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Botrytis Cinerea: How to Prevent and Control It

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.

This common fungal pathogen goes by many other names as well. Grey mold, ghost spot, ash mold, or botrytis bunch rot are just a few! But to most of us, it’s that cursed grey-black mold that turns fruit into mush and wreaks havoc on our plants.

You don’t have to suffer through it for much longer. I’ll walk you through what botrytis cinerea is, how it develops, and what to do if and when you find it. With good care and some preventative tactics, you can free yourself of the grey mould for good!

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Good Products To Combat Botrytis Cinerea:

  • Neem Oil
  • Bonide Copper Fungicide
  • GreenCure Fungicide
  • Serenade Garden
  • Down To Earth Bio-Live Fertilizer (for mycorrhizal fungi and beneficial bacteria)
  • Mycostop

Botrytis Cinerea Overview

Common Name(s) Grey mold, grey mould, botrytis bunch rot, noble rot, ghost spot, ash mold
Scientific Name(s) Botrytis cinerea
Family Sclerotiniaceae
Origin Worldwide
Plants Affected Over 200 species of plants including many food crops and ornamentals
Common Remedies Neem oil, copper fungicide, potassium bicarbonate-based fungicides, mycorrhizal fungi and beneficial bacteria (especially Bacillus subtilis, trichoderma and cladosporium). Also good plant maintenance, high airflow, low humidity environment.

What Is Botrytis Cinerea?

A close up of botrytis cinerea
A close up of the pathogen showing its branch-like shape topped with spores. source

Botrytis cinerea is a grey, fungal mold which grows on more than 200 species of plants. This single mold causes crop losses of $10 billion to $100 billion worldwide each year. It’s also the most common pathogen responsible for the post-harvest decay of fruits and vegetables.

In most circumstances, botrytis cinerea is bad. It’s destructive to any sort of fruit crops, including strawberries, raspberries, tomatoes, and many more. Even after harvest, it can be a major problem. Those strawberries that go black and moldy after harvest are infested with botrytis cinerea, for instance.

However, to a small handful of people, it can be quite useful. Winemakers who produce late-season grapes have discovered that this mold causes the sugars in their grapes to become concentrated. This results in a much more concentrated flavor to the finished wine, along with added honeysuckle-like flavor notes.

Life Cycle Of Botrytis Cinerea

This plant disease overwinters in sclerotia in plants. Sclerotia are thread-like hard masses inside the tissue of plants such as berry canes or grapevines. In the spring, as humid conditions and rain reinvigorate the plant, it will form spore masses which are spread through air currents, by workers, or through splashing raindrops.

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

On the bright side, temperatures over 80ºF make it less likely for these spores to germinate and spread, and over 90ºF, it will stop development entirely. Hot summers are a godsend for those who suffer from this disease.

Temperatures lower than 73ºF are far more common in the spring. This means that it’s likely to become an early-season issue and require rapid treatment to prevent spreading and plant damage.

Higher humidity increases the risk for this fungal disease. The higher the humidity is, the more likely that botrytis spores will germinate. This means that people who grow inside greenhouses may find this a common problem, as well as those who grow hydroponically.

Foliar feeding or misting of plants can make this problem even worse, as free-standing water on plants creates an even higher likelihood of spore germination. Watering as the temperatures begin to drop in the evening can also be hazardous, as this enables the spore to take advantage of cooler temperatures.

Symptoms of Botrytis Cinerea

Botrytis on Strawberry
Botrytis on Strawberry. source

Botrytis cinerea can strike at any part of plant development. You have to be on the lookout for it from the time you start your seeds all the way through flowering and fruiting.

Early Botrytis Detection

While different plants will have slightly different symptoms, one of the earliest signs of botrytis cinerea is water-soaked spots on the leaves. These might appear to be white or off-white in color. Over time, these spots will turn brown. They’ll eventually cover most of the leaf and cause it to wilt.

Later Botrytis Progression

As leaves start browning out, it can be easy to misidentify botrytis with other fungal pathogens. However, the real danger comes when the humidity rises. Greyish and bumpy-looking spore patches will appear.

It is at this time that botrytis cinerea becomes most dangerous, as it now can easily spread spores around your garden. The slightest breeze, splash of water, or even pruning the damaged leaves can pass microscopic spores on to other plants.

