Marijuana’s Evil Enemy: Gray Mold

Picture yourself going into your hydroponics marijuana grow room a couple of weeks before harvest and noticing that some of your buds are rotting.

If you’re an experienced grower, you know what it is: Botrytis Cinerea, the dreaded gray mold that can eat up your beautiful buds within a couple of days.

Interesting to note this “mold” is caused by a fungus, and a tough one at that. It can survive temperatures from 28°-90°F.

Gray mold goes dormant to defy eradication efforts and harsh conditions.

It travels on wind, interior air movement, tools, clothing and plant parts, and it survives attempts to remove it via mechanical methods, such as trimming out affected parts of your marijuana plants.

Botrytis Cinerea loves temperatures in the range of 70°-77°F, which is bad for you as a marijuana grower, because that’s also the ideal range for your cannabis plants.

Gray mold can tolerate temperatures below 70°F better than it tolerates temperatures warmer than 70°F.

Gray mold loves high humidity, and this gives you the opportunity to use venting, dehumidifiers or air conditioners to keep your marijuana grow room humidity between 44-53%.

It’s harder for gray mold to get established in a marijuana grow room with low humidity.

Another way to block gray mold is to prevent it from getting inside your grow room.

The most effective way to do that is build a sealed grow room or retrofit your existing hydroponics set-up so that it’s a sealed system.

Building a sealed room takes quite a bit of work and financial investment, but it pays off because molds, fungi, pests and other attackers can’t easily get at your plants.

You also get the payoff of controlled climate and increased C02 levels- both of these contribute to faster growing hydroponics plants that yield more.

A sealed room is more secure, and less likely to have avenues for gray mold and other pathogens to get in or for odors to leak out.

Can marijuana clones vector gray mold into your hydroponics room? It’s unlikely you’ll be bringing gray mold into your room via clones that you purchased commercially, but carefully inspect all clones before you allow them into your hydroponics room anyway, because clones are a vector for pests and diseases other than gray mold.

The reason that clones are not often a vector for gray mold is Botrytis Cinerea shows up inside phat buds, and cannabis clones you just purchased are not likely to have phat buds on them yet.

On the other hand, if you were growing outdoors and had blooming cannabis plants that needed to come indoors because of too much rain or cold, those plants might vector gray mold into your hydroponics room.

Gray mold loves over-watered and over-fertilized blooming marijuana plants. The excess moisture and nutrients that transpire out of your plants form a great environment for gray mold, especially inside dense buds close to the main stems.

I recently saw an article in a hydroponics magazine that recommended “foliar spraying” as a fix for gray mold.

This seemed like really bad advice to me. You really shouldn’t foliar spray your crops when they’ve formed large buds.

Obviously, the foliar spray just adds to the moisture in your buds, which gives gray mold more of what it wants!

Sometimes, no matter how hard you try to keep gray mold spores out of your hydroponics grow room, they will get in and start to work destroying your crops.

It’s most likely to happen during bloom phase, after your cannabis flowers are large and dense.

Carefully examine your flowers (especially the densest ones) to look for signs of gray mold. It’s hard to detect because it starts at the inner stem and eats its way out.

The main signs you’re looking for are individual, isolated flower leaves or hairs that rapidly turn brown, curl up and dry out, or a portion of a flower that turns black or grey, or becomes gooey or webby overnight.

It’s ugly, and it’s scary- because gray mold undetected can rot a grow room full of flowers overnight.

When you find a telltale ruined leaf or other signs of gray mold, use tweezers or some other probing implement to peel back the outer layers of floral cluster so you can see the extent of the damage.

In most cases, the moldy area will be less than 50% of the total flower mass.

Some growers try to approach this as a doctor approaches a cancer tumor- cut all of it out, and hope you got all of it, and that it doesn’t spread.

Unfortunately, that hope is often futile, and the safest thing you can do when you detect gray mold is remove the infected plant to another room fast.

Turn all the fans off. Cut the entire affected branch or branches. Throw them in a clear alcohol such as gin or vodka, which will extract THC from the buds and eliminate the gray mold too.

Or put the cut plant parts in plastic bags and dispose of them safely as soon as possible.

Quarantine affected marijuana plants for several days to see if mold returns.

Never allow plants that have been infected with gray mold to come into contact with plants that have not yet been infected.

Any time you work with a plant that has gray mold, sterilize all your equipment that touches that plant.

Change your clothes and shower after coming in contact with gray mold, before you go back into your grow room.

Some people advocate using fungicides or other industrial agriculture materials to fight gray mold. But you’ll be smoking what you’re spraying, and it will hurt you.

As a preventive very early in bloom cycle you can foliar spray your plants with Piranha liquid beneficial fungi, but don’t be tempted to use fungicides and other toxic materials on your marijuana flowers.

Another preventive tactic is to research marijuana strains to find out which ones are prone to gray mold, and which are resistant.

Many hydroponics cannabis growers have a generic and somewhat accurate understanding of this: dense, phat Indica buds are prone to gray mold, but long, thin Sativa buds aren’t.

If you live in a high humidity area, haven’t got a sealed grow room, see high humidity (above 55%) in your hydroponics grow room and are growing Indicas that are known to be gray mold magnets, you’ve got some serious thinking to do as you proceed in your marijuana growing.

You might be best off to change the strains you grow, invest in humidity control and other sealed room equipment, and pay closer attention to your buds from mid-bloom phase onwards. I hope you never experience the heartbreak of gray mold. It can rob you of your marijuana in a particularly cruel and immediate way!


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