Pest & Disease Factsheet - Fungal leaf spots on ornamental crops

Fungal leaf spots are probably the most common of all plant diseases and on deciduous trees may appear dramatic -- such as acer tar spot - but do little long-term damage.

Tar sport - image: Dove Associates
Tar sport - image: Dove Associates

However, year-round selling of trees, shrubs and herbaceous perennials in containers means even biologically superficial leaf spot diseases must be controlled because they will cause economic loss through downgrading a crop in leaf or even rendering it unmarketable.

Fungal leaf spots and tip blights can be a problem on many ornamental crops including Aesculus, camellia, Ceanothus, Chamaecyparis, Cordyline, Cornus, Garrya, hebe, Hedera, Helleborus, iris, Juniperus, phormium, rhododendron, Thuja, Viburnum, Vinca and Yucca.

How to distinguish them

There is no hard and fast way of telling by the naked eye whether a leaf spot has a fungal or bacterial origin, but any tiny, dot-like bodies associated with the spots or lesions are fungal spore-bearing structures.

Leaf spots that appear to have a marked directional or one-sided progression across a plant or a crop, or which affect adjacent but unrelated species, may have a physical cause, such as spray damage. However, some spot diseases show directional spread patterns associated with irrigation sources. In the case of tip blights they often occur at a leaf axil or stem branch and cause a blackening and constriction of the stem which causes the foliage to wilt above it. 

Biology

Fungal leaf spot symptoms are caused by the death of cells in patches around the initial infection site. The patches often enlarge and coalesce as the pathogen spreads. Tip blights girdle the stem cutting off the water supply to the foliage above it.

So-called pycnidial leaf-spot diseases — such as Pestalotiopsis, Coniothyrium, Phomopsis and Phyllosticta — show leaf spot symptoms on the upper leaf surface while the fungus is most active and produces spore-bearing pycnidia inside the leaf, making them hard to control with contact fungicides. In the case of the tip blights (for example Phoma) you can often see the spore bearing pycnidia at the base of the damaged stem area, making it difficult to reach a thin stem target with contact fungicides. Translocated or translaminar fungicides are required.

In many fungal leaf spot and tip blight diseases, the pathogen produces its spores from infected fallen leaves and stems. Outdoors, spore-producing bodies mature in spring, ejecting and dispersing spores by splash or wind in time to infect newly emerging leaves. Most need leaves to be wet for a critical period of time for infection to occur, then warmth and humidity to develop.

Symptoms

Discrete dark-coloured spots, which may expand and join as the disease progresses, are characteristic of a range of leaf spot pathogens. Darkened and constricted stems characterise a tip blight source of infection. Branch wilting then occurs and may the first sign of an infection.

Treatment: biological control

Evidence suggests that PAS 100-graded green compost in growing media and the use of compost tea have important roles in the integrated management of leaf spots on selected crops.

Treatment: cultural control

• Use sub-irrigation wherever possible. If overhead irrigation is used, try to avoid irrigating during late afternoon or evening so that leaf surfaces do not stay wet for long.
• Maintain good plant spacing to encourage airflow within the crop. Avoid high humidity in protected crops.
• Collect and dispose of all leaf litter and prunings, which may carry disease over from one season or one crop to the next.
• Thoroughly clean and disinfect beds and benches etc. between crops.
• Grow resistant varieties, if available, that not only reduce the need for on-nursery control measures but may offer performance benefits for customers.
• Use balanced fertilisers to avoid lush growth.

Treatment: chemical control

Active ingredient Azoxystrobin
FRAC code 11
Formulations Various including Amistar* (Syngenta)
Action(s) Systemic, translaminar, protectant broad-spectrum fungicide — compatible with biological controls.

Active ingredient Bacillus subtilis strain QST713
FRAC code 44
Formulation Serenade ASO* (Bayer)
Action(s) Protectant bio-fungicide — compatible with biological controls. Promotes a competitive environment on foliage with pathogens.

Active ingredients Boscalid + pyraclostrobin
FRAC code 7 + 11
Formulation Signum* (BASF)
Action(s) Systemic, protectant and
curative fungicide — compatible with some biological controls.

Active ingredient Captan
FRAC code M4
Formulation Alpha Captan 80WDG* (Makhteshim/Adama)
Action(s) Protectant fungicide — compatible with biological controls.

Active ingredient Chlorothalonil
FRAC code M5
Formulations Bravo 500* (Syngenta)
Action(s) Protectant fungicide for selected outdoor crops — compatible with some biological controls.

Active ingredients Cyprodinil + fludioxinil
FRAC codes 9+12
Formulation Switch (Syngenta)
Action(s) Systemic, translaminar fungicide with long residual activity — incompatible with biological controls.

Active ingredient Difenoconazole
FRAC code 3
Formulations Difcor 250EC* (Belchim), Plover* (Syngenta)
Action(s) Systemic, protectant and
curative fungicide — compatible with biological controls.

Active ingredient Fenpropimorph
FRAC code 5
Formulation Corbel (BASF)
Action(s) Systemic fungicide — not compatible with biological controls.

Active ingredient Iprodione
FRAC code 2
Formulation Rovral WG* (BASF)
Action(s) Protectant, contact fungicide — compatible with biological controls.

Active ingredient Kresoxim-methyl
FRAC code 11
Formulation Stroby WG (BASF)
Action(s) Systemic, protectant fungicide — compatible with biological controls.

Active ingredient Myclobutanil
FRAC code 3
Formulation Systhane 20EW (Landseer)
Action(s) Systemic fungicide — compatible with biological controls.

Active ingredient: Prochloraz
FRAC code 3
Formulation Octave (Everris)
Action(s) Broad-spectrum protectant and eradicant fungicide — compatible with some biological controls.

Active ingredient Propiconazole
FRAC code 3
Formulation Bumper 250 EC*
(Makhteshim/Adama)
Action(s) Systemic, protective and
curative fungicide — compatible with biological controls.

Active ingredient Tebuconazole
FRAC code 3
Formulation Folicur (Bayer CropScience)
Action(s) Systemic fungicide — compatible with some 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.

For pest and disease alerts, fact sheets and management, see
www.HorticultureWeek.co.uk/pests.


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