Fungicide Resistance Management

Apple Scab resistance to the Sterol Inhibiting (SI) fungicides (Nova, and Nustar) has been reported in many apple growing regions including New York, and Michigan since the early 1990's. While SI's are still very effective in managing scab in most orchards in Ontario, preliminary research from a small set of samples collected in a few Ontario orchards during 2004, suggests that SI resistant populations of apple scab may be present. Agriculture and Agri-Food Canada, OMAFRA, and private consultants are working together to confirm whether SI resistance is present in these orchards and if so why. Further research is required to verify and quantify the levels of resistance in these orchards, and to determine the extent of SI resistance in Ontario.

So how do apple growers find out if SI resistant populations of apple scab have build up in their orchards? Unfortunately, testing is expensive and not available commercially. As a general rule, in orchards that have had repeated failures from using SI's , which can't be explained by application timing, coverage, rates, calibration, or weather, it would be best to avoid using these materials for the 2005 growing season and in the short term.

If SI resistance has been confirmed in your orchard, you can not rely on the post-symptom protection of fungicides in the SI class and this possibly includes the strobilurin class of fungicides (Sovran and Flint). Scab infections that occur early in the season can explode into major problems during a wet season. This is of concern where highly susceptible cultivars like McIntosh are grown and where inoculum potential is high. Preventing the establishment of scab is critical in orchards with suspected SI resistance. Growers with confirmed SI resistance should avoid using Nova, Nustar to manage apple scab. Although the SI's will still be effective tools for managing powdery mildew in orchards with SI resistance to apple scab, some research suggests that these fungicides may stimulate sporulation in resistant scab lesions when applied to control powdery mildew.

There ae several strategies that all apple growers should be implementing regardless of fungicide resistant scab populations in their orchards. These strategies can be used to help growers avoid the development of fungicide resistance.

Always rotate between fungicide families in a control program. Never apply more than 4 applications of SI's or strobilurins/season.Always use full rates of SI fungicides with ½ rate of protectant fungicide. Start fungicide sprays at green tip before the first infection period. As a rule of thumb, apply fungicides preventatively not curatively. Using fungicides curatively, adds selection pressure on the fungus, and you may be selecting for fungicide resistant populations. This means that the orchard floor must be cleaned up (prunings) and made sprayer ready in advance of green tip. Always use full label rates of fungicides. Reducing the rates of fungicides, will likely increase the number of resistant colonies or resistant individuals present in an orchard.

• Alternate row spraying is not recommended, as this technique can leave a shadow of unprotected canopy
• Spray in a light rain (with materials like Captan Sulfur or an EBDC) if necessary to protect new foliage during infection periods lasting more than 2-3 days. SI fungicides and strobilurins should never be applied in the rain because they must dry on the leaf and be absorbed to be effective.
• Don't extend your spray intervals beyond the label recommendations. This could mean a 5-7 day interval in a wet spring like we had in 2004.
• Urea can be applied to leaf litter on the orchard floor in November or early spring to accelerate leaf decomposition and reduce the over-wintering apple scab inoculum. New York recommends 40lb/ac. of urea sprayed onto the ground. Ascospore production can be cut by more than 75% according to Dr. David Rosenberger. Make sure you add in the nitrogen component in urea when tallying N fertility inputs for the treated acreage. Flail mowing may also reduce inoculum levels by destroying leaf litter.
• Re-calibrate your sprayer and check for coverage of the entire tree.

SI's are still very effective for managing scab in most Ontario apple orchards, and they remain an important tool for apple growers to manage other diseases. Implementing a fungicide resistance management strategy will delay or help growers avoid the development of fungicide resistance, and keep our recommended fungicides working effectively.