Evaluation of Rapid Apple Scab Fungicide Resistance Monitoring

Two methods for determining the level of resistance to apple scab fungicides were tested in Ontario orchards. One method screens conidia from primary lesions collected from unsprayed trees and takes 7 days to give results. The assessment of practical resistance is assessed by determining the proportion of the population that is not inhibited by a specific dose of fungicide. When more than 40% of the isolates have reduced sensitivity, the orchard is considered to be at “practical resistance” and unreliable control will be provided by the fungicide in question. 

The other method screens ascospores collected from artificially or naturally overwintered infected leaves and takes 3 days. This method relies on comparisons between reference orchards (where the fungicide does or does not work) and test orchards. 

The first two years of the trial were a learning experience with several technical set-backs so no data were generated. The apple scab pathogen population was screened for resistance to myclobutanil (Nova) and dodine (Equal) in 2007 when 7 orchards from Niagara, Middlesex and Durham counties were successfully screened using the 7-day method. The populations in 3 of the 8 orchards had shifted from sensitive to fungicide tolerant. A 4^th site was still in the sensitive range but had experienced a significant shift. All 7 of the orchards were at baseline sensitivity to dodine. 

The 3-day method was used on only 6 orchards as the artificial overwintering protocol was unsuccessful and a second sample of leaves had to be collected in June, after the leaves from the wild-type (no fungicides sprayed) had been thoroughly mowed. It was difficult to interpret the results from the 3-day method because no reference orchards had been designated. However, the trend for the two methods was the same: orchards with a larger proportion of resistant isolates using the 7-day method also had a higher growth on the fungicide than those with a lower proportion of resistant isolates. 

Both methods can differentiate “sensitive” and “tolerant” orchards. The challenge is deciding what to do with “intermediate” rankings, the ones where control may break down with high disease pressure. However, this is a challenge with both techniques. 

How can the information generated by either method be used? When a grower feels that a particular fungicide is no longer effective, a screen for resistance can tell whether or not the population has started to shift and how close it is to “practical resistance”. When trying to decide whether to use a fungicide such as dodine in the spray program, a grower could have the orchard screened before the season started to determine whether the pathogen population is sensitive to the fungicide. This is what is being recommended in NY. When control failures occur, screening the pathogen population could be used to determine whether the failure was due to a buildup of fungicide tolerance or disease management problems. 

The main difference between the two techniques is the time frame when results can be provided. The Koller method relies on collection of primary infections. Even with the more rapid turnaround compared to the original method, the grower would not have results for most of the primary scab season, when critical decisions regarding fungicide use must be made. The line intercept method can provide this information before the growing season begins. 

The 3-day method provides a rapid and timely answer to the question of relative fungicide tolerance in an orchard, and orchards are ranked the same way as in the 7-day method. In order to use this method, “reference” orchards would have to be identified and used to rank test orchards. The artificial overwintering protocol is still being perfected. 

Acknowledgements:

The assistance of Margaret Appleby and Kathryn Carter of OMAFRA was invaluable in completing this project. Financial support by Dow AgroSciences and Cheminva Inc. and product donation by NORAC Concepts are also gratefully acknowledged.