C9 - Bacterial Diseases

Introduction

Bacterial diseases on herbs can be divided into those causing distinct leaf symptoms and those causing more general soft rots. The most common bacterial disease on herbs is leaf spot on coriander caused by Pseudomonas syringae pv. coriandricola, which can severely reduce yield and seed quality. This species of bacteria does not affect any other member of the Umbelliferae. Bacterial leaf spot on coriander can occur in any herb production system as it is seed-borne, but may be more prevalent where crops are overhead watered. The bacterium P. syringae pv. syringae occasionally causes leaf spotting on fenugreek in the UK and is suspected to be seed-borne. Another Pseudomonas species, P. cichorii, is an opportunist pathogen of many herbaceous plants and has been reported as a cause of leaf spots on lavender in the UK, and on parsley and basil in other countries.

Bacterial species such as Pectobacterium carotovorum (previously known as Erwinia carotovora subsp. carotovora) and Pseudomonas marginalis are ubiquitous and can cause soft rots on a wide range of herb species, for example crown, root and leaf rots on parsley. Bacterial soft rots are rare during production of protected herbs except when soil-grown herbs become water-logged, but can be problematic in transit and storage.

  

Symptoms

Coriander plants grown from seed infected with P. syringae pv. coriandricola may die as seedlings. More often, the disease is seen on young coriander plants as brown leaf spots (2-5 mm in diameter) which may be surrounded by a water-soaked area (Figure 1). The leaf spots are often angular, being limited by the veins, and can be seen clearly on both leaf surfaces. Leaf spots may merge to cause a more extensive blight. On mature plants, there may be blackening of leaf veins, or black edges on leaves followed by leaf death. In severe infections, the bacterium can infect vein endings and spread down via the vascular system, resulting in dark longitudinal streaks on petioles and stems. If plants are left to bolt, then flowers and seed heads on infected plants shrivel, turn black and collapse. Seed capsules become dark and many seeds are aborted. It is important to obtain an accurate diagnosis of symptoms, as bacterial leaf spot may be confused at an early stage with physiological symptoms of ‘oedema', ‘blue spot' or ‘tip-burn' on coriander leaves (Figure 2).

Typical symptoms of bacterial leaf spot on fenugreek include angular, water-soaked areas visible on both the upper and lower leaf surfaces (Figure 3). These may progress to become dark brown or black lesions with yellow haloes. In severe cases, the lesions progress throughout entire leaves, leading to leaf death and defoliation.

When plant tissues are invaded by soft rot bacteria, the material binding cells together is dissolved, and the tissue collapses into a soft mass. During production, this could occasionally affect soil-grown parsley if there has, for example, been severe water-logging. Symptoms include soft rot of the roots and crown, plus water-soaked lesions on basal leaves and petioles, which subsequently become brown, sunken and sharply defined. Under favourable environmental conditions, post-harvest soft rots can develop on packed cut herbs, particularly following injury by fungal diseases, insect damage, or physical wounds. Secondary decay organisms can subsequently invade tissue partially degraded by soft rot bacteria.

  

Disease sources and spread

Bacterial leaf spot on coriander is known to be seed-borne and transmission to seedlings has been demonstrated. It is also likely that the pathogen may survive on crop debris, but duration of survival has not been defined. Once leaf spots have developed, the disease may spread rapidly between plants by water splash. Bacterial leaf spots (Pseudomonas species) on other herb crops are likely to have a similar biology. 

Soft rot bacteria (including some Pectobacterium and Pseudomonas species) are widespread in soil, crop debris and surface water. They can enter plants through wounds and natural openings and rapidly degrade tissue under favourable conditions. Plant to plant spread is unlikely during production. Post-harvest, these bacteria may be spread between plant products if contaminated washing water is used. For cut herbs that have been packed, soft rots may spread between plants in contact with each other.

  

Conditions for infection

Development of bacterial leaf spot on coriander is associated with cool temperatures and excessive moisture. The disease may remain symptomless until plants become stressed, for example, physical damage on outdoor crops due to frost and hail is thought to trigger symptom development.

