University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Papers in Plant Pathology Plant Pathology Department 2017 Understanding Fungicide Resistance Tamra A. Jackson-Ziems University of Nebraska-Lincoln, [email protected] Loren J. Giesler University of Nebraska-Lincoln, [email protected] Anthony O. Adesemoye University of Nebraska West Central Research and Extension Center, [email protected] Robert M. Harveson University of Nebraska Panhandle Research and Education Center, [email protected] Stephen N. Wegulo University of Nebraska-Lincoln, [email protected] Follow this and additional works at: http://digitalcommons.unl.edu/plantpathpapers Part of the Other Plant Sciences Commons, Plant Biology Commons, and the Plant Pathology Commons Jackson-Ziems, Tamra A.; Giesler, Loren J.; Adesemoye, Anthony O.; Harveson, Robert M.; and Wegulo, Stephen N., "Understanding Fungicide Resistance" (2017). Papers in Plant Pathology. 481. http://digitalcommons.unl.edu/plantpathpapers/481 This Article is brought to you for free and open access by the Plant Pathology Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Papers in Plant Pathology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Understanding Fungicide Resistance Tamra A. Jackson-Ziems, Loren J. Giesler, Anthony O. Adesemoye, Robert M. Harveson, Stephen N. Wegulo Extension Plant Pathologists Introduction where tillage is reduced, since this is a residue-borne disease. The primary source for this disease is infested Fungicide resistance has developed in some diseases residue, infected seed and airborne spores. of row crops as well as specialty crops. This implies that fungicide applications to control such diseases may no In 2010, resistance to strobilurin fungicide (QoI) was longer be effective. Scientific studies have shown that reported for the first time to this pathogen in Tennessee. fungicide resistance develops through natural selection of Since this time there has been significant spread in the a mutant strain of a pathogen in a population that is Mississippi valley but we have not observed this in resistant to fungicides. Resistance is very difficult to Nebraska yet. Current distribution of confirmed resistant eliminate but can be delayed through appropriate populations are located on the IPM PIPE website management practices. The availability of inexpensive (http://frogeye.ipmpipe.org/cgi-bin/sbr/public.cgi). options with single mode of action products being Resistance to QoI fungicides in C. sojina is a result of a available makes this an important issue so we do not repeat single site mutation. This mutation is not known to have what was done in weed management. any fitness cost and has resulted in it being held in the population once it occurs. The organization known as the Fungicide Resistance Action Committee (FRAC) was established by industry General Management of Frogeye Leaf Spot and research scientists to be an overseeing group to monitor fungicide resistance and provide guidelines for Resistance development of products with long term utility. This committee established the FRAC code which identifies Soybean varieties vary in their resistance to Frogeye Leaf different target sites within specific modes of action for all Spot and there are several genes commonly used for resistance. This will reduce inoculum and exposure to active ingredients. Usually, there is a small rectangular fungicide for selection of resistance. box on every fungicide label where the FRAC number is located (Table 1). When the FRAC code shows only one Cultural Practices number, it implies that the fungicide contains a single active ingredient but if a fungicide contains two active Frogeye Leaf Spot is more severe in continuously cropped ingredients, two numbers will be shown. For example, a soybean fields. Reduced tillage systems will tend to have FRAC code shown as ‘group 7’ indicates that the fungicide more as the pathogen overwinters in residue. This will is a succinate dehydrogenase inhibitor (SDHI) whereas reduce inoculum levels and exposure to fungicide for group 11 are Quinone outside inhibitors (QoI which selection of resistance. includes strobilurins). However, if both 7 and 11 appear in the label, it means the fungicide has active ingredients Fungicide Application belonging to the two groups. Some specific examples of fungicide resistance that have been seen in different crops Application of fungicides to manage frogeye leaf spot in Nebraska is typically not warranted in most fields. Fields and are discussed below. If a fungus is resistant to a with a history of frogeye should be watched carefully and specific fungicide active ingredient within a FRAC Code, if disease develops application of a strobilurin fungicide