I have been receiving more than usual number of calls and emails about powdery mildew this year.  It seems that consensus is that it started to show up on clusters last week and just keep going.  We have passed critical time for cluster infection at this point, thus, you may wonder how it happened?  Since it takes 7-10 days to develop symptoms (powdery appearance on the surface, i.e., spores), and it probably takes a few generations prior to the explosive outbreak, thus, the initial infection events probably took about 20-25 day ago when berries are still susceptible.

Management options on on-going powdery mildew infection is limited.  As with other diseases, we do not have real curative or eradication material against powdery mildew.  Also, it is not recommendable to apply protectant fungicides because there will be higher risk of fungicide resistance development; however, as I mentioned in the previous post, there are a few options that can help you out.  All of them requires through coverages of target tissues since they need to contact with powdery mildew pathogen to be effective.  Also, please keep in your mind that none of them can provide near 100% efficacy as a protective fungicide does.

1) Armicarb or Kaligreen or Nutrol: These are potassium bicarbonate materials, and often recommended during this time of the season because its efficacy and an ease of use.

2) Stylet Oil: The rate is 1-2% or 1-2 gal of Stylet Oil/A with 100 gal of water.  It can provide as much (or even better) efficacy against powdery mildew as Armicarb or Kaligreen; however, you will need be aware of some precautions for the usage.  As with any other oil-based materials, there is some compatibility issues, thus, the general recommendation is to apply by itself, and you cannot apply some types of chemicals within a few weeks after oil application.   For example oil and captan can cause phytotoxicity.  In addition, you need to watch the temperature at the time of application.  Furthermore, there was a study showed the ill effect (delayed ripening) by Stylet oil application late in the season, so, it is not recommended to apply more than two times during fruit ripening.  For more detailed information, please visit this site.


3) Oxydate:  It is hydrogen dioxide material. It is recommended to apply back-to-back for three times.  Based on other studies, it does not provide as much efficacy as other two options.

Please make sure to read the label for more detailed information (rates, timing, etc).

We are about to have 3-4 days of high temperature conditions in next few days.  Hopefully these hot days slow down powdery mildew a bit.