C5 – Rusts

Introduction

Rust diseases occur on a wide range of herb crops and many members of the Lamiaceae family are susceptible. Peppermint is particularly prone to rust, although spearmint can also be very severely affected, depending on the rust strain. Rust attacks other herbs crops particularly tarragon, chives and sorrel. Rust not only affects plant appearance, which is important for culinary and pot herbs, but also reduces crop and essential oil yields. Table 1 lists herbs that are commonly affected in the UK and the pathogens causing rust on these crops. Note that even where the same rust species is listed for more than one herb species, different pathogen races exist which may be specialised to only one host species. Rust diseases may affect protected herbs under any production system but are most likely to occur on perennial species or long-season crops, and where overhead irrigation is used.

Table 1. Examples of edible herb crops affected by rust diseases

Herb crop Rust species
Chives Puccinia allii, Uromyces ambiguus (rare)
Marjoram Puccinia thymi
Oregano Puccinia menthae, Puccinia thymi
Peppermint Puccinia menthae
Sorrel Several species e.g. Puccinia acetosae & Uromycyes acetosae
Spearmint Puccinia menthae
Summer savory Puccina menthae
Tarragon Puccinia tanaceti
Thyme Puccinia thymi

 

Symptoms

A common feature of rust diseases is the production of large numbers of spores (often orange, brown-black or reddish-brown) in structures known as pustules. These are usually first seen as small blisters, sometimes accompanied by localised yellowing of the plant tissue. The spore mass eventually ruptures through the surface cells. The pustules are most commonly found on the underside of leaves, although sometimes they develop on upper leaf surfaces and stems. Growth of the fungus within the leaf may result in spotting of various colours on the upper leaf surface.

The life-cycles of rusts can be complex, involving up to five different spore types. The symptoms that develop on individual herb species vary depending on the stage in the pathogen life-cycle. Some rusts need to infect a second plant species in order to complete their life-cycle, however the species described here for herb crops do not need another host.

Puccinia menthae on mint has a complicated life-cycle, involving five spore stages produced in sequence on the same host (Figure 1), and resulting in a range of different symptom types. During winter, the fungus survives as tough-walled dark brown to black spores on crop debris, often near to, or within buds on stolons and rhizomes. These germinate in spring, producing tiny colourless spores, which infect young shoots as they emerge through the soil surface resulting in localised systemic infection. Affected stems become abnormally thickened and distorted (‘bull’ shoots or stems) with elongated pale internodes bearing distorted yellowing leaves (Figure 2). Pustules on these stems develop and rupture, releasing large numbers of pale yellow spores (Figure 2). These spores infect nearby mint leaves, resulting in the production of distinctive pustules on the leaf undersides containing orange/brown spores (Figures 3 and 4), sometimes with associated dark brown/purplish lesions on the upper leaf surface. Premature leaf death and defoliation occurs when the attack is severe. The orange/brown spores infect mint leaves via the stomata and this phase of the life-cycle may repeat many times during the summer, representing the most damaging phase of the disease. In the autumn, the fungus produces the over-wintering phase of dark brown to black spores.

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[]Figure 1. Life-cycle for mint rust (Puccinia menthae). (Adapted from: C.E. Horner. 1957. Control of Peppermint Diseases. Station Bulletin 547, Corvallis, OR)
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[]Figure 2.
Swollen and distorted (‘bull’) mint shoots with spore clusters, after early spring infection by rust (Puccinia menthae)
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[]Figure 3.
Orange spore clusters typical of the summer phase of rust (Puccinia menthae) on the underside of peppermint leaves
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[]Figure 4.
Close-up of spore clusters of rust (Puccinia menthae) on peppermint

On thyme and oregano, only one spore stage of the rust caused by P. menthae is usualIy seen. Round or elongated spore clusters, which are black, then cinnamon brown, develop under the surface cells occasionally on leaf midribs, but more usually on stems and petioles. Mycelium (fungal strands) of P. thymi can survive within the plant between seasons, and may lead to development of an annual ‘witches’ broom’ symptom. On thyme, the internodes are lengthened, the leaves fewer and stems sterile. On oregano, the internodes are slightly shorter than those of the healthy shoots and at most nodes, two side shoots are produced.

On tarragon, yellow rust pustules develop in early summer on the lower side of leaves and stems, with leaf tissue turning yellow around the rust colonies. Later, whole leaves may wither and fall off. In autumn, dark brown colonies of over-wintering spores develop on leaf undersides and stems.

On chives, two different rust symptoms may be seen. Raised orange pustules (1-3 mm long) develop under the surface tissue on leaves, eventually rupturing to release orange spores (Figure 5). Dark grey/black spore clusters of variable size may also be observed on the same plants, developing under the epidermis (Figure 5). Leaves may be killed if symptoms are severe.

