Soybean Disease Update: September 5, 2015

WeSoybean rust symptoms as observed on the upper leaf surface. Keep in mind the best diagnostic feature is on the underside of the leaf.

Soybean rust

As of Friday afternoon (9/4/15), soybean rust had been observed in 13 counties stretching from southern MS (geographically west of Hattiesburg) to Ripley, MS in Tippah County along the TN line. At present, active soybean rust has been observed in Amite (sentinel plot), Chickasaw (commercial field), Clay (commercial field), Franklin (commercial field), Hinds (sentinel plot), Lee (research plots), Lincoln (commercial field), Oktibbeha (research plot), Pike (commercial field), Pontotoc (research plot), Tippah (research plot), Walthall (sentinelplot), and Washington (research plots) counties on soybean. The soybean plants in all situations were beyond R6 in just about all of the most recent finds. In most cases light amounts of soybean rust were observed; however, some heavily infected leaves were observed near Tupelo, MS. Historically, this is the earliest we have observed soybean rust in the most northern counties in MS. Infection likely occurred several weeks ago, but has taken time to build up enough inoculum to become observable since environmental conditions have not generally been conducive for soybean rust development in MS this season.

Scouting for soybean rust can be difficult. Symptoms on the top of the leaf can be confused with bacterial pustule as well as Septoria brown spot and even spider mite feeding on the soybean leaf. The pustules, formed by the fungus that produce sporulation and result in additional infection, are formed on the underside of the leaf. The pustules appear like small blisters, are light tan in color, and typically can be observed quite well with a 20× hand lens. To confirm soybean rust, use the hand lens to observe the pustules on the bottom of the leaf.

Current (9/5/15) soybean rust distribution map of MS.

Fungicide application as a measure to prevent yield loss as a result of soybean rust is most effective when carefully timed based on growth stage as well as presence of the disease. However, a field’s yield potential is an important determining factor, especially this season. The growth stages of soybean fields in northeast MS ranges from as young as R3/R4 to R5+ depending on geographic location. Fields as young as R3/R4 could benefit from a fungicide application since soybean rust prefers cooler temperatures as well as high humidity and frequent afternoon showers. However, before making a fungicide application determine the yield potential of the field. A soybean field with a low yield potential  Product selection should be based on a curative mode of action (e.g., triazoles) and/or a dual mode of action product or tank mix that contains a strobilurin + triazole (or strobilurin + carboximide). Stay away from stand-alone strobilurin-based products as these only offer a preventive mode of action and if soybean rust were present in a field a stand-alone strobilurin will not be beneficial. Consider the potential economics in your particular situation before making a fungicide application. In addition, be mindful that there are likely several triazole-based fungicides available. Flutriafol (Topguard), propiconazole (e.g., Bumper, Propimax, Tilt), tebuconazole (Monsoon, Toledo, and several other generic products), tetraconazole (Domark) and would be good options for soybean rust management if a fungicide application is believed to be warranted. However, finding some of the products mentioned above may be difficult at this point in the season.

Severe aerial blight will matte leaves throughout the canopy and smells like rotten plant tissue.

Aerial blight

Active aerial blight can still be observed in some scattered fields in east MS. Aerial blight moves up the plant from the soil. Scout fields accordingly by parting back the plant canopy. Scouting fields in the morning will be easier to determine whether the disease is present. Typically, when dew is still present on leaves the leaf tissue will be matted together and some fungal growth can be observed between leaves. When the dew burns off, leaves will look dried out, burned, scorched, and can still be matted together in some instances. However, lesions can occur on leaves that will oftentimes be confused with frogeye leaf spot based on the general shape and size of the lesion. But, the lesions will lack the fungal growth/sporulation on the underside of the leaf as is commonly observed with frogeye leaf spot lesions. Fields that have reached the R5.7 growth stage are generally considered to be safe from yield loss as a result of aerial web blight. However, fields that are in younger growth stages should be sprayed with a fungicide that contains a strobilurin active ingredient. Applications should be made by ground, in as much as 20 gallons of water/acre (much preferred, but 15 will suffice) and as high a pressure as possible (65 psi preferred) to carry the application to parts of the canopy where the fungus is more often encountered. Aerial applications not only don’t provide enough water, but can’t get the product to the lower portions of the canopy where the fungus occurs.

Severe Cercospora blight will discolor pods, petioles, main stems, as well as leaves.

Cercospora blight

Since hot and dry conditions have been the norm for the 2015 season, a tremendous amount of Cercospora blight has been observed throughout the state. Cercospora blight can turn leaves a bronze to purple color, turn petioles and stems a purple to black color, and also turn pods a purple to black color depending on the response of the variety. Fungicide applications, made at the more general application timing of R3/R4, likely provide some minimal response at reducing the observable Cercospora blight. However, fungicides are likely not beneficial when applied at a time whereby Cercospora blight is observed in a field. Minimal differences in susceptibility of some varieties exist within the commercially available germplasm. However, based on the observations made while rating the soybean OVTs for the past three seasons, no variety has good resistance to Cercospora blight. Even though an extensive study of the fungal population in MS has not been conducted, based on information from LA it is likely that the majority of the fungal population in MS is resistant to the strobilurin fungicides.

