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Stewardship of Powdery Mildew Fungicides

Stewardship of Powdery Mildew Fungicides in Perennial Crops

Gary Grove, Mark Nelson, and Chang-Lin Xiao
Washington State University
Irrigated Agriculture Research and Extension Center, Prosser, WA and Tree Fruit Research and Extension Center, Wenatchee, WA

Apples, apricots, cherries, grapes, hops, peaches and nectarines are plagued by the disease powdery mildew in the Pacific Northwest. With the recent advent of strobilurin and quinoline fungicides, further development and registration of members of the DMI (sterol-inhibitor) fungicide class, and the utilization of narrow range petroleum oils, Washington growers currently have a wide variety of chemical management tools at their disposal. While many of these new tools are highly effective, many of the most valuable fungicides carry an inherent risk of resistance problems. It has been known for some time that resistance to DMI fungicides (Ypema et al, 1997) has developed in some crops, but resistance to strobilurins has only recently become a commercial problem in grapes (Wilcox et al, 2003).

Resistance to quinoline fungicides has been documented on cereals in Europe (Hollomon et al, 1999). At risk are the DMI (Rally, Rubigan, Procure, Elite, Orbit, and Bayleton), strobilurin (Abound, Cabrio, Flint, Pristine, and Sovran), and to a somewhat lesser extent quinoline (Quintec) fungicide classes. Resistance develops differently in the former two classes. Resistance to DMI fungicides manifests itself as a gradual loss of control as the disease-causing fungi becoming more tolerant to the fungicides. Compounds typically perform brilliantly when first released, and then over a period of years slowly deteriorate in efficacy. As a result, growers must apply thecompounds at higher rates and/or tighter spray intervals in order to maintain control. Conversely, resistance to strobilurin fungicides is “all or none”.

Strobilurin compounds may perform brilliantly for a period of time, and then fail miserably. For example, a strobilurin fungicide may be effective during the current season but not during the subsequent season. Agriculture cannot afford to lose these new, recently registered fungicides. Powdery mildew is the most important disease of most of our perennial crops. The DMI, strobilurin, and quinoline fungicide classes comprise our first line of defense against these diseases. The risk of resistance to DMI and strobilurin fungicides is considered to be high. The mechanism of resistance to quinolines is not fully understood, but the resistance risk is currently considered low to moderate (Hollomon et. al, 1999). Add to this the fact that very few new powdery mildew fungicides are currently in the development “pipeline”, it becomes readily apparent that sustained profitability of our hop, tree fruit, and wine grape industries depends on the immediate application of fungicide resistance management strategies. We cannot afford to lose the old- and new DMI, strobilurin, and quinoline fungicides.

What can the grower do? Diversity and moderation in the fungicide program are keys to product preservation. Diversity and moderation mean 1) the use of multiple fungicide modes of action and moderation means 2) use the resistance prone DMI, strobilurins, and quinoline fungicides in a rational manner, respectively. It is important to limit the total number of applicationsof high-risk compounds per season and to limit the number of sequential applications of any single mode of action.

Fungicide resistance management is facilitated by use of multiple fungicidemodes-of-action. Each color represents a different mode-of-action. The spray programshould be as diverse and “colorful” as possible. In general, DMI resistance can be managed by limiting the number of applications to no more than 3 per season and no more than 2 sequential applications, tank mixing withother modes of action (oils, sulfur, carbonates), and by application at maximum labeled rates. Resistance to strobilurins can be managed by limiting the number of strobilurin applications per season (no more than 3 per season and no more than 2 sequential applications), use in single or block application in alternation with fungicides from a different group and use early in the season before powdery mildew is well established. Do not alternate strobilurin products as cross-resistance has been documented in thefungicide class. Good spray coverage is essential.

The most important concept to appreciate with regard to fungicide resistance management is that only a very small percentage of individuals in a pathogen population have the potential for resistance to any given fungicide mode of action. The objective of resistance management are to reduce the populations of pathogens exposed to a given mode of action as well as reduce the duration and frequency of that exposure, thereby reducing the opportunity for those few individuals with resistance potential to become predominant in the population.The bottom line for the grower or consultant:

• utilize cultural practices to reduce pathogen populations whenever possible. For example, removal of overwintering infected terminals by dormant pruning helps reduce the primary inoculum level of apple powdery mildew. Delaying the initial irrigation of cherries early in the spring may help to delay the onset of powdery mildew epidemics. Limiting grapevine vigor reduces powdery mildew pressure in vineyards.

• use (either as alternations or tank mixes) as many fungicide modes of action (classes) as possible in the disease management program.

• use fungicides protectively before powdery mildew has become a problem.

• do not use resistance-prone compounds to attempt to get a powdery mildew epidemic under control. Narrow range petroleum oils and carbonates are the best eradicative fungicides.

• limit the number of applications of resistance-prone fungicide classes to no more than 3 per growing season.

• adjust spray volume per acre based on the size and volume of the crop to attain excellent spray coverage.

• apply fungicides at rates specified on the fungicide label (do not reduce rates).

• avoid making more than 2 consecutive applications of DMI, quinoline, or strobilurin fungicides.

• include low resistance risk compounds (sulfurs, carbonates, and petroleum spray oils) inthe spray program as much and whenever possible.



Hollomon, D.W., Wheeler, I., Dixon, K., Longhurst, C., and Skylakakis, G. 1999. Defining the resistance of the new powdery mildew fungicide quinoxyfen. PesticideScience 51(3): 347-351.

Wilcox, W.F., Burr, J.A., and Riegel, D.G. 2003. Practical resistance to Qol fungicides in New York populations of Uncinula necator associated with quanitative shifts in pathogen sensitivities. Phytopathology 93:S90.

Ypema, H.L., Ypema, M., and Gubler, W.D. 1997. Sensitivity of Uncinulanecator to benomyl, triadimefon, myclobutanil, and fenarimol in California. PlantDis. 81:293-297.