Grape Disease Risks:6/20 – 6/26/2024, PHI, and Zirum

Here are snapshots from the last week, showing the number of days with the risk of each disease: Ph = Phomopsis, PW = Powdery mildew, BR = Black rot, and DM = Downy mildew. N/A means there is no data or some issues with the NEWA. This year, we have been using a different downy mildew model due to inconsistency with the NEWA model. This model uses the past five days of weather to estimate spore production and the next seven-day forecast to estimate the infection risk.

Due to relatively low mean temperature and humidity during the nighttime, risks of downy mildew have been low in the past month or so. However, several growers mentioned to me that they saw powdery mildew. If you have severe powdery mildew issues, the best approach is to use Sulfur, Copper, Potassium Salt, or Oil. If you use oil, do not mix it with sulfur or captan (or have at least two weeks in between sprays). Do not use DMI or SDHI fungicides against ongoing powdery mildew. You will increase the risk of fungicide resistance at your vineyard.  Please see the post-bloom grape disease management slides here.

Speaking of the slide, I have to change the next Virtual Viticulture meeting, which was originally scheduled for 7/4. I have a feeling nobody will come. 😉 We will postpone it until 7/11/2024. Later today, I will send emails to the Viticulture email list and the registered meeting participant list.

Pre-Harvest Interval

Please keep an eye on the PHI. Mancozeb products have a 66-day PHI; at this point, you cannot use it for cultivars harvested in late August. Please see our Pest Management Guide for the list of the PHI and REI, or better yet; please use, which automatically calculates both PHI and REI once you enter your spray record.

Zirum (and captan) registration

As we discussed at the VVA meeting, the EPA is reevaluating the uses of Captan, Ziram, Ferbam, Thiram, and Acephate (link below). They indicate they want to cancel all uses for some or all of these materials. EPA is now offering a comment period for the public to counterbalance this initiative. I invite you to please provide your feedback on the utility of these materials for local and national grape growers.

Go to this website:

In the middle right, there is a green button that says “Submit a Formal Comment”. When you click on this link, there are options – thiram, ferbam, ziram registration reviews; click on “Ziram Registration Review” – click on “select”. Then there is an open box to submit comments – add email – then click the box at the bottom that says “I read and understand the statement above”, then click “Submit Comment”

These are examples for comments:

I will come up with my own comments, but please spend some time voicing your opinions. One for Captan is coming up soon (if it hasn’t opened already).

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