Over time, it will appear as though your entire plant becomes covered in a fuzzy grayish growth. Fruit will rot where it hangs, and your flowers will appear to be grey in color. The plant will eventually wilt and die entirely, falling victim to the mold.

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.

Understanding The Noble Rot

Noble rot on grapes
Grapes infected with the noble rot. Note the spore growth. Source: davitydave

But what of this “noble rot”, or botrytis bunch rot? While it can be incredibly useful in concentrating the sugars in the winemaker’s grapes, it really depends on the variety of grapes you’re growing.

Eating grapes like ruby seedless or flame seedless varieties tend to be highly susceptible to damage from botrytis. So too do a wide variety of wine grapes such as chardonnay, zinfandel, petite syrah, white riesling, pinot noir and sauvignon blanc.

In these more susceptible varieties, botrytis is only desireable right before harvest. It will begin to concentrate the sugars just before the pressing period or optimal eating period, and any damaged grapes can easily be removed.

There is some evidence that reducing the density of the foliage will help conditions improve and prevent the spread of botrytis. Often, vineyards trellis their plants in an open style which promotes good airflow to the entire plant and reduces the chances of fungal growth.

The more resistant varieties of grapes can hold out for longer against the spread of this plant disease. Both Thompson and crimson seedless eating varieties are more resistant. Winemaking varieties with better resistance include semillon, cabernet sauvignon, merlot, muscat of Alexandria, rubired, sylvaner, and ruby cabernet.

More resistant strains tend to only succumb to the fungal spread on damaged plants or fruit, or in conditions where they are not pruned to remove excess leaf development. They also tend to have spore blooms late in the season, at the ideal times for that extra boost in flavor. But no grape variety is fully immune.

If good care is not given to one’s vines, they will fall victim to the disease. Flavor concentration in your grapes is useless if your vines end up dying off. So it’s a very tricky balancing act!

Controlling Botrytis Cinerea

Botrytis on snap bean
Botrytis cinerea on a snap bean. Source: NYSIPM Image Gallery

As you can see, controlling this fungal disease is absolutely necessary. But botrytis cinerea can be tricky to combat. Here are some effective tips for managing your impacted plants and avoiding the fungus in the future.

Preventing Botrytis Cinerea

The first stage of treatment is prevention. Good garden stewardship will eliminate the need for various different forms of treatment.

Prune or stake your plants to provide optimal air circulation around them. Depending on the type of plant, you may need to regularly trim random overgrowth to be sure that air can readily flow around the stem and leaves.

Be sure that caged plants like tomatoes do not fill their enclosure completely. Airflow is crucial to prevention.

When pruning, clean your pruning shears between cuts with a mixture of bleach and water. A solution of one part bleach to four parts water is sufficient to keep your shears sterilized.

Keep the soil beneath your plants cleared of plant debris. Any fallen leaves, dropped fruit, etcetera can promote fungal growth, and you want to eliminate the chances for that as much as is possible.

When bringing new plants into the garden, temporarily keep them separated from your existing plants. This quarantine offers you the ability to keep any potential pests and diseases out of your garden by identifying and treating them right away.

Use a soaker hose to provide direct-to-the-roots watering and avoid moisture on plant leaves. If you don’t have an irrigation system set up, water early in the morning so the sun will dry the leaves.

These tactics will also help you to prevent other fungal diseases such as powdery mildew or downy mildew. They also make the garden look a lot nicer overall, so it’s beneficial aesthetically!

In addition, the healthiest plants are usually not the ones that develop fungal diseases. Keeping your plants healthy and in the best condition will protect them against a wide number of problems.

Preventing Botrytis Cinerea In Greenhouses or Indoor Grows

Those growing indoors will need to add a couple more preventative measures.

Inside greenhouses and garages, air circulation is key. Adding fans and vents is an essential step, both to keep the environment at an optimal temperature and to ensure the plants get all the circulation that they need.

Vent out humid air from indoor grows. Reducing the humidity will reduce the likelihood of disease spread. It may increase the likelihood of spider mites, however, so keep a watchful eye for alternate pests. This venting is especially important at night when temperatures drop and humidity rises.

