C2 – Grey Mould

Introduction

Grey mould or Botrytis (caused by Botrytis cinerea) is common on a wide range of herbs including woody members of the Lamiaceae family (lavender, rosemary and sage), flowering crops (e.g. chamomile) and especially on delicate herbs such as basil and dill. The fungus is non-host specific and is an opportunist or wound pathogen as it readily colonises damaged or ageing tissues. For this reason, crops that are repeatedly harvested (e.g. cut basil) are particularly at risk, although the disease is common in all types of herb production. Flowers and leaves are often affected first, although Botrytis may attack any plant part, particularly under cool, humid conditions. As well as developing during production, symptoms may occur when herb products are in cold storage or after dispatch, especially if high humidity or condensation occurs.

 

Symptoms

Botrytis first appears as a white ‘mould’, often near the plant base, but darkens to grey as air-borne dispersal spores (conidia) are formed. Black resting structures (sclerotia) may also develop in affected tissue, enabling long-term survival of the fungus in the absence of a host plant.

Symptoms vary depending on the species attacked and the growth stage. On flowering herbs such as chamomile, typical symptoms include water-soaked brown spotting on flower petals. On non-woody herbs, shoot die-back, and leaf and stem rot are common. Grey mould may first be visible near stem bases (Figure 1). For woody herbs, stems are often discoloured or necrotic but may not develop the typical grey spore mass (Figure 2).

Botrytis is commonly encountered on basil grown as a cut herb, particularly after harvests late in the season. Infection occurs via cut stems soon after harvest, and the disease progressively kills all leaves and secondary shoots. Stem material becomes reddish brown and woody in appearance, often with typical grey sporulation visible (Figure 3). When infection in a side shoot reaches the main stem at the bottom of the plant, the entire plant dies.

Botrytis may also develop on herb products that appear healthy when they are dispatched from a nursery. The disease can affect packaged cut herbs and also sleeved pot herbs such as basil, sage and thyme, resulting in rapid deterioration of the product (Figure 4).

Disease sources and spread

Grey mould on herbs can originate from a range of sources and be transmitted via several mechanisms (Figure 5). The main sources of Botrytis cinerea are diseased plants and infected crop debris. Other sources of the disease include long-lived survival structures (sclerotia) in soil, infected cuttings and occasionally, infected (or contaminated) seed.

Disease spread occurs most commonly via the dispersal spores (conidia) which are produced in abundance on affected plant tissues. These spores are spread by air currents or splashing water. Human activity within a crop or sudden fluctuations in relative humidity (RH) will result in spore release. Infection may also arise from fungal hyphae growing from either dead plant parts or from crop debris in contact with a host plant. Inoculum of B. cinerea in dead tissue is an important source of infection, contributing to localised disease spread. Long distance spread between glasshouses and different nurseries can occur through the use of infected (or contaminated) seed, plants or cuttings. Spore spread via insects may occur but is unlikely to be a common mode of transmission.

Conditions for infection

B. cinerea thrives under cool humid conditions although it is active over a wide temperature range.

Spores of B. cinerea can germinate and infect plant tissue immediately, or may remain dormant on plant surfaces for up to 3 weeks. High RH favours rapid spore germination and infection. Germination can occur over a very wide range of temperatures (5-25ºC). A complete infection cycle, from spore germination through symptom development to new spore production, can be as short as a few days.

Results from experiments on container-grown ornamentals (HDC project PC/HNS 121; HortLINK project 25) demonstrated that humidity is a controlling factor for germination of B. cinerea spores, whereas leaf wetness encourages disease expression once the plant has been infected.

A high humidity level, greater than 95% for a period of more than 3 hours, is the critical threshold for the germination of spores of B. cinerea. Once this period is exceeded then spore germination will continue even if the humidity is reduced to 80% or less. However, where high humidity periods are kept at 3 hours or less, there is little germination or disease development, even after a period of fluctuating high and low humidity periods.

