Vinews

No. 15 — July 15, 2019

Contents:

• Downy Mildew
• Cumulative Growing Degree Days for the Seven Grape Growing Regions of Missouri from April 1 to July 15, 2019

Downy Mildew

Downy mildew has infected a number of vineyards. The weather conditions have been conducive for infection and spread. Ideal nighttime and morning temperatures of 65 to 77°F, humid nights, and frequent rains in some areas.

Weather conditions over the next few days will slow downy mildew secondary development. Even though heat indexes will be over 100 the next few days, active downy mildew infections will continue to persist and sporulate when environmental conditions are once again conducive for growth and development. Within shaded grape canopies the leaf temperature is similar to the ambient air temperature. Therefore if the air temperature is 95°F then the leaf temperature is also 95°F. The spores (sporangia) produced by these secondary infections will likely die when exposed to the sunshine over the next few days. However, if you have active downy mildew colonies then you need to control these colonies.

Active secondary downy mildew infections should be controlled with a phosphorous acid fungicide. The phosphorous acid fungicide will “burn out” sporulating downy mildew colonies. Improved control can be enhanced by tank mixing a phosphorous acid fungicide with a protectant fungicide such as Captan. Remember that phosphorous acid fungicides only have activity against downy mildew. Do not apply more than two sequential applications of phosphorous acid fungicides. When active colonies are present use the higher rate of phosphorous acid fungicide.

Grape berries develop resistance to downy mildew at the pea-size stage of development. All other green tissue remains susceptible to downy mildew infections. This includes the berry stems (pedicels). If the pedicels become infected with downy mildew, the hyphae can grow into the berry resulting in leather berry. Leather berry symptoms are a hard dry berry void of downy mildew colonies. The pedicels also develop age related resistance to downy mildew but resistance occurs 2 to 3 weeks after the berries develop resistance.

Downy mildew outbreaks this season have a common theme. In most cases the growers have been dependent on Captan as the only fungicide. Your first thought may be that downy mildew has developed resistance to Captan. Resistance is unlikely and most likely the failure of protection is from wash-off under heavy rains and/or the lengthening of time between cover sprays. This growing season we have experienced a lot of heavy rainfall events. It is during these wet seasons that cover sprays need to be applied more frequently to provide continued protection from infections.

Besides applying protective sprays on a regular time interval, get out and scout the vineyard. If you find two leaves with downy mildew colonies after scouting 50 vines then you need to react. Minor or limited colony development can be cleaned up with an application of phosphorous acid fungicide. If rainfall and or weather conditions — heavy dew, cloudy and high humidity are predicted then consider a tank mix of phosphorous acid fungicide plus Captan. If you encounter a downy mildew explosion in the vineyard do not apply strobilurin fungicides (Abound, Pristine, Sovran). Downy mildew has a propensity to develop resistance to strobilurins. Other options for managing downy mildew are listed in the table below.

Cumulative Growing Degree Days for the Seven Grape Growing Regions of Missouri from April 1 to July 15, 2019

¹ Growing degree days at base 50 from April 1 to July 15, 2019. Data compiled from Useful and Useable at https://mygeohub.org/groups/u2u/tools. Click on link below to determine growing degree days in your area.

To determine the number of growing degree days accumulated in your area since April 1, use this tool.