As your grows get bigger and yield more cannabis, the last thing you need is for crop loss to strike. That’s why you have to educate yourself about bud rot.

“It’s very common, especially outdoors,” said Dan Kibbin, a Maine-based cannabis grower with 25 years of experience. “But I’ve experienced it in indoor grows, too.”

“You lose a considerable amount of your best product because it attacks the denser part of the plant,” Kibbin explained. “Prevention is the best method because when you have it, it can become unruly. It can be quite costly to your most high-quality product.”

Specifically, Kibbin said, bud rot attacks your best buds.

Jeff Jones, dean and a horticulture faculty member at Oaksterdam University, a cannabis college located in Oakland, California, described bud rot on cannabis as “one of the more notorious ruining molds, because you don’t realize it until the plant is basically dying.”

He said one of the things that makes bud rot tricky to treat is the fact that it’s internal and systemic.

“If it gets into one plant, watering systems or root beds, they can get the same infection. It can passively spread in the air,” Jones said.

What Exactly Is Bud Rot?

Bud rot is another term for gray mold, a necrotrophic fungus that’s also known by the scientific name Botrytis cinerea. Bud rot loves cool temperatures and humidity, and it hits more than just cannabis plants—it’s been known to attack wine grapes and strawberries, too.

While plants can’t get bud rot unless they come in contact with the fungus spores, the spores get around. They travel in the air, on clothing, even through water. And once they germinate within your grow, the fungus can quickly move through an entire crop.

How to Avoid Bud Rot on Cannabis

While treatments exist that can smother or kill the fungus, applying chemicals to your plants can be risky.

For instance, there have been cannabis recalls due to one particular fungicide treatment that, when burned, turns into what Jones described as a “hydrogen cyanide type gas.” Chemical use also can be an issue if you grow organic or medical-grade cannabis.

For those reasons, and for cost and efficiency’s sake, most large-scale growers will simply destroy plants affected by bud rot.

That’s why most experts recommend focusing on bud rot prevention. To that end, four factors affect your susceptibility to bud rot.

  1. Temperature. The ideal temperature recommended for your grow varies depending on several factors, but keep in mind that bud rot prefers cooler temperatures. Most experts recommend temperatures above 68°F to keep bud rot at bay. In a greenhouse, try to avoid cold nights by using a heater.
  2. Presence of Spores. If you grow indoors, it’s much easier to avoid Botrytis cinerea spores, but you still should be careful about what you let come indoors. Change clothes before you enter your growroom, and never let pets inside.
  3. Food Source. Remember that bud rot likes the parts of your plants that have the most moisture, so you’ll need to keep an eye on your denser colas. Of course, if you see infection remove it immediately and don’t let the infected parts of your plants touch those that haven’t been affected.
  4. Humidity. For indoor growers, limiting humidity is one of the easiest ways to avoid problems with bud rot. (More on this critical topic below.)

Don’t Forget Strain Selection

Of course, if problems like bud rot keep cropping up, Jones suggested it may simply be that you’re growing the wrong strain for your conditions.

“When in doubt, pick a different plant,” Jones said. “If it happens too much, the density of the flower may be the problem, and selecting a different strain may do more than anything you can do with treatment and prevention.”

The How and Why of Humidity Control

There are a number of ways to think about humidity control relative to crop problems such as bud rot. For outdoor growers, simple tips include avoiding planting in the shade or too close to walls, which can restrict ventilation.

Indoor grows require a bit more environmental care, especially once the operation goes beyond a few plants for personal use.

“If you’re growing indoors, you might not have problems when you first start out in your home, because the home environment is already controlled, said Clif Tomasini, business director at Quest Dehumidifiers. “Your house already has a cooling system, a ventilation system. But you can get to a tipping point where you exceed the capabilities of your home—your home wasn’t designed for all those plants.”

The challenge often comes from growers who go from hobbyist to commercial grower who don’t understand the science of climate control.

“They don’t know why they’re having problems like bud rot,” Tomasini said.

What Is the Ideal Humidity Level?

The ideal humidity level for growing depends on a number of factors—including who you ask.

“If you ask the environment guys, we’re going to tell you any time you’re below 50 percent relative humidity and you have good air circulation, you’re safe,” Tomasini said. “But someone who is really focused on optimal plant growth may recommend something higher, because some plants reach their full potential under different conditions.”

“On the hobby side of things, the two easiest, most affordable things are to make sure you have good ventilation and good air circulation,” Tomasini added, as stagnant air can trap humidity, especially around densely-leafed areas.

Encouraging ventilation can be as simple as having a good fan in your grow room.

Of course, whether you need dehumidification depends on where you live, the time of year and grow technique. In more humid climates, such as the Pacific Northwest or New England, dehumidification is critical. However, if you live somewhere like Arizona, your need for dehumidification is very different, Tomasini said.

RELATED: The 2017 Soil vs. Hydro Ultimate Grow Guide
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