Pest and disease factsheet - Rust affecting ornamentals crops

Infections can spread swiftly and damage crops.

Rust affecting bamboo - image: Dove Associates
Rust affecting bamboo - image: Dove Associates

Pustules produced by rust fungi on foliage and stems make it an easily identifiable leaf disease. Many rusts are host-specific, although some need two unrelated plant species (usually a host plant plus a weed) to complete their life cycle — for example, fuchsia rust spends part of its life cycle on Abies. Some crops can be affected by more than one rust pathogen. There are more than 100,000 different species of rust.

Abies, Althaea, antirrhinum, Bellis, Betula, Buxus, Campanula, centaurea, chrysanthemum, cineraria, dianthus, fuchsia, geranium, heuchera, hypericum, iris, mahonia, Malus, Mentha, pelargonium, Populus, prunus, Pyrus, rhododendron, rose, Salix, sweet william, trollius and Vinca are particularly vulnerable.

Infections can be symptomless early on so rusts can spread quickly. Damage takes the form of spotting, defoliation and reduced growth. On plants such as hypericum, pustules only develop on older leaves. Non-European Gymnosporangium rust fungi species are EC-listed diseases of conifers and members of Rosaceae, most likely introduced on dormant juniper plants. Poplar rust is also notifiable and caused by Melampsora larici-populina.

Rusts change genetically so it is easy for resistance to occur and important to alternate fungicide groups in spray programmes. Some rusts can also occur as early as mid February (mahonia). Some strains of chrysanthemum white rust are resistant to azoxystrobin and propiconazole fungicides. Where edible crops are attacked, fungicide control options are limited. Hot-water treatment (44°C for 10 minutes) can effectively reduce rust infections on bare-root plants.

How to recognise it

Symptoms vary with each rust type but in general they are characterised by brightly coloured pustules, which are masses of spores, and can be yellow, orange, pink, white, brown or black depending on the fungal species, coupled with yellow spotting or blotching on leaf uppers. Lab examination is needed to distinguish European from non-European listed Gymnosporangium rusts.

Biology

After penetrating leaf tissue, rust fungi infiltrate between plant cells, sending hyphal outgrowths into cells to "feed". This causes yellow marks on upper leaf surfaces. Pustules bearing spores are produced on the undersides of leaves. Spores are spread by air currents and in water splash, and need several hours of moisture for germination.

Some members of the rust group of fungi produce just one type of spore and are restricted to one host. Others produce up to five different types to complete their life cycle. These types of spores are spermatiaspores (spring), aeciospores (spring to early summer), urediniospores (summer), teliospores (late summer to autumn) and basidiospores (autumn). Some types are more tolerant to fungicide applications. Some products only target one type of spore and not others, resulting in patchy control.

Gymnosporangium spores on conifers can infect only the alternate rosaceous host and vice versa. On Rosaceae, infection is usually shed with leaves in autumn. Each fungal species has a specific combination of hosts. Rose and raspberry rusts (Phragmidium species) survive winter as black spore pustules in leaf litter and on stems. White rust of chrysanthemums (Puccinia horiana) is spread by airborne spores and carried symptomless on infected cuttings.

Symptoms

On roses, coloured pustules appear in summer in groups about 5mm across on lower leaf surfaces. Leaves can become distorted. Pustules or lesions develop around the stem, cutting off and killing the shoots. As the disease progresses, yellow spots appear on upper leaf surfaces, leading to defoliation. In autumn, more orange spores are produced followed by black spores. The pathogen survives the winter. Rosa laxa rootstocks are highly susceptible.

White rust infection on chrysanthemums causes yellow to pale-green spots, up to 5mm in diameter, on upper surfaces of younger leaves. Spots go brown as they age. These correspond to buff pustules on the lower leaf surface that gradually whiten. In severe attacks, leaves wilt, roll or twist and dry up, remaining attached to stems. White blister is sometimes referred to as white blister rust but is an entirely separate disease. Brown rust infection (Puccinia chrysanthemi) takes the form of brown pustules on undersides of leaves with yellow/green spots on upper surfaces.

Uromyces infections cause small brown or orange blisters usually on the lower leaves of broad beans, dianthus, geranium and Sisyrinchium.

On conifers, overwintering Gymnosporangium rusts produce round galls, swellings or proliferation depending on fungal species. Galls are brown, up to 3cm in diameter and in spring produce yellow/red or brown fruiting bodies. Swellings in spring may produce red/brown or orange fruiting bodies. Infections cause dieback or stem cankers. On Rosaceae, symptoms are small, swollen, yellow/red or brown spots on leaves or coloured lesions on soft shoots or on fruits. Defoliation and dieback occur in severe infections.

Treatment: biological control

Beneficial Verticillium spp. can act as hyperparasites of rust but this control is patchy and only works (to a limited effect) in situations of prolonged high humidity.

Treatment: cultural control

• In protected crops, avoid leaf wetness and prolonged high humidity to help prevent infection.
• Good crop hygiene, such as a thorough end-of-season clean-up, plays an important role in control.
• Hot-water treatment at 44°C for 10 minutes on stools prior to potting or planting.
• Maintain disease-free stock plants and use rust-resistant varieties where possible.
• Control weeds such as white campion, groundsel, willowherb and ivy-leaved speedwell, which can act as alternative hosts.
• Keep nitrogen levels balanced in crops to avoid very soft growth being produced, which is then prone to rust infections.
• Rotate FRAC codes as much as possible to reduce resistance risk.

Treatment: chemical control

Azoxystrobin† (various including Amistar*) — systemic, curative, protectant activity.
Boscalid + pyraclostrobin† (Signum*) — systemic, curative, protectant activity.
Chlorothalonil† (Bravo 500*) — protectant activity.
Difenoconazole† (Difcor 250EC*, Plover*) — contact, systemic, curative, protectant activity.
Mancozeb† (Dithane 945*, Karamate Dry Flo Neotec) — protectant activity.
Myclobutanil† (various including Systhane 20EW) — systemic, protective and curative activity.
Penconazole† (various including Topas) — protectant activity.
Picoxystrobin† (Galileo*) — systemic, translaminar, protectant activity.
Propiconazole† (Bumper 250 EC*) — systemic, protectant, curative activity.
Tebuconazole† (Various including Folicur) — systemic, protectant, curative, eradicant activity.
Tebuconazole + trifloxystrobin† (Nativo 75WG) — systemic, protectant, curative activity.
Trifloxystrobin† (Swift SC*) — systemic, translaminar, protectant activity.

Fully updated by Dove Associates.

* EAMU required for use in ornamental plant production outdoors and/or under protection.
† Compatible with most biological controls.
Use plant protection products safely. Always read the label and product information before use. 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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