Sunday, March 25, 2012

Flag shoots symptom of powdery mildew

In reply to the previous post about powdery mildew, Anton (Baudoin of VT) made a very good comment about "flag shoots" of powdery mildew.  This is a case of infection where powdery mildew pathogen survives on buds, and infect developing shoots from the get-go.  The result is infected shoots with retarded growth, hence people call it flag shoots.  You can see a picture from this Australian department of Ag site.

Why this is important? Since it bypasses the ascospore stage, the infection event I mentioned in the previous post will be irrelevant, and you will have conidia in your vineyard from the beginning of the season, i.e., mass production of spores can happen quickly.

Based on conversations I have had with growers in VA, flag shoots do not occur in VA growing conditions, and I have not seen this symptom yet.  This is primarily due to low winter temperatures; however, the winter we had was very mild, plus we had a dry summer (before Irene) to promote powdery mildew in 2010.  Thus, I think there is a chance of flag shoots infection in VA.  Please scout your vineyard carefully, and if you find suspicious shoots, please let either me or Anton know.

What should you do if you have flag shoots?  It will depend on the degree of infection, but if you have many flag shoots, one thing I can think of is the use of a potassium salt product (Armicarb, Kaligreen, etc).  It is a contact material, so, you will need a good coverage (note: probably not a good idea to mix a potassium salt with other products).  Then, you can resume (or start) your regular Phomopsis spray (mancozeb or captan) + sulfur for powdery mildew to protect healthy tissues.

Speaking of Phomopsis, the rain from yesterday also counts as a Phompsis infection event. I forgot to mention it!

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