Numerous trials were conducted during the 2013 season to determine the efficacy of popular commercially available fungicides on frogeye leaf spot in a susceptible MG IV soybean variety. The majority of the trials were conducted in Stoneville, in an irrigated situation with fungicides applied at R5 and in one case at R5.5. However, one additional trial was conducted in Starkville on the same variety, in a dryland situation with fungicide applications made at R5.
Data from five trials are included to provide information regarding the response of fungicides on a frogeye leaf spot susceptible variety in the presence of frogeye leaf spot at the time of fungicide application. Ratings were conducted using a 0-9 scale where 0= no disease and 9=90% leaf coverage. Information regarding the specific trial parameters are included in each of the reports.
Full-season soybean trials:
-2013 Cheminova protocol (R5 application timing) (2013 Cheminova)
-2013 Starkville trial (R5 application timing) (2013 Starkville)
-2013 Stoneville (R5.5 trial under heavy frogeye pressure) (2013 Stoneville)
Soybean following wheat trials:
-2013 BASF protocol (R5 application timing) (2013 BASF)
-2013 Bayer trial Stratego YLD phytotoxicity comparison (R5 application timing) (2013 Bayer phytotox)
The Bayer trial was initiated to determine how different adjuvants and different rates of Stratego YLD (4.0 or 4.65 fl oz/a) might influence phytotoxicity. However, little to no phytotoxicity was observed in the trial due to the environment at the time of application (cooler, less humidity).
Some key points regarding frogeye leaf spot and fungicides used to manage frogeye leaf spot:
1) If frogeye is observed in a field, consider the population present to be resistant to the strobilurin class of fungicides. Therefore, stand-alone strobilurin fungicides should not be applied to frogeye susceptible soybean varieties for either the R3/R4 application OR in the presence of frogeye leaf spot.
2) Fungicides are NOT systemic within the plant. The fungicide product will only be systemic within an area around where the droplet lands. Therefore, new nodes added to the plant post-application will not be protected from the fungus.
3) The fungal population in every field likely differs a good bit since there is tremendous variability within the frogeye leaf spot fungal population. Due to tremendous variability a fungicide should be expected to perform differently across multiple fields even planted to the same variety and in close proximity.
4) Phytotoxicity can be associated with the application of some fungicides, specifically those that contain some triazole active ingredients. Keep in mind, that stand-alone triazole formulations likely contain some adjuvant so adding additional adjuvant may result in a phytotoxic response. Always read and follow label instructions. Based on 2013 trial data, even in the presence of phytotoxicity, yield loss was not believed to result from the phytotoxicity observed.
5) Frogeye leaf spot in all situations, regardless of product applied, visually appeared to get worse following a fungicide application to a frogeye susceptible variety. Keep this in mind when scouting fields post-application.
The difference between applying a fungicide when the maximum number of nodes are exposed and fungicide failure is quite different. The application of a fungicide will protect the leaves present at the time of the application. Therefore, if new growth occurs following the fungicide application there is a likelihood that frogeye leaf spot will continue to infect the plant as new leaves emerge. This is holistically different than fungicide failure. A fungicide failure would occur as a result of the fungicide application in the presence of disease, and the fungicide not controlling the fungus. The resulting situation would mean an increase in the disease on leaves that were present at the time of the application. One would expect fungicide failure in a situation where all nodes were present on the plant and a stand-alone strobilurin fungicide were applied in a field where the frogeye fungal population was resistant to the strobilurin chemistry. To observe how fungicides perform in the presence of a strobilurin-resistant frogeye leaf spot isolate, look at the table related to the fungicide trial conducted in Starkville, MS. Leaf samples were collected from the trial during 2013 and the frogeye leaf spot fungus was confirmed to be resistant to the strobilurin fungicides in laboratory assays. Currently, 46 counties have been confirmed to contain strobilurin-resistant frogeye leaf spot (see associated map in the Disease Monitoring section).