Assessing Fungicide Resistance in Your Vineyard

Gabriel Torres, UCCE Tulare & Kings Counties

Powdery mildew (PM), is a globally important disease of grapes, which is caused by the fungus Erisyphe necator. This fungus only infects living cells and cannot be cultivated on artificial media which makes it difficult to study. It highjacks the plant’s cells and diverts their food resources to the fungus. However, if the plant cells die, then the fungus colonizing those cells will also perish, which is why it is considered an “obligate parasite”. In this way PM is different from many other fungi which first kill their host, and then feed on the dead material.

Table, wine, and raisin grapes are all vulnerable to PM. Powdery mildew infects all green tissues of the plant, including leaves, unlignified canes, the rachis, and unripe berries. Colonization of the berries can cause a range of damage depending on severity, and colonization of the rachis can inhibit ripening and diminish aesthetic quality of table grapes. Severe powdery mildew infections in berries before veraison can cause cracking and result in severe sour rot infections at maturation. Leaf infections can also lead to a reduction in the photosynthetic capability of the leaves. A PM infection can harm the grapevine and the grapes from a variety of different angles.

Sulfur has long been used to control PM, and it is still the most commonly used fungicide in California vineyards. In 2016 sulfur accounted for 61% of fungicides applied in the state (Figure 1). Copper (another broad spectrum fungicide) is the second most used, accounting for 13%. There are also various synthetic chemical fungicides available for use in California, including the following: myclobutanil (150,109 acres), quinoxyfen (133,192 acres), trifloxystrobin (126,015 acres), tebuconazole (118,240 acres), boscalid (112,279 acres) and pyraclostrobin (111,919 acres).

When comparing synthetic fungicides to sulfur and copper based fungicides there are a couple major differences. Synthetic chemical fungicides can be more effective and longer lasting than sulfur and copper based fungicides. However, unlike sulfur and copper, synthetic fungicides work by interfering in a specific biochemical pathway. This means that if the PM has a new mutation in that pathway it can enable it to become partially or completely resistant to that fungicide.  As this mutation allows the PM to better survive a fungicide treatment, it can quickly spread within a vineyard. Leading to the observable loss of the effectiveness of that fungicide.

This effect becomes compounded as many different synthetic fungicides can all target the same biochemical pathway. This means that a mutation leading to a loss of effectiveness in one fungicide often leads to the loss of effectiveness of many fungicides. This has led the Fungicide Resistance Action Committee (FRAC) to group together fungicides which demonstrate potential for cross resistance. These FRAC groups will be displayed on the label of all fungicides. Among the FRAC groups available for PM, resistance has been reported for FRAC 3, DeMethylation Inhibitors (DMIs); FRAC 7, SuccinateDeHydrogenase Inhibitors (SDHI); FRAC 11, strobilurines; and FRAC 13, azanaphthalenes. The most common and widespread resistance found in grape PM occurs in strobilurines.

In order to reduce fungicide resistance development, growers need to rotate among the different groups. The loss of one group can result in the necessity to use more intensively from the remaining groups. This led to an increased risk of developing resistance in the remaining groups. From 40 PM samples collected in the San Joaquin Valley in 2019, only 5% were sensitive to strobilurins (FRAC 11), meaning 95% had developed resistance. The 2019 data also showed how quickly resistance can spread within a vineyard. Thirteen samples had mixed populations, meaning some individuals within the vineyard had strobilurins resistance and some did not, before veraison. At harvest, all samples from previously mixed vineyards now only had strobilurins resistant powdery mildew. While these results are based on less than 50 samples, it does demonstrate the speed at which resistance can spread.

If you want to understand what PM resistances have developed in your vineyard and help your local UCCE viticulture advisors collect data on PM resistance across the SJV, we are looking for PM samples. In collaboration with the FRAME Network (Fungicide Resistance Assessment, Mitigation and Extension), your local UCCE viticulture advisors are evaluating PM samples from the SJV. The 2020 goal is to sample all counties in the SJV to have a better understanding of PM resistance across the Valley.

How can you contribute?

Three samples per year are suggested. The first sample should be collected before any spray against PM has been conducted, to have a baseline of the new growing season population. The second is recommended at bloom to see if resistant PM populations are increasing and implement corrective actions to reduce fungicide resistance at this stage. Finally, at veraison or at harvest to see the final population status and determine what to expect during the next season. Specific fields’ results will be confidential and only provided to each grower. Individual results will be used to generate county wide data that will be made available to the public. These results will be used to track the extent and any expansion of fungicide resistance within the San Joaquin Valley and throughout CA.

Sample kits for the 2020 growing season will be free of charge to growers. The only cost associated is that of the sampling, as it needs to be done by each grower. This process is fast and only requires the use of disposable gloves (provided by the grower). With the gloves on, the sampler needs to rub the canopy and cluster zone with the gloves throughout the sampling row. The sampler should make sure to touch areas of the canopy where PM normally develops or has already started to develop. Without removing the glove, the sampler, or another individual, should swab the glove with the provided cotton swab. When done swabbing the glove, the cotton swab should be resealed in the provided container, and the gloves disposed of properly. Collected samples can be shipped directly to the USDA laboratory in Corvallis or sent to the UCCE office in Tulare, Fresno, or Madera.

Thank you for your support, cooperation, and contributions.

UC IPM Fresno/Madera Powdery Mildew Index

The UC IPM is currently redesigning its website to a more mobile friendly version, and Powdery Mildew Index (PMI) has not been updated in the old UC IPM site. Please refer to the new PMI index here:

https://www2.ipm.ucanr.edu/weather/grape-powdery-mildew-risk-assessment-index/