Once infection has occurred, leaf wetness is important for symptom expression, with disease severity increasing with leaf wetness duration. Symptoms may develop within hours, or the infection may remain dormant and symptomless, not becoming active until days or even weeks later. This is known as latent infection. Due to the potential for latent infection by B. cinerea, cuttings taken from apparently healthy but infected stock plants can subsequently develop grey mould symptoms. Similarly, produce that appears healthy at the time of harvesting and packaging can later develop symptoms of grey mould during transit or storage.

Research from Italy has shown that for cut basil crops grown in polytunnels, outbreaks of grey mould are more likely to occur when harvesting takes place on rainy days rather than dry days. This can occur due to limited vent opening during rainy weather and subsequent high RH. Moreover, condensation of water on the inner side of the polyethylene covering may drip onto the plants, contributing to leaf wetness.

Different herbs may be more prone to infection at particular growth stages. For example, basil stem cuttings are highly susceptible to infection immediately after harvest, but this diminishes with time, so that by 48 hours after harvest, cut stems are no longer susceptible to infection by B. cinerea.

 

Integrated disease management

Botrytis is a persistent, adaptable fungus that can cause crop damage under a wide range of conditions via several infection routes. Therefore, a combination of management strategies that each act by different means is more likely to provide effective and sustainable control than any single measure.

 

Cultural control

The biology and control of grey mould on protected crops has been well researched (see Further Information). The availability of detailed information on conditions that favour Botrytis development provides the opportunity for disease management through careful nursery hygiene, avoidance of plant damage and manipulation of environmental conditions, with minimal need for chemical intervention. The following information is adapted from HDC Factsheets 23/02, 24/02 and 25/02.

Glasshouse and crop hygiene

• B. cinerea is a common seed contaminant on a wide range of species and can occasionally be present on seed as a more deep-seated infection. Ensure that seed is obtained from a reputable supplier.
• Inspect plants arriving on the nursery for Botrytis and if disease is present then reject plants, or quarantine and treat.
• Plants may carry latent infection for several weeks, so continue to monitor new plants for this time, in particular checking foliage that is dense or in contact with the growing medium.
• Infected plant debris should be removed promptly from plants, benches, floors and beds, and disposed of in covered bins or skips outside the glasshouse.
• Thoroughly disinfect the glasshouse structure if there has been a severe outbreak of Botrytis.
• Avoid taking cuttings from affected plants.

Crop management

• Manage production in order to minimise fertiliser/pesticide scorch, nutrient deficiencies or excesses and soft growth, as these factors can predispose plants to infection by B. cinerea.
• As far as possible, avoid plant wounding at transplanting or potting, spacing, and dispatch.
• Results from Israeli research demonstrated that modification of irrigation water, by halving standard nitrogen (N) content and doubling standard calcium (Ca) content could suppress grey mould development on protected basil. It was stressed that the ideal N and Ca concentration should be determined specifically for each production system, aiming for a N content of ~2.9% and a Ca content of ~1.3% in leaves.
• Where space is available, separate plants in order to improve air circulation and also to reduce the risk of losses due to contact spread of the disease between adjacent plants (this may differ according to the growth form of a particular species or variety).
• Overhead irrigation should only be done where there is time for foliage to dry before nightfall.
• If using drip and sub-irrigation, avoid overwatering of compost as this creates an environment favourable to Botrytis around the plant base.

Environmental conditions
It is important to restrict high humidity periods to 3 hours or less in order to reduce the risk of infection from newly deposited spores of B. cinerea. The key objective is to prevent moisture lying on plant tissues for several hours or an atmosphere of still, very humid air persisting around plants. Even lowering the humidity slightly can have a significant effect. Details on measuring humidity and guidance on humidity control to prevent Botrytis are given in HDC Factsheet 25/02. In summary, good humidity control needs to manipulate both the ventilation and heating system, to give the correct humidity and temperature conditions without excessive energy use. Practical guidelines include the following:

• Use fans to keep air moving over the plant surface.
• Ensure that humidity measurement is accurate and that climate control computer set points consistently prevent high humidities from occurring.
• Consider the use of heat boosts (drying periods) during periods of high risk.
• Avoid rapid rises in air temperature which leave a plant cold and increase the risk of condensation on lower leaves.

