Management of botrytis in cut flower peony crops

Botrytis is one of the most economically significant pathogens affecting peony, and can result in crop losses in excess of 20% during wet years.

Management of the disease is reliant on cultural control, site hygiene measures and fungicide inputs. Several botrytis species are known to infect peony, including Botrytis paeoniae, Botrytis cinerea, Botrytis pseudocinerea and Botrytis euroamericana. Generally, B. paeoniae is responsible for early blight and is considered specific to peony, while B. cinerea and B. pseudocinerea are responsible for late blight.

Developing in the spring, botrytis is a recurrent problem for all peony growers, especially during wet periods where the disease can be very prevalent. Infections affect new shoots and foliage leading to bud abortion or unsightly and unmarketable blooms. As a perennial crop, inoculum levels will accumulate over time where not addressed, increasing losses. Severe outbreaks can lead to the loss of a large proportion of young stems, as well as affecting the developing buds/blooms. Post-harvest losses are also a problem where symptoms develop on buds/blooms in retail stores.

Disease symptoms

Botrytis can develop on almost all plant tissues, at every developmental stage. Early in the season, the presence of inoculum sources at stem emergence enables early blight infections to develop almost immediately. Infected young shoots suddenly wilt and fall over due to lesions, which can fully girdle stems. On well-developed lesions, under conducive climatic conditions, spore-bearing structures emerge, shedding spores for further infection.

Flowers are the most susceptible tissue type to botrytis infection and can be infected from the earliest stages of bud development. Bud blast (early blight) develops on small young buds, where infection causes them to blacken and fail to open.

Mature buds can also become infected, failing to open, or only partially open. Individual petals may become infected, starting as brown spots before coalescing to cover the entire petal, eventually taking on a papery, dry appearance. During favourable conditions of leaf wetness, a velvety grey mass of fungal spores will develop which can rapidly cover part of, or all of the bud. Further infection can develop down the flower neck, and after time the infected bloom may fall away.

Infected stems which do not collapse early, develop a tan appearance with concentric rings which can fully girdle the stem. In situations where leaves become infected, large irregular concentric brown leaf spots develop, often from the leaf tip spreading inwards.

Late blight infections differ from those of early blight. Typically affecting late opening flowers, the fungal growth is more diffuse than the tight fungal growth in early blight infections (see images).


a close-up of a flower

Infected stems with a tan appearance and lesions which fully girdle the stem base, Source: Michelle Grabowski, University of Minnesota Extension - Horticulture, Bugwood.org

Brown and papery petals as a consequence of botrytis infection, Source: ADAS Horticulture


a close-up of a brain

Velvety grey botrytis sporulation at the flower base, Source: ADAS Horticulture

a lizard on a leaf

Early botrytis sporulation and tissue browning on infected peony leaf, Source: ADAS Horticulture


Sources of infection

Botrytis is a near ubiquitous fungus producing huge quantities of airborne spores which are present in almost all environments. Despite the abundant presence of this organism, infections occur predominantly on dead, weakened and stressed plants. The perennial nature of peony cultivation allows for accumulation of botrytis species over time and this must be managed to limit losses. In the UK two species of botrytis are confirmed to infect peony, however a greater diversity of botrytis species may also cause disease.

Botrytis paeoniae: This pathogen is responsible for early blight (bud blast) and is believed to be species specific to peony. The initial source of B. paeoniae is likely to be from propagative materials, or transmission from equipment and machinery used at other peony production sites. Longer term inoculum sources include crop debris.

Botrytis cinerea: Responsible for late blight, B. cinerea can overwinter in debris. B. cinerea inoculum sources may also be introduced to peony from nearby host species.

Botrytis pseudocinerea: This species of botrytis is morphologically identical to B. cinerea. It is present in the UK and is anticipated to be a pathogen of peony.

Management strategies

Similar to all botrytis outbreaks on ornamental plants, management of peony botrytis is most difficult during, or following wet seasons. Once emerged, peonies grow rapidly and are not resilient to sudden environmental changes. Cultural and site hygiene measures in combination with fungicide and bioprotectant applications, provide the most effective control strategy.

Variety selection

Variety choice is a key component of any control strategy and the use of tolerant, or resistant varieties can significantly reduce the requirement for fungicide inputs. However, there is little information on the relative susceptibilities of peony varieties to botrytis. The popular variety ‘Sarah Bernhardt’ claims full resistance by some, but is described as moderately resistant by others. Several growers claim that vigorously growing varieties, and those with thicker stems, are more susceptible to botrytis, however no published work investigating this is available. Intersectional varieties, hybrids between herbaceous and tree peonies are also considered to have greater resistance, but are less suitable for cut flowers, as blooms from these types have a shorter vase life. However, for most large commercial plantings, the main factors impacting varietal selection are yield and cost of crowns, rather than disease susceptibility.

Cultural control and site hygiene measures

A range of cultural control and site hygiene measures are available which can reduce sources of botrytis inoculum and limit crop damage. The following lists best practice recommendations for managing botrytis in peony.

Leaf wetness: Prolonged leaf wetness is essential for disease development, avoiding this represents the single most effective tactic to manage botrytis in peony, although leaf wetness management will obviously be problematic with outdoor crops.

Start clean: Ensure all propagative material is disease free to avoid introducing the pathogen at planting, where possible use stock that is certified free of disease. Ensure that suppliers have sterilised crowns before delivery.

Site selection: Plant new peony crops during cool weather in the autumn, before the first frost. If feasible, plant crops away from potential sources of botrytis inoculum, reduce plant density, or site crops in an open situation which will increase airflow, reducing periods of leaf wetness. Avoid areas which frequently receive late spring frosts or high winds to reduce crop damage.

