C7 - Leaf Spots

Introduction

A wide range of fungal pathogens cause leaf spot symptoms on herb crops. The majority of these cause only minor or sporadic problems on UK protected herbs. Examples are listed in Table 1. Bacterial leaf spot diseases are considered in a separate best-practice guideline (Section C.9). In general, infection by fungal leaf spot pathogens is favoured by conditions of high relative humidity, with disease spread by water splash. Many of these pathogens are seed-borne. In the UK, the most prevalent leaf spot diseases of protected edible herbs are caused by Septoria species and these are described in more detail below.

Table 1. Examples of leaf spot diseases on UK protected herbs

Herb	Leaf spot species recorded	Seed-borne
Chives	Alternaria porri	Yes
Coriander	Ramularia coriandri	?
	Septoria species	Yes
Dill	Alternaria radicina	Yes
Lavender	Septoria lavandulae	?
	Phomopsis lavandulae	?
Lemon balm	Septoria melissae	?
Parsley	Alternaria dauci	Yes
	Alternaria radicini	Yes
	Alternaria petroselini	Yes
	Septoria petroselini	Yes
Rosemary	Phoma species	?
St John's wort	Colletotrichum gloeosporioides	Yes

  

Septoria leaf spots (Septoria species)

Septoria leaf spot is an important disease of parsley caused by Septoria petroselini. The disease also occurs occasionally on other herbs such as coriander (caused by a Septoria species), lemon balm (caused by S. melissae) and lavender (caused by S. lavandulae). Septoria species affect other crops of the Umbelliferae family (e.g. celery) but the host range for each species is limited. For example, S. petroselini only infects parsley and S. apiicola only infects celery.

The fungus can be seed-borne for parsley and coriander, and may also survive in plant debris and on volunteer plants. Under favourable environmental conditions, the disease can spread rapidly, affecting both yield and quality. Because of its seed-borne nature, the disease can potentially develop on crops under any production system, although those that are overhead watered are more at risk.

  

Symptoms

Typical early symptoms on parsley and coriander are brown sunken leaf spots on leaves and cotyledons (Figures 1 and 2). Occasionally, the lesions have yellow haloes. As the leaf spots age, the centres turn tan or light grey and tiny black fungal spore cases (pycnidia) are often visible, which is a useful diagnostic feature for this disease (Figure 3). When infection is severe (Figure 4), leaves may die and drop off, and lesions may develop on petioles.

On lemon balm, septoria leaf spots are dark brown/black, up to 3 mm diameter and angular in shape (being constricted between leaf veins) (Figure 5). Spore cases are sometimes visible within leaf spots on the underside of the leaf.

Disease sources and spread

The fungus is seed-borne (at least for parsley and coriander) and transmission from seed to growing plants has been demonstrated experimentally for parsley. Spore cases are sometimes visible on the seed surface but infection may also be more deep-seated within seeds. The seed coat attached to the cotyledon can act as a source of infection during propagation so that the fungus infects seedling leaves.

In addition to seed-borne inoculum, Septoria can survive on parsley crop debris for at least three years and also on volunteer or overwintered plants. There is also a risk of cross-infection between neighbouring crops (e.g. with sequential planting).

  

Conditions for infection

Crops are most at risk after long periods of leaf wetness, particularly at warm temperatures and high relative humidity. Disease development is highly dependent on the presence of water for the spore cases to swell and release spores, for splash dispersal of spores between plants and for leaf infection to occur. Spores are readily spread by irrigation and also by people (e.g. on wet boots) and machinery. The following information is known about infection conditions for parsley leaf spot:

• Mature parsley leaves are more susceptible to infection than young leaves.
• Infection and symptom development can occur over a wide temperature range. Optimum temperatures for disease development are around 20-25^oC.
• At 20-25^oC, increasing leaf wetness duration results in greater disease severity, with most disease occurring after 72 hours of leaf wetness.
• Under optimum conditions, symptoms can develop on parsley after only 9 days.

