Wednesday, May 13, 2020

Disease management after bad frost events?

Unfortunately, many of our vines suffered from frost events that happened over the past weekend (and this morning for some of us). I think I lost about 30-40% of Chardonnay shoots. :( 
Here are some disease management tips for dealing with frost-damaged vines, especially if your vines have shoots with several to many leaves open.

  • Minor damage: E.g., < 15% of shoots affected.
    • You probably do not need to alter your spray schedule.
  • Moderate damage: E.g., 20-50% sporadic damage throughout a block. Some shoots are heavily damaged, but others are OK.
    • You may keep your regular spray schedule. If we have an extensive rain event(s), there is some risk of Botrytis infection on the damaged shoots. This pathogen can produce spores on the dead tissues. If you are concerned about a potential Botrytis infection (i.e., you have a Botrytis-prone cultivar and the weather forecast says there will be a big rain event coming soon), you may want to use either captain or copper instead of mancozeb for next spray, since these two have some efficacy against Botrytis.
    • If you keep using either captan or copper as one of your backbone material, please keep in your mind that you need to add a QoI (i.e., strobilurin) or DMI (i.e., sterol inhibitor) for black rot management, when we get close to blooming time because these two materials won't work against black rot. 
    • In addition, it is probably a good idea to plan for good protection against Botrytis at bloom. In addition to the risk of Botrytis infection on dead shoots, inflorescences on shoots from the secondary buds may make the blooming period last longer than usual. 
      • E.g., You can use two Botrytis materials with different FRAC groups to protect flowers, maybe use one group at trace bloom, and the second group at 50% bloom. Plus, make sure to mix a Botrytis material with either captan or fixed copper for the fungicide resistance management. 
    • Also, you probably need to spend more time on canopy management to avoid crowded canopy due to extensive lateral shoot development from the damaged shoots. 
    • As of today (5/13/20), I do not see any extensive rain events (crossing fingers). It looks like we will have showers here and there in the next 3-4 days, and we are expecting days above 80F, thus, I am hoping that damaged shoots desiccate soon so that we don't need to worry about Botrytis too much. We will see... 
  • Severe damage: widespread and extensive. E.g., >50%
    • If you think there is a chance of having fruits, please see the comment above. You will most likely have more dead tissues remaining in the canopy, thus, the risk of Botrytis can be high. 
      • Whether you can expect fruit or not depends on the location of the damage and other factors. E.g., If you see more damages on the upper part of the shoots and clusters/inflorescences seem to be intact, you may get lucky, but I must say that the damage on the inflorescence is very difficult to see.
    • Even if you think it will be a total loss (sorry), you still need to protect your vines. Once shoots start to develop from the secondary buds, you need to protect them. If you are not planning to have any crop, you can have a lean disease management program. E.g. use mancozeb plus sulfur, or captan plus sulfur, or fixed copper for the rest of the season. (plus you do not need to spray for grape berry moth) 
      • Spraying every 10 to 14 days may be enough, but it also depends on the weather. If there are many rain events, you may need to either shorten the spray interval or add a phosphorous acid product or another downy mildew material. (note: do not mix a phosphorous acid and copper, which cause phytotoxicity)
      • You do not need to worry about the 66-day PHI of mancozeb since you do not have harvest; thus, you can use mancozeb late in the season. However, keep eye on the limit of total mancozeb used, which is 19.2 lb a.i./acre/year, but please see the label for your product to make sure.
  • I can only cover potential disease management, and I understand that there is more to it. For more frost-damage related information, I found a note from Dr. David Lockwood (University of Tennessee), which he posted last month, very useful (this link will open a new window with a pdf file).
Now I am hoping that the periodical cicada in our area won't be so bad...

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