at then it will most likely be resistant to all fungicides with the R3 (pod set) – early R4 growth stage are considered the the other active ingredients in the same FRAC Code. most effective. Avoid applying products when disease development is significantly developed. Frogeye Leaf Spot of Soybean Frogeye leaf spot caused by the fungus Cercospora Gray Leaf Spot of Corn sojina is becoming a common foliar fungal disease in The disease, gray leaf spot of corn, is a common Nebraska. The disease is most severe when soybean is fungal disease in much of Nebraska. The causal agent, grown continuously in the same field, particularly in fields Cecospora zeae-maydis (Czm), is closely related to the 2017 Proceedings : Crop Production Clinics 185 fungus causing frogeye leaf spot of soybean, being in the in the upper plant canopy during grain fill reduces its same genus, Cercospora sojina, albeit a different species. impact on yield. These pathogens have many biological characteristics in common, such as survival in infested plant debris from the Integrated management previous season(s) and have similar weather conditions that are favorable for disease development, namely warm Deploying a combination of management strategies is temperatures and high relative humidity. Whereas more likely to provide satisfactory results. Planting more fungicide resistance to strobilurin fungicides (QoI resistant hybrids in high risk production systems and fungicides) has been well-documented in the soybean monitoring disease development and progression up the frogeye leaf spot pathogen in other parts of the U.S., there plants in susceptible hybrids to make fungicide application have not been confirmed reports of fungicide resistance for decisions can more effectively manage gray leaf spot. the gray leaf spot pathogen of corn in the field. But, fungicide resistance has been documented in the laboratory Fusarium Head Blight of Wheat in vitro tests where the fungus can utilize alternative respiration pathways to overcome the effects of the Fusarium head blight (FHB), also known as scab, is a fungicides allowing for spore (conidia) germination. destructive disease of wheat. In North America, it is Baseline QoI fungicide sensitivities were identified for the caused primarily by Fusarium graminearum. The disease gray leaf spot fungus collected from several states causes premature bleaching of spikelets, causing sterility (including Nebraska). The results of these experiments or production of discolored, shriveled kernels commonly indicated that resistance is possible in naturally-occurring referred to as Fusarium-damaged or “tombstone” kernels. populations, but that it may be less likely than in other In addition, F. graminearum produces trichothecene closely related species. However, frequent applications of mycotoxins, mainly deoxynivalenol (DON) and nivalenol, QoI fungicides over a large area of corn increases the which contaminate grain and are harmful to humans and probability that fungicide resistance may develop. animals. FHB epidemics occur sporadically in Nebraska Populations of the fungus should continue to be monitored due to a variable climate. The disease tends to occur during over time to assess for a reduction in fungicide sensitivity. years with high rainfall before and during flowering. The most recent major epidemics occurred in 2007, 2008, and Management of Gray Leaf Spot 2015. FHB is controlled by applying a triazole fungicide to Hybrid resistance the heads during the flowering growth stage. Triazoles used for FHB control include tebuconazole, Corn hybrids vary widely in their resistance to gray leaf prothioconazole, and metconazole. In 2011, the first spot, which reduces the size and number of lesions. isolate of F. graminearum resistant to tebuconazole was Disease immunity does not exist and highly resistant collected from a wheat spike during a survey in Steuben hybrids may still develop some lesions. Consult ratings County, New York. It is the first tebuconazole-resistant provided by seed companies to help predict how the hybrid field isolate of F. graminearum reported in the Americas. will react to gray leaf spot and position more resistant F. graminearum resistance to triazole fungicides has not hybrids in fields with a history of severe disease and other been documented in Nebraska. However, the discovery of high risk factors, such as continuous corn and minimum a tebuconazole-resistant isolate in New York indicates that tillage. the potential exists for resistance to develop in Nebraska isolates. Cultural practices Management of FHB Residue management with tillage may provide some benefits for disease reduction, but is not practical for all Cultivar Selection production