an extended period of time (warm, humid conditions).  One of the best ways to manage brown spot is to rotate fields out of soybean for several years as the organism is more problematic in fields of continuous soybean.  If an R3/R4 fungicide application was made prior to the observation of brown spot a second application is not warranted.  Quite frequently brown spot will stay lower in the canopy.  Only rarely have I observed the disease in the upper canopy.  Fields with brown spot in the upper most parts of the canopy (through the entire plant canopy and not just in the mid-canopy) may benefit from a fungicide application. However, obtaining efficacy data in MS has been difficult as severe brown spot rarely occurs.

Target spot is a common occurrence in the low to mid-canopy of many soybean fields.

Target spot

Similar to the information above regarding brown spot, target spot continues to be observed in the low to mid-canopy. Target spot can cause severe defoliation of the lower plant canopy.  Defoliation as a result of target spot is normal.  Similar to the comments made above regarding fungicides target spot will generally remain in the lower canopy and is generally a result of the prevailing environment.  Monitor fields for the presence of target spot.  Defoliation of the lower canopy at later growth stages (R5+ to R6) will not result in yield losses since the upper canopy leaves feed the plant. Fields that stay wet following rain or where the dew holds for extended periods of time or exhibiting rank growth can oftentimes have excessive target spot.