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Botrytis cinerea: How to identify and deal with this highly infectious mould 

Theme: Pest guides

Overview

What is Botrytis?
What is the impact of Botrytis cinerea?
How to identify Botrytis cinerea?
How do I get rid of Botrytis cinerea?
Conclusion and future direction

Botrytis cinerea is a highly infectious grey mould that causes disease in crops in outdoor and indoor environments. This pathogen commonly appears as a grey fuzz on fruits and vegetables and can be a serious issue for grape yield and cause damage to other agricultural plants. Early stages of Botrytis cinerea infection can be difficult to notice and may appear similar to different types of crop damage, such as water stress or other pathogens. Later stages are easier to identify, though, by this stage, significant damage is already likely to have been done. Botrytis cinerea adapts in response to diverse climates and develops resistance against conventional fungicides, making it a challenging pest to manage. However, solutions to this fungus include biological controls, which are backed up by scientific studies

In this blog, we cover the biology, impact, and management strategies of Botrytis cinerea, offering insights into its lifecycle, how it infects crops, and strategies required to combat it. 

A close-up image showing a bunch of grapes infected by Botrytis cinerea, with grey fuzzy mold - a distinctive symptom of this fungus.
A close-up of grapes infected with Botrytis cinerea showing the distinctive grey mould. Credit: Phil Taylor, CABI

What is Botrytis?

Botrytis is a group of fungi, some of which damage crops and can destroy agricultural fruits like grapes. The most common Botrytis fungus is the species Botrytis cinerea, which is also known as Botryotinia fuckeliana, and grey mould. Botrytis cinerea appears as a grey fuzz on crops and can damage leaves, stems, and fruit. The grey fuzz is actually caused by the asexual conidia spores of this fungus. 

Botrytis cinerea has a global geographic distribution and there are many management strategies to prevent the disease it causes. Despite these strategies, the pathogen has become resistant to some chemical fungicides, which limits control options. 

Botrytis cinerea is prevalent in diverse climates and is capable of surviving in broad temperature ranges. It is present throughout the continental US, though it is more likely to cause infection in humid conditions, either after prolonged rain or in over-irrigated fields. The fungus can feed on dead matter in addition to crops, and therefore, crop residues that have not been cleared away can aid the formation of active infections. 

Spores of this fungus can remain dormant in dead matter and soil over the winter before emerging in more favourable conditions. Sclerotia of Botrytis resemble small black structures. Botrytis cinerea spores can transfer between plants via wind, water, and close contacts, making transmission challenging to manage. 

Infections can emerge in different stages of crop development, including from infected seeds or during flower blooming. Fruits infected with Botrytis cinerea can act as reservoirs for spores, which are rapidly dispersed into the local environment when the fruit splits, leaks, or bursts open. 

A close-up of strawberries revealing the distinctive grey mold, a symptom of Botrytis cinerea.
Grey mould on a strawberry fruit (Botrytis cinerea) – Credit: Francesco Faretra

What is the impact of Botrytis cinerea?

Botrytis cinerea is a necrotrophic, pathogenic fungus, which means that it kills the crops it attacks to obtain nutrients for growth. Once it finds a suitable host in the correct environmental conditions, the fungus produces virulence factors such as enzymes that attack and degrade cell walls. This impacts molecular structures and is central to the pathogenicity of Botrytis cinerea. The mould causes discolouration and crop wilting, leading to rotting and death. For crops like grapes, this can mean a significant loss of yield. 

In grapes specifically, Botrytis cinerea causes fruit clusters to rot before ripening. This is known as bunch rot. Notably, low-level infections of this fungus are used in the production of some types of sweet wine. In such cases, the mould is referred to as noble rot and the grapes must be picked at a distinct stage in the course of the infection. 

Is Botrytis cinerea toxic to humans?

Botrytis cinerea is not considered pathogenic in humans but can cause allergic reactions, such as hypersensitivity pneumonitis upon repeat high exposure. It can also trigger irritation within preexisting respiratory conditions such as asthma. 

How to identify Botrytis cinerea?

A display illustrating the various stages of the fungus Botrytis cinerea on a garlic bulb, highlighting the contrast between a healthy bulb and one affected by the fungus.
Various stages of Botrytis cinerea in garlic bulbs. Credit: Lindsey du Toit, Washington State University/via Bugwood.org – CC BY 3.0 US 

Botrytis cinerea infections can be difficult to identify during the early stages, but late stages display easily identifiable characteristics of their pathogenicity. Signs of infection vary depending on crop type and which part of the crop is affected.  

Early signs of Botrytis cinerea include the formation of translucent spots on crops. These spots increase in size and can consume most of the affected parts. Close analysis of a plant may reveal tissue with a brown, water-soaked appearance that can split apart or rupture. These characteristics are more prevalent on stems and succulent tissue, while leaves typically turn brown, wilt, and drop off. The typical grey fuzz appears at the later stages of the infection and is the most obvious indicator of Botrytis cinerea. Importantly, the mould is not always grey but can sometimes be brown or reddish, depending on the host crop and environmental conditions. 

The durability of Botrytis cinerea and its infectiousness make it a challenging problem to deal with. However, several strategies exist to prevent Botrytis cinerea growth and mitigate its harmful effects. 

How do I get rid of Botrytis cinerea?

Different climates have different risk levels for Botrytis cinerea. For instance, warmer, wetter climates with less wind will have a higher risk than others. These factors should be considered when deciding what Botrytis cinerea control measures to take. 

Biological control

Botrytis cinerea is susceptible to biological control methods, including biopesticides like bacterial and fungal strains, that can prevent its growth. A review article from the University of Salamanca of around 200 studies highlights the potential for biological control of Botrytis cinerea through multiple mechanisms. ​​Such strategies include the introduction of yeast strains and bacteria like Bacillus subtilis and natural plant products like thyme oil. This type of intervention is called augmentative biocontrol.  

Cultural control: 

Careful design of farms, orchards, and vineyards can go a long way to preventing Botrytis cinerea. Distancing crops to facilitate good air circulation and providing sufficient irrigation to avoid overwatering can help prevent Botrytis cinerea from emerging. 

After crops have grown, carefully trimming back the canopy, and promptly removing dying or dead material like leaves and blossoms can eliminate niches for Botrytis cinerea growth. If, after close analysis, you identify a crop with Botrytis cinerea, remove the infected part as carefully as possible without allowing it to come into contact with healthy plants. 

Chemical control:

Fungicidal treatments are only available as a preventative measure to Botrytis cinerea growth and are less likely to be effective in high-risk areas that receive a lot of rainfall. 

Some strains of Botrytis have developed ways to defend themselves from fungicides by using particular genes. These genes can make proteins that help the fungus resist chemicals, almost like activating a shield for a given strain. Thus, chemical control of Botrytis is of limited value because of pest resistance development. 

Conclusion and future direction

Botrytis cinerea is a persistent and highly infectious fungus that poses significant challenges to agriculture, particularly in the USA where it thrives in diverse climates. The fungus can survive in varied environmental conditions and develop resistance to chemical fungicides. Farmers and gardeners can significantly reduce the impact of Botrytis cinerea by prioritizing early detection, adopting careful cultural controls, implementing biological controls, and ensuring good agricultural practices. 

Grey mold, a symptom of Botrytis cinerea present on a tomato leaf
Botrytis cinerea on tomato leaf. Credit: Scot Nelson via Flickr

For more information on different types of pests and control strategies, you can consult the CABI BioProtection Portal resources. To help find the best solution for your Botrytis cinerea problem, visit our products page.  

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