Pest & disease Factsheet - Powdery mildew affecting ornamentals crops

Accurate identification is key to tackling this serious plant disease.

Powdery mildew - image: Dove Associates
Powdery mildew - image: Dove Associates

Powdery mildews thrive in warmer and drier conditions than most other fungal diseases. They are among the most serious of all plant diseases, attacking both protected and outdoor ornamental crops.

Lonicera, clematis, spiraea, maple, oak, rhododendron, roses and thorns are among the main nursery stock hosts. Susceptible bedding and pot plants include pansies, trailing petunia, verbena, myosotis and poinsettia.

Accurate identification of powdery mildew is vital because not all chemicals are equally effective against all genera. Alternating products from different fungicidal groups reduces disease resistance.

How to recognise it

Powdery mildew tends to grow on the upper surfaces of the leaves and it is usually white in colour. The leading edge of a powdery mildew infection is irregular and consists of multi-branching fungal growth.

The poinsettia powdery mildews Leveillula clavata and Leveillula taurica are unusual because they grow on the underside of the leaf and cause chlorotic spotting on the upper surfaces of leaves, forming yellow patches that eventually turn brown. L. clavata can be confused with downy mildew because the symptoms are also similar.

Rhododendron powdery mildew (Erysiphe sp.) causes yellow patches on the upper leaf surface, corresponding to areas of pale-brown mould beneath, with defoliation in severe cases. A second species (Sphaerotheca sp., similar to rose mildew) may attack glasshouse-grown rhododendrons.

Biology

Powdery mildews are spread by two kinds of spores. Asexual conidia are commonest and cause the powdery appearance. The disease overwinters as conidia, mycelia or sexual ascospore-producing bodies in crop debris or on weeds, or as dormant infections on stems or bud scales. Some are seed borne.

Mature conidia contain a water reserve, so are resistant to drying over short periods, and are wind-dispersed. Most can germinate in drier conditions compared with other plant pathogens and some do not even need free water on leaf surfaces or prolonged high humidity.

However, there is still an initial requirement for high humidity for several hours for spore germination. Subsequent development is favoured by drier conditions and growth is most rapid on warm days and cool, damp nights.

There are five key genera of "native" powdery mildews that can affect ornamentals — Erysiphe, Microsphaera, Sphaerotheca, Podosphaera and Oidium.

Symptoms

Infection begins as spots or patches of white, felty mycelium that spread and merge over the surface of the leaf or other parts of the plant. It is most prevalent on young foliage, which can become distorted.
Surrounding tissues may develop a pinkish tinge. Plants can become chlorotic, discoloured and distorted, and the plant declines in vigour. Leaves die and fall prematurely. The whole plant wilts or droops.

Treatment: cultural control
?    Grow resistant cultivars.
?    Regularly monitor crops.
?    Regularly remove and dispose of crop debris and infected material.
?    Disinfect between crops.
?    Control weed hosts including willowherbs.
?    Avoid water stress.
?    Maintain adequate airflow.
?    Spectral filter claddings may help minimise sporulation and slow the spread of outbreaks.

Treatment: chemical control

Active ingredient Azoxystrobin
FRAC code 11
Formulation Various including Amistar* (Syngenta)
Action(s) Systemic fungicide. Controls Erysiphe, Sphaerotheca and
Podosphaera spp.

Active ingredient Bupirimate
FRAC code 8
Formulation Nimrod (Adama)
Action(s) Systemic fungicide. Controls Podosphaera, Sphaerotheca, Erysiphe and Oidium spp.

Active ingredient Cyflufenamid
FRAC code U6
Formulation Cyflamid*, Takumi SC*
(both Certis)
Action(s) Translaminar fungicide. Will control Podosphaera, Erysiphe and Sphaerotheca spp.

Active ingredients Cyprodinil + fludioxinil
FRAC codes 9+12
Formulation Switch (Syngenta)
Action(s) Systemic fungicide. Controls Erysiphe spp.

Active ingredient Fenbuconazole
FRAC code 3
Formulation Indar 5EW (Landseer)
Action(s) Systemic fungicide. Controls Podosphaera spp.

Active ingredient Fenpropimorph
FRAC code 5
Formulation Corbel (BASF)
Action(s) Systemic fungicide. Controls Erysiphe spp.

Active ingredient Kresoxim-methyl
FRAC code 11
Formulation Stroby WG (BASF)
Action(s) Systemic, protectant fungicide. Controls Erysiphe, Podosphaera and Sphaerotheca spp.

Active ingredient Mepanipyrim
FRAC code 9
Formulation Frupica SC* (Certis)
Action(s) Contact fungicide. Protects against Erysiphe and Sphaerotheca spp. Forest nurseries only.

Active ingredient Metrafenone
FRAC code U8
Formulations Attenzo*, Flexity* (BASF)
Action(s) Protectant fungicide against Erysiphe spp. Forest nurseries only.

Active ingredient Myclobutanil
FRAC code 3
Formulation Systhane 20EW (Landseer)
Action(s) Systemic fungicide. Controls Erysiphe, Microsphaera, Sphaerotheca and Podosphaera spp.

Active ingredient Penconazole
FRAC code 3
Formulation Topas* (Syngenta)
Action(s) Systemic fungicide against Erysiphe, Sphaerotheca, Podosphaera
and Oidium spp.

Active ingredient Food-grade potassium bicarbonate
Action(s) A basic substance with
contact fungicidal activity. Controls Microsphaera, Sphaerotheca, Podosphaera and Oidium spp.

Active ingredient Quinoxyfen
FRAC code 13
Formulation Fortress* (Dow)
Action(s) Systemic, protectant fungicide against Erysiphe, Microsphaera and Podosphaera spp.

Active ingredient Spiroxamine
FRAC code 5
Formulation Torch* (Bayer CropScience)
Action(s) Systemic fungicide. Controls Erysiphe and Microsphaera spp.

Active ingredient Tebuconazole
FRAC code 3
Formulation Folicur (Bayer CropScience)
Action(s) Systemic fungicide. Controls Erysiphe, Sphaerotheca and
Podosphaera spp.

Active ingredient Trifloxystrobin
FRAC code 11
Formulation Swift SC* (Bayer CropScience)
Action(s) Systemic fungicide. Controls Erysiphe, Sphaerotheca, Podosphaera and Oidium spp.

Other conazole fungicides available include Bumper 250EC* and Galileo*. Repeated use of conazoles can affect plant growth. SB Plant Invigorator has some effect on powdery mildew.

Fully updated by Dove Associates

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

* EAMU required for use in ornamental plant production and/or forest nurseries 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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