Early Season Black Rot (revised)

I received three emails from different vineyards (Southern, Northern, and Central VA) with black rot pictures.  Yes, it is a little early for us to talk about black rot, but we had very warm period at the end of March, with rain event (on 24-25th) that are warm and long enough for black rot.  I counted three black rot infection events at our location so far, thus it is not a total surprise to see it.

If you happen to have black rot now, what can you do?  Unlike Phomopsis, black rot pathogen can produce spores after 2 weeks of infection under optimal conditions (> 70F), and if you see the symptoms now, chances are it is ready (or almost ready) for spore production.  Thus, even though we are not in the critical period for berry infection, the spores from this generation will be there for berries.  (yes, it is odd to talk about bloom in the middle of April, but at this rate, it will come soon. Hopefully cooler weather will hold off a bit.  We will see…)

If you only see symptoms on a few vines, then you may ask your crew to go through vineyards (maybe as a part of shoot thinning?) to remove infected shoots and/or leaves and it should reduce the risk of next round of infection.  However, if you have many diseased shoots, this method probably won’t work.

If your vines are not close to pre-bloom, you can use mancozeb to provide protection on healthy tissues.  Unfortunately, captan or copper won’t provide much efficacy against black rot.  [also, please remember that we do not have any cure for infected tissues, regardless of diseases.]

For pre-bloom or at bloom spray, you can use mancozeb to protect your healthy tissues and add Rally if needed.  Mancozeb will provide a good protection, and Rally will provide a good kick-back activity (i.e., stop on-going infection after rain).  Since Rally is a DMI (or SI) material, I am assuming that other DMI (such as tebuconazole, Mettle, Revus Top, etc) (note: in the earlier post, I was listing other chemicals by mistake, sorry!) does have a similar kick-back, but I do not think we have a good data for it.  (If I can get a culture going, I will start testing on it this summer.)  The other option is a use of QoIs such as Abound, Pristine, and Reason (note: according to Dr. Baudoin, Reason probably won’t do much against black rot).  The QoI’s are not as good as DMIs in terms of kick back activities, but it should give you a good protection and some kick-back activities.  If you decided to go with a QoI, please mix with mancozeb since QoIs are more prone to resistance development than many other classes of fungicide.

As usual, the decision to mix a DMI or QoI to the mancozeb spray depends on the weather and also what is going on with your vineyard(s).  If you had powdery mildew issues in the past few years, adding a DMI will also benefit to the control of powdery mildew too.  Also, even if you use mancozeb, a rain storm may wash it off from the tissues.  Then a follow-up DMI application may make sense.  On the other hand, if we end up having a very dry period, and you have not have much powdery mildew in the past, the addition of a DMI or QoI won’t be needed simply because the black rot fungus requires water to successfully infect grape tissues.  You still need a material for powdery mildew, but sulfur may be sufficient.

Personally, I prefer to see the use of a DMI around bloom for powdery mildew.  It has a good kick-back activity against powdery mildew and black rot, thus, it makes sense to me to use it for this critical period.  For more detailed discussion on the use of these materials, please refer to my workbook, which you should be able to find it on the right-hand side of this blog.

Also, for the future reference, please make sure to remove your infected clusters out from your vineyard.  Dropping them off to the ground won’t do much.  Infected clusters will produce black rot spores that are capable of both airborne and rain splash dispersal for the entire season.

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