Identification and characterization of fungicide-resistant strains of Parastagonospora nodorum in Kentucky
Carl A. Bradley*, Danilo Neves, and Nathan White
Department of Plant Pathology - University of Kentucky Research & Education Center, Princeton, KY
*Contact information for C. A. Bradley: tel: 859-562-1306; email: carl.bradley@uky.edu
Parastagonospora nodorum (formerly known as Stagonospora nodorum and Septoria nodorum) causes both Stagonospora leaf blotch and glume blotch. Stagonospora leaf blotch is one of the most common foliar diseases observed in Kentucky. When flag leaves are affected by leaf blotch, yield losses generally occur. Glume blotch also is a common disease in Kentucky, and can reduce yields and test weight. One of the most common ways to manage these diseases is through application of foliar fungicides.
The strobilurin fungicide class is one of the most common fungicide classes applied to wheat and other field crops. This class of fungicides contains active ingredients that control a wide spectrum of diseases, but has a high risk of selecting for fungicide-resistant strains of fungal pathogens.
The objective of the proposed research are:
Collect isolates of P. nodorum throughout Kentucky and evaluate their sensitivity to strobilurin fungicides
Characterize the mechanism of P. nodorum’s resistance to strobilurin fungicides
Develop an efficient method to identify strobilurin fungicide-resistant strains of P. nodorum
Current status of project:
A graduate student (Mr. Nathan White) was hired to work on this project and began collecting leaf and head samples affected by Stagonospora leaf and glume blotch in the summer of 2019. From the 2019 collection, there have been 20 P. nodorum isolates collected so far, and more isolations from samples are currently underway, which will add to the isolate number. In 2019, samples were collected from Caldwell, Crittenden, Fayette, Fulton, Logan, Todd, and Woodford counties. In addition, approximately 50 isolates were collected in 2018.
From preliminary assays, it is apparent that some isolates that are highly-resistant to strobilurin fungicides are present in our isolate collection. In Figure 1, isolate no. 1480 is highly resistant to azoxystrobin fungicide in petri dish assays. These preliminary assays are helping us determine a discriminatory dose of the fungicide that can be used to discriminate between fungicide-resistant and -sensitive isolates.
Figure 1. Petri dish sensitivity assays of P. nodorum isolates to varying concentrations of azoxystrobin fungicide. Fungicide concentrations are listed at the top and increase in strength from left to right. Six different isolates of P. nodorum are shown on the left, and growth inhibition of most isolates can be observed in higher concentrations of the fungicide, with the exception of isolate no. 1480, which is not affected by concentrations of the fungicide up to 10 ppm.
Preliminary conclusions:
Although much of the work is still in progress, it is apparent that P. nodorum isolates resistant to strobilurin fungicides are present in Kentucky. As we continue to assay additional isolates, we will have a better estimate of how widespread strobilurin-resistant P. nodorum isolates occur in Kentucky. In addition, as we continue to work through our methodology, we will be able to develop a quicker, more efficient single-dose discriminatory assay to identify strobilurin-resistant isolates of P. nodorum.