Botrytis Cinerea: How to Prevent and Control the Devastating Plant Disease

If you've ever checked on your garden and found weird, grey, fuzzy looking spores on your plants, chances are you've run into botrytis cinerea.

It's one of the most common fungal pathogens out there, and goes by several other names:

  • Grey mold
  • Ghost spot
  • Ash mold

If your growing conditions are humid but relatively cool, chances are good you'll run into this pathogen. So let's learn what it is, how to identify it, and best of all — how to treat it.

What is Botrytis Cinerea?

A close up of botrytis cinerea

A close up of the pathogen.

Grey mold is an extremely common pathogen that will affect your garden, especially if you are growing hydroponically due to dense planting and oftentimes poor air flow.

It can affect almost every part of your plants, from the stems all the way to the leaves, flowers, or fruit. This makes it especially damaging if not either prevented or caught early and treated.

Because it's so common, many gardeners are able to identify a botrytis cinerea attack early on, but if it's your first time dealing with it, you may lose most of your crop due to not treating it soon enough.

Did You Know...

If you're a winemaker, you actually love botrytis cinerea! One of the names for this fungus we didn't mention above is "The Noble Rot." It's called this in winemaking because it has a unique ability to aggregate sugars in grapes that are harvested late in the season. They use it to make wine much sweeter than it would normally be, upping its value in the market.

Symptoms of Botrytis Cinerea

Unfortunately, your plants can be attacked by botrytis at almost any phase of growth. This means you need to be on the looking as soon as you start your seeds all the way through the flowering and fruiting phase (if you're growing plants that reach that phase).

Early Botrytis Detection

Early on, keep an eye on your the leaves and stems of your plants. If botrytis comes knocking, you will see lesions on them, and soon after that you will see a grey fuzz appear. This is where it gets the name ash mold or grey mold.

Botrytis Progression

If left uncontrolled, these fuzzy grey lesions will keep growing and make their way around the entire stem. At this point, they weaken the stem structure and your plants are susceptible to collapsing.

You can also spot the spores on your leaves and flowers, where they will quickly spread across the surface. If the spores reach fruits, they will be decimated and turn into a mushy, watery mess.

Botrytis on Strawberry

Botrytis on Strawberry. source

You're not even safe from botrytis cinerea after you harvest. If you store your harvest in a cool but high humidity area, the spores can germinate and absolutely destroy your harvest by converting it into a watery mush.

Life Cycle of Botrytis Cinerea

Botrytis struggles to develop without humid conditions and relatively cool temperatures (59-73ºF or 15-23ºC). This makes it crops grown in between winter and spring prone to developing a botrytis problem, especially with poor air flow.

The higher the humidity, the easier it becomes for botrytis spores to germinate. If there is free standing water on plants from misting or foliar feeding, spores have an even easier time germinating.

After germination, they penetrate the tissue of your plants to form mycelium. If you're not familiar with mycelium, it can be thought of as the vegetation of fungi. This mycelium will crawl into the spaces between plant cells and begins to form vessels that release botrytis spores into the air. Once this happens, they can spread rapidly across your garden, decimating your crops.

However, these spores are usually only released when the crop moves around, usually due typical gardening activities like pruning. In fact, pruning makes it even easier for the spores to get into your plants, because the open cuts are easier to penetrate.​

Prevention and Environmental Control

The easiest way to control botrytis is to make sure the environment you are growing in makes it impossible for botrytis to develop. This means that your air temperature should be above 73ºF or 23ºC, and your relative humidity should be low.

Practically, this means you should vent out humid air from your garden and vent in drier air. This becomes even more important as night falls and humid air cools and condenses on your plants — the perfect growing environment for botrytis.

Additionally, split your pruning and harvesting into two separate times of the day. If you have any botrytis spores in the garden, moving your plants will spread them around and you don't want any spores from pruning activities to settle on your harvested greens or fruits.

Controlling Plant Stress

On top of controlling your environment, it's important to make sure your plants are as healthy as possible. Stressed, weak plants are more prone to infection than healthy plants. While maintaining healthy plants is the subject of another article, you can take a look at this checklist to make sure you're doing everything correctly.

Here are a few other plant issues that can increase susceptibility to botrytis:

  • Too much nitrogen in your fertilizer
  • Too little calcium and silica in the tissue of your plants

Controlling Botrytis Cinerea

There are a few effective ways to deal with botrytis if you find your garden under attack. The two main approaches are chemical or microbial.

Chemical Treatments

Unfortunately, chemical fungicides are becoming less and less effective to control botrytis due to it developing a resistance to many popular fungicides.

However, you can still go this route if you detect botrytis early and hit it hard. The best products to use are:

  • Dicarboximide fungicide​
  • Benzamidazole fungicide
  • Chlorothalonil

If your garden is suffering from consistent botrytis infections and you want to use chemical fungicides, make sure to rotate the types that you use. This ensures that the fungus won't build up a resistance to the particular fungicide you're using.

Microbial​ Treatments

The more effective approach is to use biological control in the form of beneficial bacteria and fungi. The way this works is by introducing these bacteria or fungi into your garden, where they quickly 'crowd out' the botrytis while doing no harm to your plants.

The two most effective types are trichoderma and cladosporium​, which have been turned into products that you can buy at your local gardening or hydroponics store. These two types are fascinating in their effectiveness and often times much cheaper and healthier than a chemical fungicide.

Whichever method you choose, you must also supplement botrytis treatment by controlling your environment and keeping a watchful eye for the development of lesions and spores. As the gardener, you will always be the first line of defense against any and all garden pests and diseases.​

Botrytis cinerea goes by many names: grey mold, ash mold, or the ghost spot. Whatever you call it, it's deadly for your plants, so learn how to prevent it!
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