Split your pruning and harvesting into two different times of day. If you prune early in the day, wait a few hours before harvesting so any spores which might be in the air won’t land on your newly-harvested produce.

Avoid allowing moisture to linger on your plants, even if it means you need to add a fan blowing directly across them.

If walking into your greenhouse feels like you just entered a tropical rainforest, and everything is damp, you will have a botrytis outbreak sooner rather than later! It’s best to be safe rather than sorry.

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.

What To Do When Prevention Fails

Botrytis cinerea
A botrytis-colonized flower. Source: Flowersabc

Sometimes, no matter how clean you keep the garden, botrytis cinerea will still form.

One of the biggest difficulties with botrytis cinerea is that it has the tendency to adapt to different fungicidal methods. It can develop a form of immunity to commonly-used methods. You will also need to adapt your methods to treat the disease in a way which makes it difficult for the fungus to develop that immunity.

Varying your treatments between organic and microbial methods is a good way to do this. Organic foliar treatments can defend your plants and wipe out growth early. Adding soil microbial treatments provides added protections built right into the soil which can help your plants.

Organic Treatments

The old standby of neem oil applies here. If you can prevent fungal growth from the time you plant by regularly treating with neem oil, you may not develop fungal growth in the first place. It will also keep other pests at bay. Failing that, there’s alternatives to pick from.

Liquid copper fungicides, like Bonide Copper Fungicide, have been proven to help prevent plants from spore infestation. If the weather forecast is predicting a long period of cooler, wet weather, it may be time to prepare. Apply this fungicide every seven to ten days from the onset of flowering through harvest to protect your plants.

Potassium bicarbonate is also effective against botrytis blights. One variety which is in wide organic use is GreenCure Fungicide. This powdered formula, when mixed with water, can be sprayed regularly to reduce fungal growth and deter diseases. It is considered safe by the FDA and is often used in organic gardening.

Mycorrhizal and Bacterial Treatments

To avoid fungal disease becoming immune to other organic fungicides, you can alternate use with a product made with bacillus subtilis. This beneficial bacteria will help prevent many different fungal growths, and is marketed under the name Serenade Garden.

Don’t rely just on foliar spraying. Ensure your soil and plants are healthy and well-defended by infusing the soil with beneficial mycorrhizal fungi and beneficial bacteria. Not only will these help prevent diseases, they will also help your plant’s roots take up nutrition more readily.

I like to add a little Bio-Live Fertilizer to my soil when planting new plants. This 5-4-2 fertilizer is filled with a huge list of healthy soil dwellers as well as being a solid organic fertilizer choice.

For people doing hydroponic grows, a product like Mycostop may be a good option provided that you have continuously circulating water. It can also be done as a soil drench or a foliar spray for in-ground growers. This provides control or suppression of many common fungal diseases.

The two most effective beneficials for preventing botrytis cinerea are trichoderma and cladosporium, so be sure whatever you use includes them.  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.​

Frequently Asked Questions

Botrytis cinerea
Another botrytis-colonized flower. Source: Flowersabc

Q: Is botrytis cinerea dangerous to humans?

A: While most people will not have a problem, botrytis cinerea can cause an allergic reaction. Known as “winegrower’s lung”, this is a form of hypersensitivity pneumonitis. It is often not lethal, but it can be very uncomfortable and may require treatment by a doctor to resolve it.

While hypersensitivity pneumonitis can be caused by a number of factors (including aspergillus mold, which results in “compost lung” or “farmer’s lung”), botrytis cinerea inhaled in large quantities can cause lung inflammation. It’s best to avoid it altogether by eliminating any mold on your plants before they become widespread.

If you discover a large growth of grey mold, place a plastic bag over it to reduce the likelihood of the spores getting into the air, and remove it carefully. Fully infected plants should always be removed this way to avoid spore spread anyway!

Prevention of hypersensitivity pneumonitis is surprisingly simple: don’t breathe in large quantities of dust. If you’re doing something dusty, wear a mask while doing it.

Armed with this information, you should be able to make a solid stand against grey mold in the garden. And, with luck, you’ll have fewer problems with it going forward!

Are there any other questions you may have about botrytis cinerea, or diseases which you find yourself plagued with? Let me know in the comments so I can focus in on them for you!

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