In coming years, the availability of effective pesticides—especially fungicides—will be one of the biggest challenges a sugarbeet grower has to face. Increased costs for developing new chemistries, regulatory restrictions and the loss of existing fungicides will reduce the number of products on the market.
One contributing factor for losing fungicides is certainly the development of fungicide resistance within the target organism. Fungicide resistance is a widespread problem affecting a variety of pathogens in many different crops. It is hard to manage or to reverse resistance after it has developed. Therefore, preventing and delaying resistance is the most effective approach.
Different Erysiphe species have developed fungicide resistance over the years and many of the fungicides used to control E. polygoni are known to have a medium-to-high risk for resistance. Erysiphe polygoni DC, the causal agent of powdery mildew on sugarbeets, is a reoccurring disease problem in many production areas. If not controlled, powdery mildew can cause a decrease in root yield and sucrose concentration by up to 35 percent, as well as an increase in impurities. Currently powdery mildew can be controlled by applying foliar fungicides in conjunction with cultural practices such as proper fertilization, irrigation and planting of tolerant varieties. Fungicides should be applied before powdery mildew is visible in the field and depending on disease pressure and location, one to multiple applications is necessary to minimize economic losses.
In past years, individual products were screened for a potential loss of efficacy and to detect fungicide resistance in E. polygoni. In 2012, products belonging to the fungicide Group 11 (Quinone outside inhibitors = QoI-fungicides) showed reduced efficacy in controlling E. polygoni. This observation was confirmed in 2013 by testing QoI-fungicides and fungicides belonging to two other chemistries (DMI = Demethylation inhibitor of sterol biosynthesis, SDH = succinate-dehydrogenase inhibitor) for their efficacy in controlling E. polygoni.
As seen earlier, QoI-fungicides were not able to decrease the severity of powdery mildew when compared to the non-treated control. Results from the efficacy trials were confirmed by Dr. Melvin Bolton at USDA-ARS in Fargo, N.D., who was able to detect a cytb G143A mutation causing QoI resistance in E. polygoni. With the confirmed QoI-fungicide resistance in powdery mildew, it is now very important to consider good resistance management practices.
The following principles and tools are very general and can be applied to many other crops and pathogens. They are based on a solid Integrated Pest Management (IPM) program utilizing all available tools ranging from cultivar selection, cultural practices to scouting and pesticide rotation.
The foundation for a successful resistance management program is a good understanding of the different chemical groups, their mode of action and their potential for resistance. Many online resources, including Web pages of pesticide manufacturers, provide good information for selected products. More general information is provided by the Fungicide Resistance Action Committee at www.frac.com and includes recommendations and management tools to prevent fungicide resistance. The committee also grouped chemicals based on their mode of action and target site within an organism and developed a code system. The FRAC code can be found on every fungicide label and allows the applicator to easily distinguish between chemical groups.
The mode of action influences to a certain degree how fast resistance can potentially develop. Products with single-site activity such as QoI-fungicides (FRAC code/Group 11) are known to lose efficacy more rapidly, because it is easier for the target organism to overcome the chemical’s mode of action. Despite the potential for resistance, these products are widely used for many reasons. Initially they are highly effective against multiple pathogens. They have systemic properties allowing the chemical to be absorbed by and to a certain degree be distributed throughout parts of the plant. These chemicals also exhibit a longer residual activity than products with contact activity.
Nevertheless products with contact properties have the advantage of multi-site activity but need to be reapplied to the plant in regular intervals since they are not systemic and new growth needs to be protected. Some multi-site fungicides such as sulfur have never been associated with the development of resistance and therefore can serve as a tank-mix partner for at-risk chemicals to reduce resistance.
It is important to rotate or alternate at-risk fungicides with fungicides in different groups and based on different mode of actions. In addition, the risk for resistance can be reduced by tank mixing different fungicide groups or using premix products, but the amount of an at-risk product should be considered. Caution needs to be taken in case of cross-resistance. This form of resistance renders a pathogen less susceptible not only to one specific chemical but to all products with the same FRAC code.
Part of a successful resistance management program is the right application timing. Applications should be based on scouting and if available on prediction models. Scouting and monitoring fields and surrounding areas for signs of diseases and observing weather patterns allow for preventative applications and minimize the need for curative applications. Curative applications or applications after the pathogen is established of at-risk products should be avoided since this can potentially increase the risk for resistance. It is important to use at-risk fungicides early on when pathogen populations are low. Delaying as well as omitting subsequent application with other modes of action can lead to an increase in severity. In between applications, fields should be monitored and assessed for control efficacy. Product failure or reduced efficacy might have many reasons besides fungicide resistance, but it is important to investigate the cause.
Agronomic practices can help to reduce fungicide applications and therefore the development of fungicide resistance. Selecting disease-resistant cultivars, promoting a healthy crop by applying sufficient fertilizer, improving soil properties and managing irrigation will minimize stresses rendering the crop less susceptible to the disease.
Some of information given above is summarized in the product label. It is crucial to follow the instructions printed on the pesticide label to help prevent resistance. The label gives specific recommendations for application intervals, rates and potentially other useful information, such as the use of surfactants, fungicide rotation partners, or the addition of sulfur. Reducing the recommended rate or changing the application interval will increase the likeliness for resistance as do poorly maintained and calibrated equipment or insufficient carrier volume resulting in poor coverage.
The development of fungicide resistance is driven by several factors, including the biology of the pathogen, the mode of action and the management practices. Despite these challenges, it is possible to prevent and to delay resistance with the right knowledge and a solid IPM program.
Editor’s note: Neher is manager of Ag Technologies at The Amalgamated Sugar Company in Boise, Idaho.