How to Prevent and Get Rid of Gray Mold

It seems that there is no shortage of pests and diseases that want to wreck all of my hard work in the vegetable garden this year.

Where I live, it has been an unexpectedly hot and dry year. While that means we were lucky enough to not have to deal with fungi, it meant that we were subjected to a whole host of other problems. From tomato hornworms to blossom end rot, we really struggled to get a handle on things!

strawberry with brown rot
Gray mold on some strawberries. Photo by Rasbak – Own work, CC BY-SA 3.0, Link

Fortunately, those problems have more or less subsidies, and we are still enjoying a bountiful harvest from our plants nonetheless. Another benefit of the warm, dry weather was that we didn’t have to worry too much about a problem we’ve had in the past – fungal disease.

Gray mold, also known as botrytis, is one of the most common fungal diseases in the backyard vegetable garden. It is common in cucumbers, tomatoes, and green beans, along with some fruits like blueberries, strawberries, and raspberries. Flower gardeners, you aren’t immune – it can even affect African violets and some species of succulents!

Gray mold can be devastating to a garden, but luckily, there are some steps you can take to cut it off before it becomes a problem. Here’s how to prevent and get rid of gray mold before it drives you insane!

What is Gray Mold?

Gray mold is a fungal disease caused by the fungus Botrytis cinerea that can develop on more than 200 species of plants. It is incredibly destructive, particularly to fruit crops like strawberries and tomatoes. It poses problems even long after harvest.

This disease overwinters in sclerotia, or thread-like masses, inside the tissues of plants. Each spring, as humid conditions and wet weather wake up the plant (or spores in the soil), more spore masses are formed and then spread through the wind or through the rain.

Fortunately, gray mold tends to calm down once the temperatures rise. Temperatures over 80 degrees Fahrenheit (26 Celsius) make it far less likely for the spores to spread and germinate. In fact, over 90 degrees F (32 C), the spores stop completely – which is why we didn’t suffer during our super hot, dry summer.

What Plants Are Affected By Gray Mold?

Gray mold most commonly affects spring flowers such as marigolds, roses, peonies, and impatiens. However, there are several greenhouse flower favorites that can be infested too, including poinsettia, geraniums, and cyclamen.

Flowers like petunias, pansies, and snapdragons are most commonly affected in the seedling stage.

Other plants that can be affected by gray mold include grapes, blueberries, blackberries, raspberries, plums, strawberries, pears, peaches, apples, tomatoes, and beans.

Symptoms of Gray Mold

Gray mold can affect plants in a variety of ways. Often, it causes your seedlings to die before they ever really get started, in a cognition called damping-off.

Damping-off normally will make your plants seem totally healthy – but then, inexplicably, they will die. Upon further inspection, you might notice that the roots of infested plants seem soggy and black.

This fungal disease can also cause related issues like blossom blight, crown rot, shoot blight, and fruit rot.

You’ll know gray mold is to blame, though, and not some other culprit, when you see what looks like a water-soaked spot on your plants. Later, that tissue will become watery and soft as the parts of the plant that are affected collapse and then wilt.

gray mold on grapes
Gray mold on some grapes.

Should humidity remain high, you might notice that the tissue develops a gray-brown coating of fungal spores.

These spores will be produced in earnest – and they can then be blown or splashed onto healthy plant parts. Infection and germination alike can occur within a matter of hours as long as there is plenty of moisture present.

You may even notice infestation on the fruits and stems. Here, infection will look like flat, black growths that are surrounded by bits of decay.

Unfortunately, gray mold can overwinter on plants, as well as in the soil and in plant debris. The spores will develop as soon as the weather conditions are favorable and can be moved by splashing water as well as by the wind.

They’ll make their way onto young leaves or blossoms, where these spores germinate and enter the plant.

What are the ideal conditions for spores to proliferate, you might ask? Generally speaking, they’re the same ones that attract other fungal diseases – cool temperatures between 45 and 50 degrees F (7 to 10 C), and high humidity (93% or higher).

It is less common for mold spores to penetrate through healthy, green tissue, but more likely that they enter through wounds on plants that are still growing.

Therefore, plants that have recently been pruned, as well as those that are being grown from cuttings are far more likely to suffer from a sudden infestation.

Some other signs that gray mold has taken over your garden include leaf drop, wilting or decaying leaves, and spotting or discoloration on leaves.

You might notice a fuzzy or gray growth on foliage and flowers or flower buds that fail to open. Sometimes, those flower buds rot altogether.

How to Prevent Gray Mold

Space Plants for Adequate Ventilation

Your first step in preventing gray mold starts the day you plant. Make sure you have adequate spacing between all of your plants and put in some good cross-ventilation systems if you’re growing in a greenhouse or similar contained settings.

If you haven’t’ already, prune or stake your plants. This will help improve air circulation. You may also want to add a fan, which will add airflow and can be beneficial particularly if you are growing indoors.

