Control strategies for powdery mildew diseases of poinsettia crops

A combination of cultural and fungicide control measures will provide the best strategy for managing powdery mildews on poinsettia crops.

Powdery mildews require a living host plant to complete their life cycle. The asexual life stage is what is seen, with no sexual resting spore stage reported, at least with the American and Mediterranean powdery mildews.

Hygiene

Purchase plants from suppliers with good production hygiene practices to reduce the chance of introducing the disease to your nursery. Maintain good nursery hygiene standards to minimise the risk of disease spread. If the disease does arise, seek to determine the source to prevent reoccurrence.

Varieties

Although varieties with red bracts are most popular, those with white, pink or variegated bracts were shown to have more mildew resistance in the USA.

Humidity

Where powdery mildew is seen, reduce relative humidity by ventilation and lower the glasshouse temperature to around 15°C to slow disease development. Increasing plant spacing, particularly with dense canopied mature plants, will reduce humidity around leaves. 

Plants with symptoms

Where isolated mildew spots are seen on leaves or bracts, promptly remove the affected leaves, bag in-situ, and destroy/dispose of them outside the glasshouse to prevent the dispersal of spores within the nursery.

As American powdery mildew is potentially epidemic in its rate of spread, eliminate the disease by plant removal and fungicide treatment, including surrounding symptomless poinsettia plants.

Fungicides commonly used to control powdery mildew diseases on protected crops are listed in Table 1 (as at March 2021). Some are commonly used in poinsettia production and are well understood, while others are untested and should be trialled on small batches of plants before wider use to avoid crop damage.

Table 1. Fungicides and bioprotectants permitted on protected poinsettia with potential to control powdery mildew, either via on-label approval or by Extension of Authorisation for Minor Use (EAMU), together with their mode of action classification by Fungicide Resistance Action Committee (FRAC) group

¹Defence elicitor product which requires multiple treatments in advance of pathogen challenge.

²May harden leaves/bracts.

³Not tested on poinsettia.

⁴The active ingredient myclobutanil has not been supported for EU re-approval. Therefore myclobutanil is expected to be withdrawn across Europe when the approval expires on 31/05/2021. In GB, myclobutanil has automatically been granted a three year extension to its expiry date. If the active is also not supported in GB, then it may be withdrawn prior to the extended date of 31/05/2024.

⁵Tested on poinsettia as part of the Bedding and Pot Plant Centre project.

Note: Amistar (azoxystrobin), Frupica SC (mepanipyrim) and Nimrod (bupirimate) have on-label or EAMU approval for use against powdery mildew in ornamental crops, however their use has been shown to be phytotoxic on poinsettia when applied at full label rate.

Fungicide efficacy

Trial work on the efficacy of fungicides against powdery mildew diseases on poinsettia is limited. None of the products in Table 1 have a label recommendation for use on poinsettia. However, they are authorised for protected ornamental crops and would be expected to have some activity against poinsettia powdery mildew. In trials in the USA, various triazole (SBI) fungicides gave effective control of the disease on poinsettia, with Systhane (formulation and rate not stated) giving persistent control. Systhane 20EW has been commonly used on poinsettia in the UK to control fungal diseases.

Fungicide application

Programme options are greater in the early stages of crop production, especially before flower initiation and bract development. Follow these guidelines to develop fungicide programmes against poinsettia powdery mildew:

• If it is suspected that plants might be carrying symptomless infection, use a systemic fungicide (see Table 1)
• If preventative treatment is to be applied due to conditions of high disease risk, it is recommended this is done in September or early October before the bracts begin to show any signs of colour
• Determine what fungicides were used on cuttings/young plants and choose a follow-on fungicide from a different mode of action group
• Monitor crops at least weekly and in normal circumstances do not treat unless powdery mildew is suspected or seen
• Once identified, treat promptly using a minimum of two products with different active ingredients applied at a short interval (around seven days), ensuring the minimum spray interval is observed
• Use the fewest applications possible to provide effective control
• Treat the lower and upper leaf surface and the full crop canopy, adjusting spray pressure and volume to optimise coverage
• Extend the spray interval (up to 28 days) during prolonged hot weather when disease development is usually slower

Crop safety is a critical consideration because of the high value of poinsettias and the demand for a quality appearance at the point of sale. Fungicide treatments may cause scorch or other tissue damage, affect plant growth or leave deposits. Information on the safety of some of the fungicides authorised for use on protected ornamentals is shown in Table 1.

Some fungicides leave an unacceptable spray deposit on the crop

Image copyright ADAS Horticulture.

Grower observations or trials are required to provide further information on crop safety for the current poinsettia varieties grown at different ages under diverse growing conditions. From previous work:

• Systhane 20EW causes no phytotoxic damage and leaves no residues
• Amistar has resulted in a brown speckling of bracts that have become increasingly obvious during shelf-life testing
• Stroby WG has caused no damage but has left slight deposits on the crop

The number of applications, application rate, poinsettia variety, plant growth stage and the weather at the time of application all influence the risk of crop damage. Always test a small batch of plants of the variety grown before a treatment is used widely for the first time.

Note: Triazole fungicides such as Systhane 20EW or Topas can have growth regulatory activity on ornamental crops (including Chrysanthemum and Fuchsia). There is a risk they may affect growth characteristics if used on young poinsettia plants, especially if used regularly.

Fungicide resistance has developed in several powdery mildew fungi in the UK, although there are no reports of fungicide resistance in American powdery mildew of poinsettia. Good practices to avoid selecting fungicide-resistant strains include:

• Only use fungicides when needed
• Alternate products from different modes of action groups or use fungicide mixtures containing two active ingredients, which are both effective against powdery mildew
• Seek advice on how biological and chemical fungicides can be integrated into disease management programmes
• Keep to the manufacturer’s recommended dose rate
• Do not rely on fungicides alone for disease control; follow the nursery hygiene and crop inspection procedures detailed earlier
• Apply treatments before the disease becomes well established; monitor crops regularly, so outbreaks are detected early

This table has been collated using information from the Health and Safety Executive (HSE) website (pesticides.gov.uk) and from product labels and supplier technical leaflets. Important – regular changes occur in the approval status of plant protection products, arising from changes in the legislation or for other reasons. For the most up-to-date information, please check the HSE website or with a professional supplier or BASIS-qualified consultant, as information could have changed since the publication of this page.

EAMU – Extension of authorisation for minor use. LTAEU – Long term arrangements for extension of use.

Growers must hold a paper or electronic copy of an EAMU before using any product under the EAMU arrangements. Any use of a plant protection product via an EAMU is at the grower’s own risk.

Always follow approved label or EAMU recommendations, including rate of use, maximum number of applications per crop or year, and where crop safety information is not available, test the product on a small number of plants to determine crop safety prior to widespread commercial use.

If in doubt about which products are permissible on ornamentals or how to use them correctly, seek advice from a BASIS-qualified consultant with expertise in ornamental plant production.

Webpage content correct as of March 2021.

Author(s) – Dave Kaye and Erika Wedgwood, ADAS Horticulture.

Original author – Tim O’Neill, ADAS Horticulture.