Over the season we have received several calls regarding the use of iron to manage Cercospora blight.  Several search terms have also been observed on the crop situation blog statistics page that list “iron Cercospora blight” with a high volume of searches. Some trials, conducted only in LA, suggest that iron (an iron-mannitol formulation rather than a chelated iron formulation) provides some benefit in managing Cercospora blight. However, trials have only been conducted in LA and much of the information encountered in MS has been confusing. One article was released early in the season; however, the article does not list the proper iron formulation and rates of application are not listed. I have at least one trial in MS looking at iron alone and in combination with fungicides and may likely make one more application to compare the products observationally. Based on minimal data availability and limited products that contain the proper formulation I would not try to make applications to large acreages. I suspect this will be a hot topic at winter meetings and am hopeful that additional information will be released by LSU in the meantime.

Characteristic frogeye leaf spot symptoms. Sporulation of the fungus generally occurs on the bottom of the leaf. However, in this photo, sporulation can be observed on the top of the leaf.

Frogeye leaf spot

We continue to receive telephone calls regarding frogye leaf spot management. In most situations the questions are regarding multiple fungicide applications to manage the disease. Over the past several weeks more frogeye has been observed in susceptible varieties throughout the state. But, if the OVT locations are any indication, less frogeye has been observed in 2015 than was the norm in 2014. Frogeye leaf spot is most easily recognized by brown to maroon lesions on the upper leaf surface. However, in extremely rare situations, sporulation can occur on the upper leaf surface. The fungus produces spores on the underside of the leaf in a dark, grey to black clump of reproductive structures. Regardless of product, application rate, or application timing (whether R3, R4, R2, R5, or R5+), in the trials that have been conducted in MS over the past three seasons, suggest that frogeye leaf spot will increase in incidence and severity following the application in susceptible soybean varieties. A few multiple application trials have been conducted and in most cases the second fungicide application is not warranted based on yield protection. A single, carefully timed fungicide application can prevent yield loss and provide an economical benefit. Based on commodity prices and the environment encountered this season a second fungicide application to manage frogeye leaf spot may not be economically beneficial. For additional information regarding the response of frogeye leaf spot to multiple fungicide products refer to:

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

Common concentric lesions associated with target spot (left of image) as well as the smaller lesion type without the concentric appearance.

Septoria brown spot and target spot

The hot and dry conditions this year have increased the observation of brown spot throughout MS. In general, brown spot is not a disease of great concern. Brown spot is more commonly observed in the lower canopy in almost every soybean field with a history of continuous soybean production. Clustures of dark brown to black lesions with a severe yellow halo are the most common characteristic associated with Septoria brown spot. Observing the lesions on the underside of the leaf can lead to confusion since the lesions appear raised and more maroon in color. Confusing the lesions with soybean rust can be quite easy due to the raised appearance. In 2014, Illinois documented the first case of fungicide resistance within the brown spot fungal population. Due to the widespread use of fungicides in MS for the better part of a decade it is likely that certain fungicides are no longer effective on the fungus responsible for brown spot. In situations where brown spot is observed in the upper canopy (generally a rare occurrence), fungicide applications may be necessary to prevent additional defoliation as a result of the disease.

Target spot has also been observed throughout MS this season, more so than in recent years. In general, target spot is a low to middle canopy type of disease and rarely has it been observed in the upper plant canopy. Lesions are generally characterized by a round, concentric ring patter, similar to a target (see image). However, in some situations, smaller lesions (1/8 inch) can be observed whereby the concentric pattern is not present. Scout multiple leaves to determine the particular disease present. Fungicide application for target spot is likely not warranted since the disease does not appear to result in much yield loss and generally stays in the lower plant canopy.

Clockwise from left, lesion associated with stem canker in an inoculated plant, a susceptible soybean variety exhibiting severe stem canker, interveinal chlorosis associated with stem canker, sporulation of the fungus on a stem-associated canker, “flagging” of infected soybean plants in a field exhibiting stem canker.

Stem canker

Widespread stem canker has been observed throughout MS this season. Stem canker can be easily confused with several other diseases, since the main aboveground symptom is the presence of severe interveinal chlorosis. However, stem canker is usually only present in small groups of plants. Make note of the presence of stem canker in fields since the fungus responsible for causing stem canker can overwinter in fields with a history of the disease. Most of the commercially available soybean varieties have some tolerance to the disease. However, there are still some susceptible varieties (see image of plot at left) available and those should be avoided in situations where stem canker is observed. When scouting for stem canker focus not only on aboveground symptoms, but also the presence of a brown to maroon sunken lesion that can occur on the main stem, any branching points along the main stem, or where petioles attach to the main stem. Stem canker has been more of a historical issue in east MS, especially the Tupelo area; however, stem canker has been observed in multiple field situations throughout the Delta this season. Prior to 2015, stem canker had only been observed in limited incidences in the Delta. Be mindful that the interveinal chlorosis associated with stem canker can also be confused with fungicide phytotoxicity. Observe the stems of plants that exhibit interveinal chlorosis.