Clean and Disinfect All Pruning Equipment

When using pruning shears, make sure you take the time to disinfect everything with one part bleach to four parts water after each and every cut you make – yes, even on the same plant! This Can be a bit cumbersome and annoying to have to do, but it’s worth it to not have to deal with constant reinfestation.

Clear Debris

Keep the soil beneath your plants clear of debris. Get rid of fallen leaves, dropped fruit, and other plant parts immediately. This will help eliminate the likelihood of spores spreading to your plants, along with many other different pests and diseases.

Inspect New Plants

If you are adding any new plants or transplants to the garden, make sure you keep them quarantined for a while until you have time to make sure that they are not affected by any disease or pests – gray mold included.

This will help you keep problems on the plants you’ve purchased from infecting your entire property.

Practice Good Watering Hygiene

Knowing how and when to water your plants is incredibly beneficial when it comes to growing healthy crops. You will want to make sure you water first thing in the morning to give the plants plenty of time to dry out.

If you’re growing indoors, make sure this oil is well-draining so that moisture does not accumulate in waterlogged soil.

When you water, try not to splash water onto the leaves of the plant and water from below instead. Installing basic irrigation systems like drip lines or soaker hoses can be incredibly beneficial when it comes to preventing the spread of gray mold and other diseases.

You won’t have to worry about water splashing up on the plants because everything will come from below.

You can even add a thick layer of mulch. Mulch can help prevent fungal spores from splashing up onto the leaves and flowers. Organic mulch will add nutrients back to the soil as it decomposes, helping to strengthen the plant’s defenses.

If you use mulch, just make sure to keep it a couple of inches away from the base and stem of the plant – otherwise, you might have issues with rot there, too, because the mulch will hold moisture.

Carefully Monitor Greenhouse Growing

If you’re growing your plants in a greenhouse and discover that the gray model is a problem, you’ll want to up your game to help knock out the issue.

For starters, aim for a relative humidity that’s lower than 80% and install multiple fans – how many fans you will need will vary depending on the size and dimensions of your greenhouse.

Elevate the temperature of the greenhouse at sunset – this will help dry off any condensation that still remains on the leaves. If you notice plants that are infected with botrytis, get rid of them immediately.

Don’t Harvest When You Prune

Lots of growers, particularly those who grow in a greenhouse setting, recommend splitting up harvesting and pruning time into two separate occasions.

For example, if you prune in the morning, harvest at night. This will let any spores that might have been stirred up into the air to settle so you don’t plop them onto your freshly harvested produce.

Grow Resistant Varieties

Although there aren’t any flowers or vegetables that are completely resistant to gray mold, there are some that are far less likely to acquire it. For example, both ‘Tidal Wave Pink’ and ‘Fantasy Blue’ petunias are believed to be resistant to gray mold, as are tree peonies and similar hybrids.

How to Get Rid of Gray Mold

Consider a Fungicide

There are some organic fungicides that can be used to help halt or present the spread of infection. Those that are made out of sulfur or copper generally work best, and can help protect plants from re-infection.

Apply these once a week when the weather conditions are favorable for gray mold, as well as if you have had any issues with gray mold in the past.

If you use a sulfur-based spray, you should use it first thing in the morning or late in the evening. Do not apply it during the heat of the day, as this can damage populations of bees and other beneficial insects.

Try Neem Oil

Neem oil is a more natural treatment that can help prevent and reduce the spread of fungal growth. It should be applied first thing in the morning and can help keep other pests at bay too.

Encourage Beneficial Bacteria and Fungi

One antifungal treatment that many gardeners swear by is something called bacillus subtilis. This treatment is actually a bacteria but it’s a helpful one, often marketed under the name of Serenade Garden.

However, it’s not enough just to spray your plants with this treatment and be done with it. You need to encourage healthy soil life and functioning from the outset.

Adding plenty of compost to the soil can help encourage mycorrhizal fungi and good bacteria to hang out around your plants – which in turn can stop the spread of harmful pathogens like botrytis.

Remove All Affected Plant Parts

Although you can’t do much to control gray mold once it sets in, you should take the time to prevent its further spread among your other plants.

Start by removing all affected flowers and foliage from instead plants and destroy the plant matter immediately. Do not attempt to compost it, as the fungal spores can live on through the composting process. Burn or bury them instead.

Embrace Gray Mold

Well, perhaps not literally – or even figuratively, I suppose!

However, it’s important to recognize that gray mold, though annoying, does actually present some benefits to growers.

There are some winemakers who specialize in the production of late-season grapes who have discovered that gray mold causes sugars in the grapes to become more concentrated. As a result, their wines were sweet and intricately flavored.

Fortunately, gray mold isn’t usually harmful to humans. There are some people who report having allergic reactions to the mold, but this is pretty rare. While it’s not a fun problem to have in the garden, it’s not necessarily going to make you sick.

Naturally, though, unless you’re one of these winemakers, it can be hard to just “suck it up and deal with it” when it comes to gray mold in the garden.

If you’re not ready to embrace the suck, consider implementing the tips above so that you know how to prevent and get rid of gray mold in your garden if it ever happens to you.

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