Pest & Disease Factsheet - Downy mildew affecting ornamentals crops

This fungal disease relishes cool, humid conditions.

Downy mildew on buddleja - image: Dove Associates
Downy mildew on buddleja - image: Dove Associates

Downy mildew is a foliar fungal disease characterised by dark felting on the undersides of infected leaves. This trait distinguishes it from powdery mildew, an unrelated disease but one with which it is often confused. The white fungal growth of powdery mildew tends to occur on the upper sides of leaves and is distinctly more powdery because of the larger numbers of spores.

Downy mildew belongs to the same fungal order as Pythium and Phytophthora. Most of the species that are pathogenic on commercial ornamental crops are host-specific, so infection on one type of bedding plant will not spread to another.

The disease relishes cool, humid conditions and wet leaves, so is often encountered in poorly ventilated structures. It is rapidly transmitted by spores but can spread by infected but healthy looking cuttings, and even seed. Infected propagation material is the means by which impatiens downy mildew is believed to have arrived in the UK.

How to recognise it

Downy mildew is characterised by a downy or felt-like growth, which can range in colour from white or grey to purple, on the undersides of leaves. In some instances this growth is sparse. Markings occur on the upper leaf surface, tend to be angular and are usually yellow but can be brown or purple. Plant growth can be distorted, with leaves falling prematurely.

Symptoms

Peronospora parasitica Attacks cruciferous plants such as stocks, wallflowers and alyssum. Along with felting on leaf undersides, stems may swell and become distorted.

Peronospora grisea Purple/grey felting on undersides of leaves and leaf yellowing on upper surfaces. Young shoots can be distorted.

Plasmopara obducens Attacks impatiens. Fungal growth is sparse in early stages of infection but will increase to cover whole lower surface apart from main veins. Growth can be stunted and flowering disrupted.

Peronospora radii Affects Compositae family, such as Argyranthemum. Young leaves blacken and die back. Downy growth on undersides of leaves is grey to dark-brown. Flowers on infected plants may be small and distorted.

Peronospora violae Purple/brown felting on undersides of pansy leaves with light-green patches on upper surfaces. Flowering is disrupted and growth is stunted.

Peronospora sparsa Produces slight fungal growth on underside of rose leaves and red/brown or purple spotting on upper surfaces. Can also infect stems, causing extended dark-brown blotches, and sometimes attacks flowers.

Bremia lactucae White fungal growth that attacks lettuce, Centaurea, Cineraria, Helichrysum and Osteospermum.

Biology

Spores produced by the fungal growth on the undersides of leaves are readily spread in air currents and by water splash. They germinate on wet leaf surfaces, the fungal strands penetrating leaf cells. The disease will grow only on living tissue but thick-walled sexual resting spores, or oospores, can be produced inside the plant, probably as a survival mechanism, persisting in the dead leaf debris.

Treatment: cultural control

- Under protection - good humidity management and watering practice to avoid foliage getting wet.

- Space plants as much as economically possible to promote air circulation.

- As with all diseases, nursery hygiene is vital to reduce the risk of outbreaks and contain spread. Use disinfectants for routine cleaning.

- Weed control will prevent susceptible weeds from kick-starting an outbreak in a crop.

Treatment: chemical control

The following are examples of the wide range of fungicides available for the control of downy mildew in ornamental plant production. Other excellent products are available, with some containing mixtures of some of the single active ingredients mentioned, such as Previcur Energy* (fosetyl-aluminium + propamocarb hydrochloride) and Fubol Gold WG* (mancozeb + metalaxyl-M).

Active ingredients Ametoctradin + dimethomorph

FRAC codes 45 + 40

Formulation Percos* (BASF)

Action(s) Systemic, contact protectant fungicide - compatible with some biological controls.

Active ingredient Azoxystrobin

FRAC code 11

Formulation Amistar* (Syngenta)

Action(s) Systemic, translaminar fungicide - compatible with biological controls.

Active ingredients Benthiavalicarb-isopropyl + mancozeb

FRAC codes 40 + M3

Formulation Valbon* (Certis)

Action(s) Protectant fungicide mixture with good residual activity - incompatible with biological controls.

Active ingredients Boscalid + pyraclostrobin

FRAC codes 7 + 11

Formulation Signum* (BASF)

Action(s) Systemic, translaminar, broad-spectrum fungicide - compatible with some biological controls.

Active ingredient Chlorothalonil

FRAC code M5

Formulations Bravo 500* (Syngenta), Life Scientific Chlorothalonil* (Life Scientific)

Action(s) Contact-acting, protectant fungicide. Russetting may occur in some flowering ornamentals. Pittosporum foliage is also sensitive. Compatible with biological controls.

Active ingredients Chlorothalonil + metalaxyl-M

FRAC codes M5 + 4

Formulation Folio Gold* (Syngenta)

Action(s) Contact and systemic fungicide - compatible with some biological controls.

Active ingredient Copper oxychloride

FRAC code M1

Formulation Cuprokylt* (Certis)

Action(s) Contact product - compatible with some biological controls.

Active ingredients Dimethomorph + mancozeb

FRAC codes 40 + M3

Formulation Invader* (BASF)

Action(s) Systemic protectant mixture - compatible with biological controls.

Active ingredients Fenamidone + fosetyl-aluminium

FRAC codes 11 + 33

Formulation Fenomenal* (Bayer)

Action(s) Protective and curative fungicide - compatible with some biological controls.

Active ingredients Fluopicolide + propamocarb hydrochloride

FRAC codes 43 + 28

Formulation Infinito* (Bayer)

Actions(s) Protectant, translaminar fungicide - compatible with some biological controls.

Active ingredient Mancozeb

FRAC code M3

Formulations Various including Dithane 945*, Karamate Dry Flo Neotec (Landseer)

Action(s) Broad-spectrum protectant fungicide - compatible with biological controls.

Active ingredient Mandipropamid

FRAC code 40

Formulations Revus* (Syngenta), Pergado Uni* (Syngenta)

Action(s) Protectant fungicide with some curative action - compatible with some biological controls.

Active ingredient Propamocarb hydrochloride

FRAC code 28

Formulations Promess*, Proplant* (both Arysta LifeScience)

Action(s) Applied preventively as a drench or by incorporation into compost - compatible with biological controls.

Fully updated by Dove Associates

Use plant protection products safely. Always read the label and product information before use.

* Extension of Authorisation for Minor Use (EAMU) required for use in ornamental plant production outdoors and/or under protection.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.


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