Mulching: While mulching of commercial crops to prevent freezing is rarely used in the UK, where it has been applied it should be removed in spring to allow the ground to dry around newly emerging stems. Farmyard manure has been implicated with favouring botrytis establishment and should be avoided.

Weed and debris removal: Weeds should be well controlled to improve airflow around the base of plants, and to remove potential inoculum sources of B. cinerea. In early autumn (mid-September) all plant material should be cut back to, roughly 10 cm above the soil, with the debris removed and destroyed.

Removal of infected material: If practical, in the spring remove young infected shoots as soon as wilting appears. Infected buds, and other infected material should be removed when found.

Bloom avoidance: Petals are highly vulnerable to botrytis infection and once infected can act as secondary disease sources as a consequence of petal stick. Avoid flower bloom in fields, ideally removing the flower buds that will not be harvested at the same time as cutting the marketable stems, ensuring that flower petals cannot develop. This will reduce post-harvest losses through reduced inoculum levels on harvested buds.

Harvest: Use sharp, clean blades to harvest flower buds and that these are frequently disinfected with a product known to be effective against botrytis.

Post-harvest: Maintain low relative humidities and avoid fluctuations in post-harvest stores.

Fungicides and bioprotectants

Fungicides

As botrytis is able to infect at stem emergence, timely and preventative application of fungicides is essential for effective management. The degree of chemical control required will be dependent on environmental conditions, the age of the crop and the severity of the disease in the previous season.

Many fungicides are available to treat peony botrytis, covering multiple active ingredients and several modes of action. Over the course of a season, a peony crop should receive four (or more) fungicide applications for botrytis control, dependent on conditions and varietal susceptibility. The first fungicide application should be a preventative, systemic treatment, applied at stem emergence to protect developing shoots, this targets B. paeoniae.

Further fungicide applications may be applied as frequently as every 14 days after bud formation (targeting B. paeoniae and B. cinerea). However, additional applications may be necessary during wet periods, or to protect crops following damage from high winds, frost, heavy rain or hail. Fungicide application prior to harvest will suppress the inoculum present on buds, reducing post-harvest rots and supermarket rejections. Harvested plants need to be treated after cutting is complete, protecting the wound sites which are susceptible to pathogen invasion. Further fungicide use will reduce disease levels as the crop moves into winter.

Products used by UK peony growers as part of fungicide programmes to manage botrytis in peony (December 2020)

Product

Active ingredient

FRAC code and resistance risk

Notes

Amistar

Azoxystrobin

11 - high risk

Systemic, translaminar and protectant fungicide

Bravo

Chlorothalonil*

M5 - low risk

Protectant fungicide

Manzate

Mancozeb**

M3 - low risk

Protectant fungicide

Nativo

Trifloxystrobin + tebuconazole***

11 - high risk

3 - medium risk

Protectant fungicide

Systemic fungicide

Signum

Boscalid +

pyraclostrobin

7 - medium risk

11 - high risk

Protectant fungicide

Systemic fungicide

Switch

Cyprodinil +

fludioxonil

9 - medium risk

12 - low/medium risk

Broad spectrum fungicide mixture

*Chlorothalonil was withdrawn from use in 2019, with final use of remaining stock by May 2020.

**At the time of writing the withdrawal of mancozeb has been confirmed by the EU, however a withdrawal notice has not yet been issued by the Chemicals Regulation Division (CRD) confirming grace periods for sale and use in the UK.

***The future of tebuconazole is in doubt as it is a potential endocrine disruptor.

Fungicide products recently granted EAMUs for use against botrytis in ornamental production (February 2021)

Product and EAMU No.

Active ingredient

FRAC code and resistance risk

Notes

Frupica SC

(1294/2019)

Mepanipyrim

9 - medium risk

Protectant fungicide. Applications to be made between 15th May – 30th September, and must only be made after 1st flower (BBCH 49), immediately post trimming.

Luna Privilege

(0289/2021)

Fluopyram

7 - medium risk

Systemic, protectant fungicide. Latest time of application BBCH 89.

Prolectus

(0784/2019)

Fenpyrazamine

17 - low/medium risk

Protectant fungicide. Applications can be made between 1st March and 30th September. Final application: 1 day pre-harvest.

Sercadis

(4348/2019)

Fluxapyroxad

7 - medium risk

Protectant fungicide. Application must only be made between 1 April and 30 September.

Bioprotectants

Alongside chemical fungicides, bioprotectants have been demonstrated to be effective against a range of fungal species and several products are now available for use outdoor in the UK. Unlike fungicides, careful consideration on how to apply these products must be made to ensure their use is optimised. Several bioprotectants have short, or zero day harvest intervals enabling application right up to harvest as well as avoiding some of the operator exposure risks associated with conventional fungicide use.

Bioprotectants currently authorised for use in outdoor ornamental production in the UK (December 2020)

Product

Active ingredient

Type

Maximum no. applications

Notes

Amylo X WG

Bacillus amyloliquefaciens D747

Bacterial

6

Preventative, apply before, or immediately after the pathogen is first observed.

Fytosave

COS-OGA

Plant defence elicitor

8

2-3 preventative applications are required prior to pathogen challenge to elicit a good level of plant defences.

Prestop

Gliocladium catenulatum

Fungal

1

Preventative, antagonistic to other fungi (multiple modes of action).

Serenade ASO

Bacillus subtilis (strain QST 713)

Bacterial

6

Preventative, apply before, or immediately after the pathogen is first observed.


Further resources

'A review into management practices of botrytis in peony' report

For further information see ‘A review into management practices of botrytis in peony’ prepared by Dave Kaye, ADAS Horticulture available on the CFC and AHDB websites.

Read the full report

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Wayne Brough

Knowledge Exchange Manager - Horticulture (Ornamentals)
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