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[]Figure 5.
Rust (Puccinia allii) on chives, with orange pustules and dark grey spore clusters, representing two different spore stages

On sorrel, rust diseases cause reddish purple spots on the leaves (visible on both surfaces) and stems. Spore clusters of two types (both brown in colour) develop, usually on the underside of the leaf, in association with the leaf lesions.

 

Disease sources and spread

A common feature of rusts is the large numbers of spores produced, which may easily be splashed (e.g. by overhead irrigation) or carried, sometimes for many miles, on air currents and the wind. The spores of many rust species can also survive for weeks or even months on crop debris, stolons or rhizomes, enabling carry-over between crops.

All rusts have a stage known as a ‘latent period’ in which infection has occurred but visible symptoms (spots or pustules) have not yet developed. Under ideal conditions for a particular rust species this may be as short as 7 days, but if conditions are sub-optimal, then it can be several weeks. As a result, there is potential for the transport of rust diseases between nurseries on infected but symptomless plants, cuttings or rhizomes.

Mint rust can originate from the use of contaminated rhizomes as planting material. Rhizome contamination is superficial, originating from over-wintered spores, rather than due to systemic infection by hyphal strands (mycelium). It is thought that systemic mycelial infection by the germination of the over-wintered spore stage occurs in spring as the shoots emerge. The mycelium of the rust does not persist in the rhizomes. During the spring, summer and autumn, secondary disease spread occurs by the transmission of spores produced on leaves and stems to neighbouring plants by air currents and water splash.

Similarly, for different rust species on other herbs, the development of an over-wintering spore phase on crop debris or over-wintered plants, enables disease survival between seasons, while spore production during the summer months enables secondary disease spread.

Individual rust species tend to infect only a narrow host range. For example, several different races of mint rust exist that can affect different mint species/hybrids and other species of the Lamiaceae family. Because of this specialisation, rust from spearmint is unlikely to infect peppermint. Moreover, the race of P. menthae that infects oregano is specific to this host. There is, however, evidence that rust on wild mint may infect commercial varieties of peppermint to some extent. Host specificity also occurs with other rust species on herbs. For example, it is thought that the race of P. thymi causing rust on thyme is different to that which infects oregano, such that rust on one crop would probably not be a source of infection for the other crop.

 

Conditions for infection

In general, rust fungi thrive under warm, humid conditions. Low temperatures merely slow down infection and sporulation processes, allowing the fungus to persist until conditions become favourable. A key feature of rusts is that they require a period when there is free water on the leaf surfaces, in order for their spores to germinate and infect the plant. If the temperatures are optimal for the fungus (these vary between species), the leaves need only be wet for a few hours for infection to occur.

For rust on peppermint (P. menthae), the infection of leaves from spores produced during summer conditions requires a minimum leaf wetness duration of 6 hours. Infection can occur under a wide temperature range (less than 5oC to 27oC), with an optimum of 20oC.

 

Integrated disease management

Effective control of rusts on protected herbs requires ‘breaking’ of the disease life-cycle, careful management of environmental conditions and close attention to crop hygiene.

 

Cultural control

Planting material
Carefully inspect newly delivered planting material or cuttings for evidence of rust. If the disease is found, then the consignment should be rejected. If propagating from your own stock, regularly monitor the mother plants for rust. Do not take cuttings from rust-affected plants.

On mint, experiments have shown that heat therapy of dormant rhizomes at 43oC for 10 minutes is sufficient to kill various spore forms of P. menthae, (even if some spores are protected by soil, bud-scales or spore clumping) thus giving complete control of rust. Heat treatment under these conditions does not affect mint rhizome vigour. Based on this type of research data, the following protocol was developed by growers at a UK pot herb nursery to ensure the availability of clean rust-free mint stock from which to propagate (L. Reed, pers. comm). It is recommended that the procedure should be repeated annually to ensure that the nursery remains free of mint rusts throughout the main selling season. The technique can be used both in organic and conventional production.

During the autumn, stock plants (and leftover sales stock) are pulled apart for their healthy runners (stolons and/or rhizomes according to variety). For propagation in trays or pots, the runners can be left in long lengths. Some varieties (e.g. Water, Black Pepper, Pineapple and Eau de Cologne) do not always produce much rhizome material, and in these cases the stolons are used.

The runners are put into a suitable bag (e.g. plastic net bags for small amounts or polypropylene bulb sacks for larger quantities) and immersed in hot water that has already attained a temperature of 44oC±2oC. The runners should remain completely immersed for 10 minutes. A laboratory water bath with stirrer is ideal for this purpose. The runners should not be packed too tightly otherwise the heat will not properly penetrate.

After 10 minutes the treated material is removed, drained, placed onto and covered with potting substrate containing control release fertiliser in clean trays (e.g. mesh-bottomed rooting trays or shallow crates). The trays are given bottom or all-round heat (e.g. 23oC for 1 week) to generate clean shoots for further propagation by tip cuttings. Cuttings may be taken about 3-5 weeks after treatment, depending on conditions and variety. Supplementary lighting improves production. The stock trays produced by this method are productive for 12 months; the process is then repeated even if rust symptoms were not observed throughout the season.

