I’m receiving reports of powdery mildew observations in Wisconsin vineyards. While we do not often associate drought conditions with grape diseases, powdery mildew is an important exception and can be a serious problem during periods of dry weather. The reason for this has to do with the biology of the powdery mildew pathogen. It does not require water on leaf or fruit surfaces to cause infections. While it does require humidity for spores to grow and infect, it can still spread and thrive in drought conditions. The pathogen often hides in the shadier areas of the canopy, so infections can be missed if you are only looking at the most outward growth when scouting. If control measures are not taken on susceptible varieties, it can reduce fruit quality and yields, and ultimately reduce vine growth and winter hardiness. Reducing dense canopies will decrease powdery mildew by increasing air circulation and increasing UV light and fungicide penetration.

With growing concerns of fungicide resistance development, many WI grape growers may consider expanding their current management tool kit. In 2021, my lab submitted a few powdery mildew samples for fungicide resistance testing by the FRAME Networks team and we received confirmation that we do have strobilurin (FRAC 11; QoI fungicides) resistant isolates in Wisconsin. While these results only represent a very small subsample, this data will help inform how we continue to manage this disease in the future. In fact, upcoming research in my program will monitor fungicide resistance in vineyards across the state of Wisconsin. FRAC 11 chemistries are classified as high-risk for fungicide resistance development. This mode of action class is effective against several grape diseases we commonly see in Wisconsin; however, it should be used strategically to avoid overreliance and resistance development. This class of chemistry is an effective disease management tool, and we want to extend its efficacy for as long as possible. When using FRAC 11 products to manage fungal diseases remember to 1.) avoid back-to-back applications from this FRAC group (this principle applies to all single site fungicides) and rotate with other chemistries, 2.) tank mix other modes of action with FRAC 11 and do not make solo applications of FRAC 11 fungicides, and 3.) consider pre-mixture fungicide products that include 2 (or sometimes 3) different FRAC groups to reduce the reliance on a single mode of action.

Over the last several years, fungicide chemistries specifically targeted for powdery mildew control have been introduced. These chemistries represent a unique mode of action for powdery mildew control, and they are not registered for other fungal diseases of grape. These products have been tested in other crops, less so in grapes, and are rated “good (G)” to “excellent (E)” for powdery mildew control in the Midwest Fruit Pest Management Guide.

Gatten (flutianil) is manufactured by Nichino America Inc. and label for powdery mildew on grape and other specialty crops. It has a unique mode of action and is labeled under FRAC group U13. This product has a 14-day PHI and a 12-hour REI for grapes. This product can be a good rotational option, but fungicide resistance management is still required. Avoid back-to-back applications of this chemistry.

Quintec (quinoxyfen) is manufactured by Gowan Company LLC and labeled for powdery mildew on grape and other specialty crops. It is a FRAC 13 product and considered medium risk for fungicide resistance development. Back-to-back applications should be avoided and should be rotated with chemistries from other FRAC groups. This product has a 21-day PHI and a 12-hour REI for grapes.

Vivando (metrafenone) is manufactured by BASF and labeled for powdery mildew on grapes and other specialty crops. It has a unique mode of action and is labeled under FRAC group 50. This product has a 14-day PHI and a 12-hour REI for grapes. Vivando is a good rotational option, but fungicide resistance management is still required. Avoid back-to-back applications of this chemistry.

Torino (cyflufenamid) is manufactured by Gowan Company LLC and labeled for powdery mildew on grape and other specialty crops. It has a unique mode of action and is labeled under FRAC group U06. This product has a 3-day PHI and a 4-hour REI for grapes. Fungicide resistance management is required when using this product; avoid back-to-back applications of this chemistry.

This article was posted in Disease, Grapes and tagged disease, Grapes, Leslie Holland, Powdery mildew.