Introduction

Grey mould or botrytis (Botrytis cinerea) is common on many herb species including woody perennials (lavender, rosemary and sage), flowering crops (e.g. chamomile) and delicate herbs (e.g. basil and dill). The pathogen readily colonises damaged (including from repeat harvesting) or ageing tissues and will also often infect flowers and leaves first. Harvested herbs in cold storage or in transit may also develop symptoms. High humidity and condensation are particularly likely to promote infection and symptom development.
B. cinerea causes crop damage under a wide range of conditions via several infection routes, so disease control is best achieved using an integrated management approach.

Botrytis symptoms and biology on herbs

Symptoms

• Botrytis produces fluffy grey-brown fungal strands bearing spore masses and causes plant collapse and death. Symptoms are often first visible near stem bases (Figure 1)
• On flowering herbs such as chamomile, typical symptoms include water-soaked brown spotting on flower petals.
• In non-woody herbs, shoot die-back, and leaf and stem rot are common.
• For woody herbs, stems are often discoloured with dead tissue but may not develop the typical grey spore mass (Figure 2).
• Black structures (sclerotia) may develop, enabling long-term survival of the fungus in the absence of a host plant.
• 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 sleeved pot herbs such as basil, sage and thyme, resulting in rapid product deterioration (Figure 3).

Biology

The main sources of infection for Botrytis are already diseased plants and infested crop debris. Other sources include long-lived survival structures (sclerotia) in soil, infected cuttings and occasionally, infected or contaminated seed (Figure 4).

Disease transmission is most common via dispersal spores (conidia) spread by air currents or splashing water. Fungal strands growing from infected plant parts or crop debris can also spread the disease locally. Long distance spread occurs via infected (or contaminated) seed, plants or cuttings. Spore spread via insects may occur but is less common.

B. cinerea thrives under cool humid conditions but is active over a wide temperature range (5-25ºC). Spores can germinate and infect plant tissue immediately or remain dormant on plant surfaces for up to 3 weeks. A complete infection cycle, from spore germination through symptom development to new spore production, can be as short as a few days.

Relative humidity (RH) is the key environmental factor affecting spore germination and infection. High RH, greater than 95% for more than 3 hours, is the critical threshold for B. cinerea spore germination. 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 RH. Once infection has occurred, leaf wetness is important for symptom expression, with disease severity increasing with leaf wetness duration.

Botrytis symptoms may develop within hours, or the infection may remain symptomless for days or weeks (latent infection). Because of latent infection, cuttings taken from apparently healthy but infected stock plants can subsequently develop grey mould symptoms. Similarly, produce that appears healthy at harvest can later develop botrytis symptoms during transit or storage.

Cultural control of botrytis on herbs

Based on knowledge of botrytis biology, this section outlines how cultural practices, in particular crop hygiene, growing the crop well and avoiding high risk conditions for disease development, provide the foundation for botrytis management on herbs.

Crop hygiene to control botrytis on herbs

• B. cinerea is a common seed contaminant on a wide range of species and can occasionally be present as a more deep-seated infection. Ensure that seed is obtained from a reputable supplier.
• Inspect plants arriving from the propagator 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 soil.
• Infected plant debris should be removed by mowing or flailing crops, ideally with debris collected in a hopper or trailer, and disposed of by composting in a dedicated area away from the field or glasshouse.
• Unused infected transplants can be disposed of in covered skips. Gutters, benching, trays, pots and trolleys should all be part of a regular cleaning programme or used new. Where disease has occurred, there should be a deep clean of surfaces with particular attention to crop debris removal.
• Machinery used for crop destruction should be pressure-washed, away from growing areas.
• If it is not practical to remove infected plant material from the growing area, a gas burner could be used before, after, or even instead of mowing, depending upon crop architecture and the level of infection.

Crop management to control botrytis on herbs

• Plants should be handled carefully at planting to avoid damage, both by workers and by careful setting of transplanting machinery.
• Growers should follow best practice for soil nutrients by following recommendations in the AHDB Nutrient Management Guide (RB209) or use a FACTS qualified advisor to avoid over application of fertiliser. Nitrogen rates, in particular, should be carefully considered, as excess can lead to soft leaf tissue, which is more prone to botrytis infection. Check soil pH on a regular basis, aiming for 6.5-7.0 to ensure crop uptake of nutrients.

