Bud Rot (Botrytis Cinerea)

Cannabis growers in the Pacific Northwest (N. California, Oregon, and Washington) have been dealing with a new form of the “plague” for decades.  However, this “plague” isn’t black.  It’s grey and known as the necrotrophic fungus called Botrytis Cinerea.  Botrytis Cinerea is a naturally occurring fungus that affects many plant species, including our favorite plant – Cannabis.

Growers all over the PNW have had to deal with “bud rot” or other outbreaks of Botrytis Cinerea on a pretty regular basis when growing crops here.  Many of us however, fail to realize that the problem of Botrytis Cinerea is quite regional, and quite easy to manage and keep at bay.  To understand and treat the Botrytis Cinerea fungus,  you must first learn why the Pacific Northwest has heavy concentrations of Botrytis Cinerea spores and where they come from.


While it is a naturally occurring organism, in the past 30 years, we have seen a steep increase on Botrytis Cinerea outbreaks.  Regionally, these outbreaks tend to be heaviest in concentration (PPM) in the Napa Valley (California), Willamette Valley (Oregon), and Walla Walla (Washington) regions.  These areas have more spores in the air than other regions in the Pacific Northwest, but most people are unsure as to why.

As it turns out, it is the wine makes in this region who can be blamed for the extra concentration of Botrytis Cinerea and for proliferating this fungal disease.  Wineries and wine makers pay thousands of dollars per year for vials of cultured Botrytis Cinerea, and some even purchase lab equipment to culture their own on a as needed basis.  The reason for this culturing of the Botrytis Cinerea fungus is late-harvest wines.  Wine makers will “paint” their grape clusters with the spores to infect their vines on certain species (usually Riesling) to make Late-Harvest Riesling and/or Ice Wine.

Late-Harvest Riesling is very sweet in nature, as is the dessert wine known as Ice Wine.  These are traditional German style wines made from the Riesling grape.  The reason that the wine makers will treat the grapes with the Botrytis Cinerea is due to the fact that the Botrytis Cinerea fungus will sap water content out of the grapes, raising the “Brix” (pronounced “Br-EE”) level, the sugar content of the grape, making for more concentrated juices that produce much sweeter wines.  The problem with this is the fact that the spores are very mismanaged and often spread easily through the air and wind to infect plants and crops that do not enjoy the fungus as much as the grapes do.


There is very little we as farmers can do to stop the spread of the Botrytis Cinerea fungus, but we can treat and manage our crops properly to prevent any outbreaks or loss of product.  Using a few simple steps and procedures, you can easily keep Botrytis Cinerea at bay.

First, you must keep the air flowing.  Botrytis Cinerea loves stagnant, moist air.  It is what it thrives on.  To make its environment as unfriendly as possible, lower your humidity below 45% and keep air flow moving.  You can do this with oscillating fans, intake and exhaust systems, and even dehumidifiers.

Secondly, preventatively spray your garden.  While some growers believe the myth that getting water on your buds causes bud rot, that is certainly not the case.  You cannot get bud rot without the Botrytis Cinerea being present.  Preventatively spraying your plants with a solution that kills the Botrytis Cinerea fungus in a natural and organic method is the way to go about doing it without fear of hurting your plants.

Some growers have reported success using a 1:10 ration of Milk/Water in a foliar application. I do not recommend this for anything other than ornamental plants, as sprayed milk might produce bacteria.  There is very little data on this to be found it seems. Instead, a better and even more organic method is available which kills the Botrytis Cinerea spores by lowering the PH, rather than using a poison or fungicide.  Simply mix 1 TBS of Baking Soda (Sodium Bicarbonate), and 3-5 drops of an organic dish soap (used as a spreading agent), mixed with 1 gallon of room temperature water.

  • Spray the solution on your plants up to week 5 in flower to kill any Botrytis Cinerea spores that may be resting on the plant waiting for water.  Spray once a week or as needed, but make sure that your airflow is good as to not allow standing water to rest inside of flowers.  This spray also works very well for Powdery Mildew treatments as well!

So while there isn’t much we can do to stop the wineries from spreading the Botrytis Cinerea fungus, we can certainly do our not help spread it around and let it affect our lives and the medicines of our patients.

Brought to you by WA Seed Bank

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