The whole procedure must be done under clean conditions, ensuring there is no spore contamination from hands, clothing or tools. Keep the treatment area away from the runner preparation area to avoid any contamination. All potential sources of re-infection should be cleared as described under ‘crop hygiene’

Growers observed that where mint variegation is due to viral origin (e.g. in variegated peppermint), variegation may be lost following hot water treatment of runners. Variegation in pineapple and ginger mint is stable to heat.

Varieties
Research in different countries has demonstrated that Puccinia menthae exists as several races, some of which infect spearmint varieties and some which infect peppermint varieties. Breeding programmes to develop mint varieties that are resistant to a range of rust races has been limited to date.

Glasshouse and crop hygiene
Thoroughly clean the relevant glasshouse or polytunnel and benches of crop debris before introducing new batches of plants. If rust occurs on a few plants in a crop, these plants should be removed and disposed of, taking care not to spread spores around the glasshouse or polytunnel.

Debris or over-wintering plants from a previously affected crop, whether in pots or beds, can be a major source of re-infection. All old mint plants and crop debris should be disposed of carefully in covered bins or skips. If pots are to be re-used, they should be disinfected, as should standing beds and benches.

For permanent stock beds or over-wintered cut herb crops, flaming with a propane burner in autumn, is an effective way to kill spores in crop debris, although spores on rhizomes may survive. It is important to flame as early as possible in the autumn whilst most of the rust spores are still in their pustules. If delayed, then there is an increased chance of soil surface and thus rhizome contamination. This technique can help to minimise rust carry-over and ensure that shoot re-growth in the spring will be in a rust-free environment. Weed control is also improved. Some researchers have advocated spring, rather than autumn flaming, in order to kill rust at the stage where it causes a systemic infection in young shoots. However, this can cause significant disruption to crop growth and reduced yields.

Environmental conditions
Avoid periods of prolonged high relative humidity. It is particularly important to reduce to a minimum the time when free water is present on the leaf. Reduce humidity by ventilation (with heat as appropriate) and air circulation.

Avoid overhead irrigation if at all possible, particularly in the evening when prolonged leaf wetness can result. Consider sub-irrigation where this is practical.

 

Biological control

No biological fungicide products are currently approved for the control of rust diseases in the UK.

 

Chemical control (protected herbs)

Cultural controls should be used to minimise the risk of disease outbreaks. For situations where risk of rust infection is high, or where it is necessary to prevent secondary spread from a disease outbreak, Table 2 lists the fungicides that have activity against rusts and are currently permitted for use on protected herbs. The activity of these products will vary, sometimes even between rusts. Rust control is likely to be most effective when the fungicides listed in Table 2 are applied as preventative treatments.

Before selecting products to use for rust control on protected herbs, test treat a small batch of plants before widespread application if using a fungicide for the first time, to ensure crop safety. Be aware that for Amistar, the harvest interval is 28 days for crops harvested in November to April and 14 days for crops harvested in May to October.

Resistance to some of the commonly-used fungicides (not available for protected herbs) has been reported in chysanthemum white rust and carnation rust. These cases highlight the potential risk of fungicide resistance developing in rusts on other crop species. The products listed in Table 2 are arranged in fungicide ‘groups’ such that products with a common mode of action, and a common risk of fungicide resistance are grouped together. If more than one fungicide treatment is applied to a crop, alternate products from different groups. See Section A – Principles of IPM for further details on strategies to minimise the risk of selecting resistant strains.

Table 2. Fungicides with activity against rust diseases and permitted on protected herbs (March 2013)

Fungicide group and active ingredient(s)

Example products

Compatibility with pest biological control

1. DMI
Prochloraz

 
Scotts Octave

 
Moderately harmful*

2. QoI
Azoxystrobin

 
Amistar

 
Safe

3. Carboxamide + QoI
Boscalid + pyraclostrobin

 
Signum

 
Not yet known

*Adverse effects on certain biological agents

  • Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants.
  • Pesticide approval information in this guideline is current at 31 March 2013.
  • Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/.  General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
  • Always follow label recommendations or statutory conditions for use on Extension of Authorisation for minor use (EAMU, previously SOLA) notices of approval.
  • Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
  • Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with an EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
  • Use pesticides safely.

 

Further information

Edwards, J. 1999. Control of mint rust on peppermint. Epidemiology and chemical control. RIRDC Publication No. 199/122. Australia: RIRDC.

Fletcher, J.T. 1962. Experiments on the control of mint rust. Plant Pathology 11: 115-120.

HDC Factsheet 23/00. Rust diseases of bedding plants.

HDC Factsheet 09/02. Control of rust diseases of protected bedding plants.