Friday, June 13, 2014

Disease risks from last few days, and a plan for next spray

Boy, what a week!
Winchester area received about 17 hours of wetness with average temperature of 20 C (68F) on the 11th, 14 hours of wetness with average temperature of 22 C (72F) on the 12th, and right now on-going wetness with 17hour + with an average temperature of low 20C (68F).

These events are warm and long enough for downy mildew, Botrytis, Black rot and Phomopsis.  On top of that because of evening rain events, night time conditions have been favoring downy mildew development.  Our main concern is downy and black rot since our clusters are currently in critical period.  These berries are susceptible to the infection by these pathogens.

It looks like the weekend weather is promising, and a 10-day forecast is calling for a chance of rain again on Tuesday... what can we do?

For our vines, the last application was made 10 days ago, and we received a total of 1.47 inches of rain at the site.  Last application was mancozeb + sulfur + Phosphite (Prophyt, Phostorl, etc.) + Rally. Our Chardonnay is on its 6th leaf, clusters are about BB side, and the site is not good.  We have chronic issues with powdery and downy.

Plan A) mancozeb + sulfur + Phosphite + Rally. 
  • This is the second Rally (DMI) application for the season, but since they are BB-size, I am hoping that this will cover most of the critical period.  Knowing that we had 1.5 inches of rain since the last spray, I may want to be safe.  If I decided to do it, this will be our last Rally application for the season.
Plan B) mancozeb + sulfur + Phosphite + Pristine
  • We have not used Pristine this season.  I know that our downy mildew populations are resistant to QoI (one of components in Pristine), but considering potential black rot infection, this would make a good rotation partner.  
  • Please note that although a QoI has a kick-back activity against black rot, Rally has a better efficacy based on a study.
Plan C) Ridomil Gold MZ + mancozeb (with rate adjustment) + sulfur + Rally (or Pristine)
  • With all the rains we received, this may be the time to consider Ridomil.  I tend to rely on a phosphite more because of the resistance issues (relatively speaking phosphite has a lower risk), but Ridomil tends to have better kick-back activities, and it is always nice to mix things up.
  • Yes, you can mix Ridomil and a phosphite too!  I typically do not recommend it, simply because it tends to become an expensive application.   I rather hit it with a phoshite first and then hit it again with Ridomil.
Please note that these are just examples, and the actual decision needs to be based on what and when you have sprayed in the past, variety, site, amount of rain you received, etc., etc.  For example, I am not considering Botrytis specific chemicals because Chardonnay passed its flowering time, but if I need to make a plan for Cab sauv. for this week, I will add something like Vanguard.

Also don't forget to check for grape berry moth!  I have heard from several people about webbing in clusters.

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