 

Biological control

Serenade ASO based on the bacterium Bacillus subtilis has an Extension of Authorisation for minor use (EAMU) for use on protected herbs, and has reported efficacy against Botrytis. Prestop and Prestop Mix (containing the fungus Gliocladium catenulatum) have on-label approval for all edible crops and provide moderate control of Botrytis.

Biological control alone may be insufficient to achieve consistent Botrytis control on a crop-scale particularly under high disease pressure, but can form a useful component of integrated disease management. Where necessary, biological fungicides can be alternated with conventional fungicides.

Chemical control (protected herbs)

Implementation of the cultural practices and biological control described above should help minimise the need for conventional fungicide use to control grey mould on protected herbs. In situations of high disease risk, however, several of the fungicides approved for use on protected herbs have activity against B. cinerea (Table 1).

Some of the older fungicides such as iprodione prevent spore production, while newer fungicides (e.g. azoxystrobin and pyrimethanil) may prevent initial infection. Once the pathogen is established within a crop it is difficult to eradicate because the spores may have already spread to other plants and initiated new infection before the disease is discovered. For this reason, preventative treatments are most effective for Botrytis control.

The best control of grey mould is likely to be achieved if plants are sprayed at a high water volume, sufficient to achieve full coverage. For example, on woody herbs grown in containers for garden centres, grey mould may develop around the stem base where the foliage is in contact with the growing medium, so it is important to get the fungicide onto foliage near the plant base for good protection.

Before selecting products to use for Botrytis control on protected herbs, test treat a small batch of plants before widespread application if using a fungicide for the first time, to ensure crop safety. Be aware that for Amistar, the harvest interval is 28 days for crops harvested in November to April and 14 days for crops harvested in May to October.

Resistance has been confirmed in B. cinerea to iprodione (Rovral WG) and, less frequently, to fungicides in the anilo-pyrimidine group (e.g. Scala). Where resistant isolates of the fungus are present, treatment with these fungicides will provide little or no disease control. Where strains of B. cinerea are resistant to one fungicide in a particular group (see Table 1), there is usually also resistance to other members of the same fungicide group. See Section A – Principles of IPM for further details on strategies to minimise the risk of selecting resistant strains.

Table 1. Fungicides with activity against Botrytis cinerea and permitted on protected herbs (March 2013)

Fungicide group and active ingredient(s)	Example product	Compatibility with pest biological control
1. Anilino-pyrimidine Pyrimethanil	Scala	Generally safe but harmful to Amblyseius swirskii (thrip control)
2. Dicarboximide Iprodione	Rovral WG	Safe
3. DMI Prochloraz	Scotts Octave	Moderately harmful*
4. Hydroxyanilide Fenhexamid	Agrovista Fenamid Teldor	Moderately harmful*
5. Microbial Bacillus subtilis Gliocladium catenulatum	Serenade ASO Prestop, Prestop Mix	Safe Safe
6. QoI Azoxystrobin	Amistar	Safe
7. Carboxamide + QoI Boscalid + pyraclostrobin	Signum	Not yet known

*Adverse effects on certain biological agents

• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants.
• Pesticide approval information in this guideline is current at 31 March 2013.
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/.  General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
• Always follow label recommendations or statutory conditions for use on Extension of Authorisation for minor use (EAMU, previously SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
• Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with an EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
• Use pesticides safely.

 

Further information

HDC Factsheet 23/02. Control of grey mould (Botrytis cinerea) in container-grown ornamentals: unheated greenhouse crops.

HDC Factsheet 24/02. Control of grey mould (Botrytis cinerea) in container-grown ornamentals: heated glasshouse crops.

HDC Factsheet 25/02. Controlling humidity to minimise the incidence of grey mould (Botrytis cinerea) in container-grown ornamentals: heated glasshouse crops.

Yermiyahu, U., Shamai, I., Peleg, R., Dudai, N. & Shtienberg, D. 2006. Reduction of Botrytis cinerea sporulation in sweet basil by altering the concentrations of nitrogen and calcium in the irrigation solution. Plant Pathology 55: 544-552.