Controlling Powdery Mildew in the Vineyard

Powdery Mildew (Uncinula necator) can be a challenging disease for vineyards to control, specifically in California wine regions, where the disease a persisted as a major pest for more than a century. Early, initial symptoms of Powdery Mildew infection are typically seen as brown patches on the top of the leaves, followed by white mycelia on the bottom of leaves. After that, infected areas produce white, powdery spores, which can also affect the berry surface. Early fruit infections can lower yields and berry flavor. Infections on leaves can lower photosynthetic rates, and as a result, slow sugar accumulation.

With all types of fungicides labelled for Powdery Mildew control, they will be most successful when deployed as a crop protectant, before symptoms appear. Once powdery spores appear on leaves or fruits, infection is already underway and infected portions of the plants will already be compromised.

Powdery Mildew Model as Management Tool

The Gubler-Thomas Powdery Mildew Risk Index, developed at UC Davis, provides a useful tool to make smart decisions regarding preventative fungicide sprays based on the weather and optimum temperatures for conidial reproduction. The model uses these temperature thresholds to calculate a daily (0–100) risk index which can be used to adjust intervals between sprays.

• An index of 0–30 indicates low risk of disease. Chemical fungicides may be used at intervals of 21 days, or label max, and sulfur may be used at 14–21 day intervals.
• An index of 40–60 indicates moderate risk of disease.Chemical fungicides may be still be used at intervals of 21 days, or label maximum, at this risk index, but sulfur,if used, should have a reduced interval rate of 10–17 days.
• Above 60 indicates high risk which should results in a 14 day interval, or label minimum, for chemical fungicides, and 7 days for sulfur.

Using the Powdery Mildew Risk Index to adjust spray intervals has been proven, in many cases, to reduce overall number of fungicide sprays compared to weekly rotations, while maintaining effective control.

Calculating Daily Risk Index Levels

The Gubler-Thomas Powdery Mildew for grapes is intended to be a daily risk index which should be recalculated, as a running total throughout the production season. The rules for calculated are listed as follows from UC IPM recommendations:

• If fewer than 6 continuous hours of temperatures occurred between 70° and 85°F, subtract 10 points.

• If 6 or more continuous hours of temperatures occurred between 70° and 85°F, add 20 points.
• If temperatures reached 95°F for more than 15 minutes, subtract 10 points.

• If there are 6 or more continuous hours with temperatures between 70° and 85°F AND the temperature rises to or above 95°F for at least 15 minutes, add 10 points. (This is the equivalent of combining points 2 and 3 above.)

The total maximum risk index level is 100 and the minimum is 0. The maximum amount of points that can be added in a given day is +20 points, and the most that can be subtracted is -10.

Temperature data can be obtained from in field thermometers, or temperature loggers or through weather stations placed near the vineyard. Sensors placed within the plant canopy, at the leaf level will typically give the most accurate data, but ambient temperatures are still very useful when in-field sensors are not practical or reliable. Some weather station models will calculate daily risk. Growers seeking an option without weather stations should consider using the Pest Prophet mobile app.

When to Begin Using the Powdery Mildew Model

One management decision regarding the Powdery Mildew Risk Index Model is when, in the season, to begin using it. For disease infection to occur, three requirements need to met: a suitable environment, a suitable host, and a pathogen needs to be present. This concept is referred to by plant pathologists as the disease triangle.

This can be a useful concept when determining when to begin calculating the Powdery Mildew Risk Index. For pathogen presence, it is easy to see after infection occurs, but because we are looking to prevent any infection, we generally assume the pathogen is present in any region where grapes are commonly grown. The host factor would refer to any susceptible plant tissue, for grapes this is any time after bud break, as new tender leaves can be very susceptible. The environment factor is what is determined with the help of the Powdery Mildew Risk Index Model.

One option for when to begin using the model is to use historical reference points for when infection has most often occurred in the season for a particular ranch. However, this method is not very accurate and is the kind of guesswork that is trying to be avoided by using weather based models. Another more conservative approach, is to assume infection has begun immediately after bud break and begin calculating the Powdery Mildew Risk Index model at this point in the season, using it to adjust spray intervals for the entire stretch that leaf tissue is present on the vine. This method does typically work well, but it can lead to potentially over-spraying early in the season, when fewer or no sprays may be needed. This  can sometimes cause phyto-toxicity (chemical spray damage) in young, emerging, leaf tissue or can disrupt natural predators.

As another option, UC IPM recommends the following calculations for beginning the index phase:

Initiating the Risk Index

After you find powdery mildew infections caused by ascospores, an epidemic will begin (conidia will begin generating new infection sites) when there are 3 consecutive days with 6 or more continuous hours of temperatures between 70° and 85°F as measured in the vine canopy.

1. Starting with the index at 0 on the first day, add 20 points for each day with 6 or more continuous hours of temperatures between 70°and 85°F.
2. Until the index reaches 60, if a day has fewer than 6 continuous hours of temperatures between 70° and 85°F, reset the index to 0 and continue.

1. If the index reaches 60, an epidemic is under way. Begin using the spray-timing phase of the index.

Powdery Mildew Model for Ascopore Phase of Infection

So far in this post, we have been referring to grape powdery mildew infection solely in the conidial phase of the fungal pathogen’s life cycle. The conidial phase refers to the reproductive cycle of Powdery Mildew wherein conidia (asexual spores) are produced on leaf surfaces and spread by wind to infect other leaves or grapes. This reproductive phase is associated with large scale epidemics, and because of this, solely using the Gubler-Thomas Risk Index model to time spray intervals is usually sufficient to predict large scale outbreaks.

At the end of the growing season, powdery mildew fungi will produce ascopores (sexual spores) which are able to overwinter in fruiting bodies called cleistothecia. (See picture below)

After overwintering period in winter, initial infection in spring is usually caused by ascopore infection. By modeling ascopore infection risk, growers are able to predict initial infection. Researchers have developed a Powdery Mildew ascopore risk model as a modification of the Mills table for Apple scab ascopore infection, by using 2/3 of the require leaf wetness hours. The modified Mills table is provided below (from UC IPM).

The first initial treatment should ideally be applied when infection environment requirements are fully met from the table above. After this the grower can transition to the conidial phase powdery mildew model. Using these two models together is the best way to fully prevent powdery mildew infection for the whole growing season without unnecessary fungicide sprays.

Using Pest Prophet to calculate Powdery Mildew Risk

The easiest way to begin using  pest and disease models on your vineyard is through the Pest Prophet app. Using the Pest Prophet app, you can have Powdery Mildew Risk calculated automatically for your vineyard daily, without needing to add any expensive weather stations or hardware. It is simple and easy to integrate into your decision making process, and will pay for itself many times over in saved chemical costs.