Rainfastness of Prothioconazole + Tebuconazole for Fusarium Head Blight and Deoxynivalenol Management in Soft Red Winter Wheat

Andersen, K. F., Morris, L., Derksen, R. C., Madden, L. V., and Paul, P. A. 2014. Rainfastness of prothioconazole + tebuconazole for Fusarium head blight and deoxynivalenol management in soft red winter wheat. Plant Dis. 98:1398-1406. Fungicides are most warranted for control of Fusarium head blight (FHB), a disease of wheat caused by the fungal pathogen Fusarium graminearum, when wet, rainy conditions occur during anthesis. However, it is unclear whether rainfall directly following application affects fungicide efficacy against FHB and its associated toxin, deoxynivalenol (DON). The objective of this study was to determine the rainfastness of the fungicide tebuconazole + prothioconazole and the residual life of tebuconazole when applied to wheat spikes at anthesis in combination with the nonionic surfactant Induce. Three field experiments were conducted during 2012 and 2013 in Wooster, OH. Simulated rainfall of a fixed intensity and duration was applied to separate plots at five different times after the fungicide treatment (0, 60, 105, 150, or 195 min). Spike samples were collected at 4-day intervals after fungicide application and assayed for tebuconazole residue. A similar set of greenhouse experiments was conducted using six post-fungicide-application rainfall timing treatments (0, 15, 30, 60, 120, or 180 min). All experiments were inoculated at anthesis with spores of F. graminearum, and FHB index (IND) and DON were quantified. In four of the five experiments, all fungicide-treated experimental units (EUs) had significantly lower mean IND and DON than the untreated check, regardless of rainfall treatment. Among rainfall treatments, EUs that received the earliest rains after fungicide application tended to have the highest numerical mean IND and DON, but were generally not significantly different from EUs that received later rain or fungicide without rain. In both years, fungicide residue on wheat spikes decreased rapidly with time after application, but the rate of reduction varied somewhat between years, with a half-life of 6 to 9 days. Rainfall treatment did not have a significant effect on the rate of residue reduction or the level of residue at a fixed sampling time after fungicide application. In this study, tebuconazole + prothioconazole mixed with a nonionic surfactant was fairly rainfast for a fixed set of rainfall characteristics, and tebuconazole residue did not persist very long after application on wheat spikes. Fusarium head blight (FHB), an important disease of wheat and other small grain crops, is primarily caused by the fungal pathogen Fusarium graminearum (teleomorph: Gibberella zeae) in North America. The economic impacts of this disease have been devastating, with a reported 3 billon dollars in losses in the 1990s alone (32). Losses are largely due to reduced grain yield and seed quality and are exacerbated by contamination of grain with mycotoxins, particularly deoxynivalenol (DON). DON has shown toxicity to both humans and livestock. It inhibits protein synthesis and causes vomiting, feed refusal, and other acute symptoms (5,24). Because of health concerns, the U.S. Food and Drug Administration has set a 2 ppm DON threshold for wheat grain and 1 ppm for finished wheat products destined for human consumption (27). Consequently, grain with DON levels above 2 ppm may be rejected completely or priced down at grain elevators. Much research effort has been dedicated to the management of FHB and DON with fungicides in an effort to minimize grain yield and quality losses. The triazole family of fungicides has been shown to be the most effective against FHB and DON (3,18,22), with the highest levels of efficacy achieved when applications are made after head emergence, around early anthesis (17). However, among the triazoles, considerable variation in efficacy has been observed from one active ingredient to another and among studies (22). An imperative need to better evaluate the overall efficacy of fungicides against FHB and DON led to the establishment of uniform fungicide trials (UFTs) through the U.S. Wheat and Barley Scab Initiative (USWBSI) (21). Based on a quantitative synthesis of results from more than 100 UFTs, researchers discerned that the greatest reduction in FHB index and DON when relying on a single anthesis application was obtained with tebuconazole + prothioconazole (Prosaro 421 SC; Bayer CropScience, Research Triangle Park, NC), metconazole (Caramba 90 SL; BASF Corporation Agricultural Products, Research Triangle Park, NC), or prothioconazole (Proline 480 SC; Bayer CropScience) (22). Prosaro is now one of the industry standards for FHB and DON management, and results from a meta-analysis of data from USWBSI uniform integrated management trials showed that when used in combination with a moderately resistant cultivar, this fungicide contributed to reducing both FHB and DON by over 70% relative to the untreated, susceptible check (31). However, despite its efficacy and very clear indications that it has to become an integral part of any FHB/DON management program, commercial utilization of Prosaro following current application guidelines has been somewhat of a challenge. In particular, producers have not always been able to apply this fungicide at the recommended flowering growth stage. This is either because wet, rainy field conditions during anthesis make it difficult, if not impossible, to make ground applications or simply because it is not always easy to determine the exact flowering date. The issue of rainfall around the time of anthesis, conditions under which a fungicide is most warranted for FHB management, is also of concern to producers from the standpoint of its effect on efficacy, and could affect their decision to make an application. What is the effect of rainfall on absorption, efficacy, and the persistence of residue of Prosaro when it is applied during, or directly before, a rainfall event? Rainfall may negatively affect the performance of a fungicide by washing it off of the plant surface or diluting it to a less effective concentration. The extent to which this occurs depends on several factors, including when and how much it rains after the product is applied, the formulation of the product, Corresponding author: P. A. Paul, E-mail: [email protected] Accepted for publication 13 April 2014. http://dx.doi.org/10.1094 / PDIS-01-14-0092-RE © 2014 The American Phytopathological Society