Optimum temperatures for development of soft rot bacteria are in the range 20-30^oC. Long periods of soil saturation are necessary for field infection and symptom development to occur. Harvest wounds, use of contaminated washing water and poor storage conditions (e.g. high relative humidity or free water on the product) can increase the incidence of post-harvest soft rot. Rotting due to Erwinia species is usually insignificant at temperatures below 5^oC. In contrast, P. marginalis can cause rotting even at 0^oC, such that decay is delayed but not halted by low temperature storage.

  

Integrated disease management

Cultural control

Planting material
For a seed-borne bacterial disease such as coriander leaf spot, use of ‘clean' seed is important for healthy crop production, however, there are no clearly defined tolerance standards for this disease. Previous research in Australia indicated that use of seed with infection levels greater than 0.5% gave a high risk of yield loss, but this threshold is being further investigated in HDC Project FV 318. Seed treatments for this disease are currently being investigated.

Glasshouse and crop hygiene
If an outbreak of bacterial leaf spot occurs, infected plants and debris should be removed and destroyed. If early symptoms are observed on a few plants, these should be bagged in-situ and disposed of outside the glasshouse in covered bins or skips. For soil-grown crops, avoid use of the same soil for at least two years. Disinfect glasshouse or polytunnel structures, benches and equipment, and re-circulating irrigation systems before re-introducing plants of the same species. For further details on nursery hygiene, see Section A - Principles of IPM.

Crop management
Ensure careful handling of the crop at harvest and packing, to minimise risk of soft rot development.

Environmental conditions
Use of sub-irrigation systems rather than overhead watering can reduce the risk of bacterial leaf spot diseases. Development of soft rot diseases during production can be minimised by avoidance of over-watering or water-logging. Use appropriate storage conditions for the product.

  

Biological control

There are no products currently approved in the UK for biological control of bacterial diseases.

 

Chemical control (protected herbs)

There are no products approved specifically for the control of bacterial diseases on protected herbs and it is unlikely that chemical control will be required under protection. Where there is a high risk of bacterial leaf spotting, Headland Inorganic Liquid Copper (copper oxychloride) has a Specific Off-Label Approval for use on protected herbs, but is not approved for umbelliferous crops such as dill, coriander and parsley. This product has limited activity as a bactericide when applied as a preventative treatment. It has a 14 day harvest interval on protected herbs and may leave visible deposits on leaves. Copper oxychloride is safe to many biocontrol agents used for pest control but is moderately harmful to Encarsia and Hypoaspis.

There is some evidence that strobilurin fungicide products such as Amistar (azoxystrobin) and Signum (boscalid + pyraclostrobin) may give protection from bacterial infection by promoting plant host defence responses. Both of these fungicides have a Specific Off-Label approval for use on protected herbs. 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. Use of Amistar is compatible with biological control of pests in IPM. There is no information available yet on any side effects of Signum on products used for pest biocontrol.

Before selecting products to use for bacterial control on protected herbs, test treat a small batch of plants before widespread application if using a product for the first time, to ensure crop safety.

Growers using strobilurin fungicides such as Amistar or Signum (even if for the management of bacterial diseases), should be aware of the need to use strategies to minimise the risk of selecting resistant fungal strains; for further details, see Section A - Principles of IPM.

• 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 2009. 
• 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 with the Information Office at the Pesticides Safety Directorate (PSD) Tel: 01904 640500; or on their website (http://www.pesticides.gov.uk/).
• Always follow label recommendations or statutory conditions for use on Specific Off-Label (SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or SOLA.
• Growers must hold a paper or electronic copy of the current SOLA before using any product under the SOLA arrangements.  Any use of a pesticide with a SOLA is at grower's own risk.  Relevant SOLAs are sent to HDC members by HDC, or are available from PSD (see above) or from consultants.
• Use pesticides safely.

  

Further information

Dennis, J. & Wilson, J. 1997. Disease control in coriander and other spice seeds. Australia: Rural Industries and Development Corporation. http://www.rirdc.gov.au/reports/NPP/DAS-40A.doc.

HDC Project 318.  Outdoor herbs:  Integrated management of parsley septoria and coriander bacterial blight.  Annual Report, April 2008.