Powdery Mildew Resistance Testing

Collections last spring launched the start of a 2-year National Powdery Mildew Fungicide Resistance Testing, which is part of a project administered by the Ontario Apple Growers (OAG). Funding for this project is provided by Agriculture and Agri-Food Canada through the Canadian Agricultural Adaptation Program (CAAP) and the Pest Management Centre, Risk Reduction Program. All apple-producing provinces (Nova Scotia, New Brunswick, Quebec, Ontario and British Columbia) participated, with 16 Ontario sites randomly selected by the OAG from the five growing districts. In total, 45 sites across Canada were tested for powdery mildew fungicide resistance, including 17 in British Columbia, 6 in Quebec and 6 in Nova Scotia.

Similar to the apple scab resistance survey, cooperating growers agreed to leave 6-8 trees unsprayed for disease until "white tips" of powdery mildew infected shoots appeared (Figure 1). Samples were collected and sent to the Okanagan Tree Fruit Cooperative (OTFC), north of Kelowna, BC for resistance testing to both sterol inhibitor (SI) (Group 3) and strobilurin (Group 11) fungicides.

Figure 1. Powdery mildew symptoms on the leaves appear as white, felt-like patches of fungal growth with a pinkish-red discoloration or hue on the leaf margins.

Figure 1. Powdery mildew symptoms on the leaves appear as white, felt-like patches of fungal growth with a pinkish-red discoloration or hue on the leaf margins.

DNA screening using a standard method was successful for all 2012 orchards tested. The method detected the presence of the G143A mutation, which indicates a high level of resistance to strobilurins (Flint, Sovran and Pristine). With guidance from Dr. Danielle Hirkala from the OTFC, results were interpreted using baseline mutant genotype thresholds set to determine the sensitivity levels of powdery mildew to strobilurins:

  • If the mutant genotype (MG) is below the baseline susceptible average MG of 10%, then the population sample is considered susceptible to the fungicide.
  • If the MG is between the baseline susceptible average MG of 10% and the resistance threshold average MG of 50%, then the population has shifted toward resistance.
  • If the MG is above the resistance threshold average MG of 50%, then the population is considered resistant to the fungicide.

An alternative resistance test using spore germination bioassays had difficulty with contamination and low spore viability. OMAFRA specialists are working with the OTFC to develop an better sampling protocol. In 2013, we will be adjusting our collection method to improve spore viability in order to re-test the 2012 orchards as well as the 2013 samples. At that time, plated media treated with a SI (Nova, Nustar or Inspire), a strobilurin (Flint, Sovran or Pristine) or an untreated control will be inoculated with powdery mildew isolates from each orchard and the amount of sporulation will be determined.

Based on the project results (Figure 2):

  • Caution should be used with strobilurin (Flint, Sovran, and Pristine) fungicides; preliminary results indicate powdery mildew populations may be shifting or resistant in some orchards.
  • Presence of the G143A mutation indicates a high level of resistance to strobilurins; 6 out of 16 orchards tested in Ontario had levels of this mutation greater than 10%.
  • Powdery mildew management should:
    • start early season when buds are opening,
    • use fungicides as protectants only,
    • include broad-spectrum mildewcides (e.g., sulphur),
    • continue from green tip to terminal bud set,
    • always include a targeted program for susceptible varieties (e.g., Gala, Honeycrisp, Cortland, Idared, Paulared, Crimson Crisp, Goldrush, Fuji, Russet)

Graph showing Presence of the G143A mutation associated with strobilurin (Flint, Sovran, Pristine) resistance in 16 Ontario powdery mildew populations, 2012

Figure 2. Presence of the G143A mutation associated with strobilurin (Flint, Sovran, Pristine) resistance in 16 Ontario powdery mildew populations, 2012 (text version)

Remember that resistance is dependent on how the fungicide was used. Though some orchards in your area may show resistance to a product, don't assume it's in yours! Each orchard needs to be individually tested for resistance and each product needs to be evaluated. Also, although an orchard may be resistant in laboratory tests, it does not mean control failure is occurring in the field. It will eventually fail, however, if use of the product is continued. Testing for powdery mildew fungicide resistance can be challenging, expensive and, at times, inconclusive. However, it can be a very useful tool to make appropriate control strategy decisions.

This work was in partnership with Ontario Apple Growers, Apple Growers of New Brunswick, Nova Scotia Fruit Growers' Association, Fédération des producteurs de pommes du Québec, British Columbia Fruit Growers' Association and the Apple Working Group of the Canadian Horticulture Council. We gratefully acknowledge Danielle Hirkala at the Okanagan Tree Fruit Cooperative. Funding has been provided by Dow AgroSciences Canada, Syngenta Crop Protection E.I. DuPont Canada, BASF Canada Inc and Bayer CropScience Inc. Technical assistance by Ken Wilson, Lindsay Pink, Brian Sutton, Frankie Cooper, Michelle Linington, Rebecca Vandertoorn, Carly Decker and Tyler Jenner.



For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca