Botrytis spp. is a fungal pathogen found in many plants, wherever they are grown around the world. The disease caused by this fungus may be known as: botrytis blight, gray mold, gray rot, blossom blight, noble rot (in grapes), or bulb rot (in onions). This fungus can infect a variety of plant tissues, and is able to grow and reproduce rapidly under ideal conditions, making it a difficult pest to manage. Understanding its life cycle and the environmental factors involved in its growth are essential for farmers and gardeners to prevent infection of this disease.

Botrytis Gray Mold Identification

Early symptoms of Botrytis infection can vary greatly on different plants and tissues, however, the mature fungal spores are fairly easy to identify. Infected areas will be covered in fuzzy, gray spores, that can sometimes turn brown, and infected fruit tissues will become soft and rot. Infected flowers can also develop the gray, fuzzy spores, and infected flower tissues will turn brown and die.

 

Infected leave tissues will typically develop lesions, and may appear sunken, soft and tan colored. On onions, lesions will first appear as white flecks on green leaf tissue. Infected leaf tissues will eventually wilt and die.

Botrytis Gray Mold Life Cycle

The gray spores, seen above, are spread by wind or splashing water and can infect other flowers, fruit or leaf tissues during the season if environmental conditions are met and a proper host is found. These infected tissues then become soft, and rot as the fungus grows until gray spores again are produced and the cycle continues. This rapid form of reproduction is how disease epidemics are found in fields.

However, if a suitable host is not found, the fungus can grow mycelia (non reproductive), or it can form sclerotia, small inactive black structures.These structures can survive on, or in, plant debris, or in soil, and can overwinter, and then produce conidia (reproductive spores) when proper environmental conditions are met.

Botrytis spp. lifecycle. George Agrios. Plant Pathology 4th ed.
Botrytis spp. lifecycle. George Agrios. Plant Pathology 4th ed.

This life cycle gives Botrytis the ability to both: survive in a wide range of environments, and reproduce very rapidly once ideal environmental conditions are met. This, along with its wide range of hosts, is the primary reason why Botrytis has become such a common, and widespread, plant disease.

How Environmental Conditions Affect Growth

The ideal conditions for rapid Botrytis growth are damp, somewhat cooler conditions (between 65° and 75° F). Large outbreaks generally occur in high humidity periods, after rain events, or following heavy overhead irrigation sets.

Botrytis Risk Infection Model

Researchers have developed a reliable risk infection model for Botrytis cinerea based on temperature and leaf wetness duration. The output of the model classifies infection risk from light to severe. This model was developed primarily for grapes, but because it based environmental factors for optimum infection and reproduction rates, it can be useful tool for any crop with risk of Botrytis cinerea infection. You just have to keep in mind, that the risk infection classifications provided by the model will be relative. Other factors such as crop stage, variety susceptibility, or the type of crop itself, when extending the model outside of grapes, will all play a factor in Botrytis infection. In terms of accounting for infection variability and risk based on weather, the model does a very good job at translating environmental factors into actionable data assessments.

Because most Botrytis controlling fungicides are preventative, the risk infection model can be a very useful tool for helping growers plan when to spray (or when to not spray). Once the fuzzy gray spores are present in the field, it will be too late for compromised plant tissues. However, if you can use preventative sprays during high infection risk periods to prevent infection from occurring. The model is also helpful for determining when not to spray. There is no sense in wasting money, labor and time, to spray when the targeted pathogen, Botrytis cinerea, is not able to complete its life cycle in current environmental conditions. Typically, extremes on either end of the risk model are intuitive, even to inexperienced farmers. After rain = high risk of infection, and hot and dry = low risk. However, the model can be very useful for everywhere in the middle and can take some guesswork out of timing preventative sprays.
The guideline for calculating the Botrytis risk infection model are outlined below, from UC IPM:

Infection Index = ln (Y/1-Y) = -2.647866 – 0.374927W + 0.061601WT – 0.001511WT2

Where: W = leaf wetness duration in hours; T = temperature in Celsius; and ln (Y/1-Y) = the logit of disease incidence where Y = the proportion of infected berries.

Additional model assumptions/implementation considerations:

  1. Split wetness periods: If the wetness sensor registers more than 4 hours dry then the model restarts wetness accumulation at next wetness event, otherwise, it combines wetness periods and notes that a split period has occurred.
  2. If T < 12 C then run the model as if T = 12 C (minimum temperature tested in experiments)
  3. If T > 32 < 40 then run model as if T = 32 C (maximum temperature tested in experiments)
  4. If T > 40 C then time interval is not conducive to infection
  5. If RH is greater than or equal to 95% then assume a wetness period is occurring due to limitations of wetness sensors.
  6. If more than 16 hours of wetness occur, regardless of temperature, then consider wetness event severe.

The disease index is calculated whenever leaf wetness is detected.

 The Pest Prophet app is another option for calculating infection risk models. User provides the location of the field, and the app will calculate risk infection level on a daily basis, making it the quickest and easiest way to start implementing disease risk models into an integrated pest management program. The Botrytis  risk infection model is coming soon.

Note on Leaf Wetness Duration as  Model Input

 Some disease risk models like the Botrytis  risk infection model use leaf wetness duration as input. The most accurate way to measure this directly is with a leaf wetness sensor such as this one by decagon sensors . These types of sensors should be placed within the leaf canopy, where they mimic a leaf and record leaf wetness presence and duration. Using sensors like this will require a data logging system (remote or otherwise) and they will require regular cleaning and maintenance to function properly. However, some weather systems such as the Pest Prophet app, are able to model the leaf wetness using a variety of weather parameters and in the case of Pest Prophet can achieve about 90-95% accuracy without the cost of the leaf wetness sensor, or any maintenance. This is an attractive option for most growers, especially those without existing weather data logging infrastructure on their field.

Botrytis Gray Mold Management

This disease can be difficult to control once an outbreak has begun, however, it is possible to manage with cultural practices and selective preventative fungicides, as a part of an overall integrated pest management program. As is the case with most fungal pathogens, control options will all be preventative in nature, because once the fungus has successfully infected plant tissue, infected cells will not be able to survive. While different crops may have different, specific, management procedures; adhering to the general practice below will help prevent, or at least minimize, Botrytis infections.

1. Before Planting

The first step towards controlling Botrytis infection comes even before planting occurs. There are, obviously, a large number of factors that go into planning where a crop, orchard, or vineyard will go, and Botrytis management will likely not be the most important determining factor. However, in an organic system, or area with especially high Botrytis risk, it should be a consideration when planning where to plant. Micro-climates with less air movement, or higher relative humidity, are going to have a higher risk of Botrytis infection than areas with more wind, where standing water droplets will dry out faster. For this reason, relatively warmer areas are also preferable.

This concept should also be considered when blocking different varieties within field, vineyard or orchard. In most cases, relative susceptibility to Botrytis infection can vary greatly by variety or cultivar. For example, Petite Sirah is highly susceptible to botrytis bunch rot, but Cabernet Sauvignon is highly resistant. So the Petite Sirah should be planted in an area on the vineyard that gets more wind, or is slightly warmer.

Orientation of planting rows can also affect air movement through orchards or vineyards. Planting trees in the same direction that the wind moves will capture this air movement, while planting perpendicular will block the wind.

2. Plant Canopy Management

Another aspect in preventing Botrytis infection is effective plant canopy management. The specific actions related to this concept will vary by crop, but there are some common goals to aim for. These are to:

  • *Increase air movement
  • *Ensure maximum spray coverage when fungicides are used
  • *Improve drainage and avoid standing water

Some common ways to achieve these goals are:

  • *Pruning
  • *Removing old leaves (especially beneficial in grapes and strawberries). Note: in some cases it might even be beneficial to remove younger, healthy leaves if canopy density is especially high.
  • *Increasing plant spacing
  • *Trellising, if crop requires
  • *Controlling plant vigor/ density through irrigation and nutrient management.

Simple canopy management procedures sometimes produce very effective results. In an 1987 study on leaf removal in grapes, researchers found that removing leaves was a more effective measure than spraying fungicides for controlling botrytis bunch rot.

One way that canopy vigor can be assessed and measured is through overhead imaging. This technique can be useful in a variety of ways, but it can also be used to determine higher botrytis risk areas. One use case may be to map canopy density and find areas where leaf removal and/ or pruning may be most beneficial, without spending the resources to manage the entire orchard.

Plsnt Vigor in Almond Orchard. Photo: Ceres Imaging
Plant Vigor in Almond Orchard. Photo: Ceres Imaging

3. Irrigation Techniques

Because Botrytis infection is driven by free moisture, careless irrigation practices can sometimes drive infection. In general, overhead watering with sprinklers can greatly increase risk of infection in all crops where they are used. Using drip tape is preferable, but if overhead sprinklers are used, irrigation sets should be timed to limit standing water on crops. Overhead watering should especially be avoided during bloom times, when flowers are most susceptible.  Over-watering with drip tape can also lead to excess moisture and increase infection risk.

4. In-Field Sanitation

Possibly the single most effective management technique in preventing large scale Botrytis outbreaks is maintaining good field sanitation by removing inoculated plant tissues (Fruit, berries, flowers, leaves, etc.) from the field. The gray conidial spores, which are easily recognizable, are the spores that will infect other flowers, or parts of the plant if spread by water or wind, so removing them completely from the field (not just dumping them in the furrow, or on the ground) greatly reduces the level of inoculating spores present. This technique is usually very easy to accomplish in a garden, but can be more difficult, or labor intensive in a large scale field, orchard or vineyard.

5. Biological Control Options

There are some biologically derived options for controlling Botrytis gray mold, for when sprays may be a necessity. Bacillus spp. (product name: Serenade) is commonly used option, that usually performs pretty well against Botrytis in most cases. Gliocladium catenulatum  and Clonostachys rosea  are two other biologically derived microbials that have shown some success in controlling the spread of Botrytis on strawberries, flowers and other crops. As an interesting alternative to spraying at all, these microbial products can also be applied to flowers by using bees.

6. Chemical Control Options

There are a number of chemical fungicides, with different active ingredients that are effective in managing Botrytis infection. Some commonly used products are Captan (phthalamide), Elevate (Fenhexamid), Pristine (Pyraclostrobin/Boscalid), Switch (Cyprodinil + Fludioxonil) and a number of other products.  However, not all are available for all crops and some may be damaging to specific plants, or at specific stages of crop development. 

Botrytis spp. is also able to develop resistance over time to certain fungicides. For this reason, it is very important to rotate different modes of action within a growing season. If certain products are not effective in an area on a given crop, resistance may have developed and that product should be avoided. It is also important when using chemical sprays, to optimize spraying in order to maximize the intended effect of the product.

Learn More about Botrytis Management for Specific Crops:

Sources:

Sclerotinia fuckeliana (conidial state: Botrytis cinerea). Ellis M.B., Waller J.M..C.M.I. Descriptions of Pathogenic Fungi and Bacteria 1974

Environmental Conditions Affect Botrytis cinerea Infection of Mature Grape Berries More Than the Strain or Transposon Genotype. Ciliberti et al. Phytopathology 2015.

Control of Botrytis Bunch Rot of Grape With Canopy Management. Gubler et al. Plant Disease. 1987.