Avoiding environmental conditions high risk for botrytis on herbs

High relative humidity (>95%) for more than 3 hours is needed for botrytis spore germination to occur. Reducing RH level and duration is key for botrytis control. This is more straightforward in protected cropping but can also be considered for field crops:

• The use of crop environment monitoring systems can help to identify when RH needs to be reduced and to target fungicide applications when they will be most effective.
• In glasshouses, fans help to keep air moving over the plant surface while heat boosts can be used to provide ‘drying’ during periods of high risk. Avoid rapid rises in air temperature which leave a plant cold and increase risk of condensation on lower leaves.
• Overhead irrigation should be applied early in the day so that foliage can dry before nightfall. Equipment using large, heavy droplets can damage leaf tissue leaving plants more susceptible to infection; apply irrigation using a boom or sprinkler where possible.
• Drip or sub-irrigation should be deployed for more susceptible crops, with the advantage of keeping foliage dry. Avoid over-application which may create high humidity favourable to botrytis around the plant base.
• To reduce humidity, avoid high density planting or drilling of the most susceptible crops, e.g. basil or rosemary.
• The use of crop covers for insect exclusion or hail damage prevention may exacerbate conditions for botrytis. Consider increasing air flow by supporting covers above crops. Assess which target is the greater risk to the crop and amend management strategy accordingly.

Biological Control of Botrytis in Herbs

Biological control alone may be insufficient to achieve consistent botrytis control on herbs, particularly under high disease pressure, but can form a useful component of integrated management. There are a number of bioprotectant options available, which where appropriate, can be integrated with conventional fungicides.

Biofungicides registered as plant protection products with Extension of Authorisation of Minor Use (EAMU) for herb crops, with activity against botrytis (May 2021) are listed below:

• Amylo X WG (Bacillus amyloquefaciens sub sp. plantarum, strain D747) can be applied as a ground spray between March to October in open field production or all year-round for protected crops.

• Serenade ASO (Bacillus subtilis strain QST 713) can be applied as a foliar spray for outdoor and protected herbs.

• Prestop (Gliocladium catenulatum strain J1446) can be applied as a foliar spray, drench or for soil incorporation for field production. In addition, Prestop Mix can be used as a drench or for soil incorporation with protected edible crops.

• To optimise biopesticide use, consider details such as product storage, application methods, environmental factors and compatibility with conventional pesticides. The AHDB AMBER project provides useful guidance on biopesticde application.

Integrating plant protection products for the control of botrytis on herbs

Use of cultural practices and biological control can help minimise the need for conventional fungicides to control botrytis on herbs. In situations of high disease risk, however, there are several fungicides with different modes of action approved for use in herb production that have activity against B. cinerea.

Growers should consult a BASIS qualified advisor to decide upon the most suitable programme. Once botrytis is established within a crop it is difficult to eradicate because spores may have already spread to other plants and initiated new infections before symptoms are seen. For this reason, protectant treatments are most effective for botrytis control. In situations of high disease risk, several of the conventional fungicides approved for use in herb production (June 2021) have activity against B. cinerea (Table 1).

Table 1. Fungicides with activity against botrytis permitted for use on herb crops (June 2021)

Fungicide group and active ingredients	Example Products
Aniliopyrimidine & phenylpyrrole
Cyprodinil & fludioxonil	Switch
Biological activity
Bacillus amyloliquefaciens	Amylo X WG
Bacillus subtilis	Serenade (ASO)
Gliocladium catenulatum	Prestop (outdoor only); Prestop Mix (protected only)
Hydroxyanilides
Fenhexamid	Teldor
SDHI & strobilurin
Boscalid & pyraclostrobin	Signum
Strobilurin
Azoxystrobin	Amistar
Strobilurin & triazole
Azoxystrobin & difenoconazole	Amistar Top (outdoor only)

B. cinerea can rapidly develop resistance to fungicides if certain active ingredients are frequently and repeatedly used; this has been reported for other crops (e.g. tomato and strawberry) to a varying extent for all the conventional fungicide actives currently approved on herbs. Strategies to minimise the risk of developing fungicide resistance can be found on FRAG and FRAC websites.

Botrytis control is most effective if fungicides are applied at a sufficiently high water volume to get full coverage. Use of water sensitive paper placed within the crop can show how well this has been achieved. Advances in nozzle technology, such as angled application and drift reduction, can all help to optimise spray coverage. Adjuvants can be used to increase fungicide coverage over the leaf target area. In all cases, read the product label carefully when deciding upon the pesticide application rate – full rate use may not be permitted depending upon the adjuvant used.

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. All products that are approved by an EAMU are applied at the grower’s risk and are not backed by the manufacturer or supplier.

Chemical approvals regularly change and the listing of a product does not constitute a recommendation for use. Check the approval status and suitability of application on the HSE pesticides website prior to any application.

 

Further information

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

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

AHDB 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.