  

Integrated disease management

Cultural control

As Septoria species can be seed-borne, use of clean seed is important for disease avoidance. Thiram fungicide can be used as a warm water soak for parsley seed (but not approved for coriander) to reduce septoria infection to acceptable levels. However, under EC regulations this is not permitted for seed being used for organic production. There is potential for reducing levels of Septoria species on seed using hot water treatment, but treatment conditions are critical and are not well defined for parsley or coriander seed. In Sweden, research is ongoing to adapt a seed treatment technique originally developed for cereals, for use on vegetable seed including parsley. The technique involves treatment of seed with hot humid air and precise control of temperature, air humidity and treatment duration, (www.thermoseed.com).

Growers can confirm the health of seed samples by testing at NIAB (NIAB, Huntingdon Road, Cambs. CB3 OLE or http://www.niab.com/).

The viability of Septoria species on parsley and coriander seed may decline over time during storage but this is not a reliable control method.

Although there are significant varietal differences in the susceptibility of parsley to S. petroselini there are no resistant varieties available.

The risk of septoria and other fungal leaf spot diseases can be reduced by using the following measures:

• As leaf spot fungi can survive in plant debris, plant trays and pots should be new, or washed and disinfected before re-use in propagation or pot production.
• New parsley for cut herb production should be planted in soil where parsley has not been grown for at least 3 years.
• In all production systems, irrigation and ventilation should be carefully regulated since overhead watering can result in splash dispersal of the fungus between plants, while poor air movement can lead to extended periods of leaf wetness that are conducive for disease development.
• If overhead watering is necessary, avoid watering late in the day when leaf wetness duration will be prolonged.
• Inspect plants regularly for early symptoms of leaf spot and rogue out infected plants.
• Remove crop debris and volunteer plants, and dispose of them carefully.
• Isolate new plantings to avoid splash dispersal from currently or recently affected crops.
• Wider plant spacing may help to improve air circulation in the canopy, thus reducing leaf wetness duration and associated disease risk.
• In cut herbs, mowing alone is unlikely to eliminate the disease as the fungus can survive on crop debris. Flaming, as practised by some cut herb growers, could be more effective.
• Restrict entry into the crop.
• Use clean footware.
• Clean planting and harvesting equipment.
• Remove surplus or reject plants from the cropped area and dispose of carefully.

  

Biological control

There are currently no biological fungicide products approved specifically for fungal leaf spot diseases.

  

Chemical control (protected herbs)

For protected herb production systems where fungicides are used, Cleancrop Mancozeb or Karamate Dry Flo Newtec (mancozeb), Amistar (azoxystrobin) and Signum (boscalid + pyraclostrobin) are likely to be highly effective against septoria leaf spot. In trials on parsley septoria, all three fungicides gave excellent control when applied 5 or 2 days before infection, or 2 days after infection (HDC project FV 318). 

Management of septoria leaf spot could involve an early application of Amistar  or mancozeb as a protectant, or an application of Signum immediately after high risk environmental conditions (e.g. high temperatures, long leaf wetness duration) or if early symptoms of the disease were observed. 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.

Before selecting products to use for leaf spot 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 should be aware of the need to use strategies to minimise the risk of selecting resistant strains. If more than one fungicide treatment is applied to a crop, alternate products from different groups. Dithiocarbamate fungicides such as mancozeb are known as ‘multisite inhibitors'; they are less likely to result in problems of fungicide resistance and are particularly useful in preventative programmes. See Section A - Principles of IPM for further details on strategies to minimise the risk of selecting resistant strains.

Use of Amistar is compatible with biological control of pests in IPM. There is no information available yet on the side effects of Signum on products used for pest biocontrol. Mancozeb is moderately harmful, with adverse effects on certain biological control 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 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 Factsheet 09/04. Management of celery leaf spot.

HDC Project 237. Outdoor celery: development of integrated strategies for the management of septoria leaf spot and other diseases. Final report, March 2004.

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