UCCE Central Sierra Agriculture

Powdery Mildew Bulletin

Powdery Mildew in Vineyards

OOPS! I have some mildew. Now what??

This season’s weather (spring rains=large canopy + cool summer= mildew conditions) has meant some of you have encountered more powdery mildew than in previous seasons. Now that we have verasion, should you be worried? I called Dr. Doug Gubler, UC Cooperative Extension Plant Pathologist, and asked him what a grower should consider IF he/she finds mildew at this stage of the season. Here is what Doug had to say:

  • Powdery mildew lesions continue to sporulate for 28 days after the lesion first forms.
  • No new infections develop on berries once verasion occurs, except on the rachis which is not a big deal for wine grapes.
  • 3% or more mildew can result in off flavors in wine.
  • At this time of the year, with cooler temps. and shorter day lengths, the mildew mycelium on berries switches into “survival mode” and becomes thick and rough which is (obviously) undesirable.
  • If a grower has 3% or more mildew at verasion, they should either prune those bunches out or consider putting on a treatment.
  • Eradicant treatments may consist of either JMS Stylet Oil or Kaligreen.
  • JMS Stylet Oil can be applied at 0.5 -2%; a 0.5% oil treatment should not result in phytotoxicity, even if temperatures exceed 90°F. (Oil can produce phytotoxicity at higher concentrations with high temperatures.)
  • Doug said Kaligreen has been an “intermediate” product in his experience. He said some people love it and some hate it, depending on who you talk to.
  • COVERAGE is key. This means high volumes and targeting the fruiting zone with your nozzles. You may need to pick leaves in the fruiting zone to target bunches. Doug said a volume of 600 gal. per acre is not unheard of if the canopy is thick, in order to get adequate coverage. (I replied no one would use 600 gal/acre here and he said “whatever it takes to get the coverage”)!!
  • What about sulfur at this point in time? Doug said the old recommendation used to be 600 gal water/ac + 5 lbs. microthiol sulfur + some soap to break surface tension.