Fungicides are essential for maintaining healthy crops and reliable, high-quality yields. They form a key component of integrated crop management, and their effectiveness must be sustained as long as possible. Fungicide resistance is a major threat to disease control in all crops, including grapes.
How Does Resistance Develop?
The development of fungicide resistance is influenced by many factors including the cropping system, pathogen, type of fungicide, climate, and fungicide use pattern. Within the vineyard, a few isolates in the fungal population are naturally resistant to fungicides. When fungicides are applied, most but not all of the sensitive isolates are killed. However, the few naturally resistant isolates survive and reproduce. When the next fungicide application is applied any remaining sensitive isolates are killed but the resistant isolates survive. This pattern continues for each fungicide application until eventually the entire population is resistant to the fungicide. Resistance can take from 3 to 10 years to occur. Overuse or misuse of fungicides with a single mode of action can shorten the length of time for resistance to develop in the fungal population.
How Do I Manage QoI Fungicide Resistance?
Ideally management strategies should be put into place BEFORE resistance is detected in the vineyard. However, all is not lost if resistance is detected. It’s important to remember that the presence of QoI resistance in the vineyard does not mean control failure is eminent. Rather, it means that the use of strobilurin fungicides should stop or the use pattern should be modified.
Several strategies can be implemented to slow QoI resistance. Every year growers should evaluate the effectiveness of their previous years spray program. When developing the current year spray program consider what worked and what didn’t work in the previous year and make modifications accordingly.
When developing a fungicide spray program consider implementing the following fungicide resistance management strategies:
1. Calibrate the sprayer regularly.
2. Reduce the use of QoI fungicides. A reduction of even one application of a QoI fungicide can slow the development of resistance.
3. Avoid applying QoI fungicides during the critical period for fruit disease if resistance is present in the vineyard.
4. Tank-mix QoI fungicides with a protectant fungicide such as captan or mancozeb.
5. Rotate modes of actions (i.e. FRAC groups). If resistance is present in the vineyard avoid spraying two sequential applications of a QoI fungicide.
6. Avoid extending spray intervals beyond the recommend interval provided on the label.
7. Do not use rates above or below the recommend range provided on the label.
A management decision table for vineyards with QoI resistance is available on the Fruit Pathology Program website. The table was developed to assist growers with making management decisions for grape powdery mildew when faced with challenges associated with QoI fungicide resistance. If resistant fungal populations are low enough, resistance can be reversed. However, reversal of resistance is not easy and requires that no QoI fungicide be used in plots with resistance for at least three years.
Lastly, fungal pathogens donot respect fence lines! Fungal spores move between vineyards by wind, which means resistance can also move between vineyards. Therefore, fungicide stewardship needs to extend to your neighbors. Work with neighboring growers to identify spray programs that reduce the risk of resistance developing in all surrounding vineyards. Talk with your neighbors about what has worked (and not worked) for controlling powdery. Be open and honest with your neighbors if resistance has been detected in your vineyard. If we hope to have the long-term conservation of fungicide effectiveness everyone will have to work together!
Powdery Mildew Fungicide Resistance in Ohio
Last year the Fruit Pathology Lab surveyed seven vineyards in Ohio and tested 27 samples for powdery mildew QoI resistance (also referred to as strobilurin or FRAC 11 resistance).While the 2019 sample size was small, QoI resistance was detected in six of the vineyards. Overall, 81% of the samples tested positive for QoI resistance and 4% of the samples had resistance and sensitivity (mixed sample) to QoI fungicides.
In addition to QoI resistance testing, a subset of the samples was tested for succinate dehydrogenase inhibitor (SDHI) resistance (also referred to FRAC 7 resistance). Approximately 7% had increased tolerance to boscalid and fluopyram and less than 3% had increased tolerance to boscalid only. SDHI testing of the 2019 samples is projected to be completed this spring.
As a continuation of the Fungicide Resistance Assessment,Mitigation and Extension Network (FRAME), the Ohio State University (OSU) Fruit Pathology Program will continue surveying Ohio vineyards for QoI resistance in 2020. A rapid DNA test has been developed by FRAME collaborators to test for QoI resistance. The sampling process is simple and requires only a few minutes to take a sample and place your sample in an overnight mailer to be sent back to OSU or directly to the testing lab in California. All the supplies needed to collect the samples are provided to the grower and shipping costs as well as test costs are free of charge. Results will be made available to growers within 7-21 days, allowing growers to make immediate management decisions and/or management decisions for next year’s growing season. A test for SDHI resistance is also available, although the sampling process is a bit more complex and results can take up to 3 months to be reported. — By: Melanie Lewis Ivey, Department of Plant Pathology, Ohio State University-Wooster
If you are interested in having powdery mildew from your vineyard tested for fungicide resistance, please email Melanie Lewis Ivey at firstname.lastname@example.org. If you had your vineyard tested last year, we are interested in testing your vineyard again this year. All results from individual vineyards will be confidential.