C4 – Downy Mildews

Introduction

Downy mildews on UK herb crops include Peronspora destructor on chives, Peronospora lamii on mint and sage, and Plasmopara umbelliferarum on parsley. Downy mildew was reported on basil in the UK for the first time in 2010. The fungi that cause downy mildew on herbs tend to be specific to one or a few hosts. In other countries, downy mildews affect herbs such as basil, coriander and savory, so disease symptoms may occasionally be seen on imported produce. Protected herbs may be affected by downy mildews under any production system, although symptoms may be more prevalent in unheated structures and where overhead irrigation is used.

Symptoms

It can be difficult to differentiate symptoms of downy mildew from those of powdery mildew; accurate diagnosis is important to ensure appropriate disease management.

Downy mildews are primarily foliar diseases. Symptoms vary with host plant but generally appear as discoloured patches, often angular in shape, on the upper leaf surface. ‘Downy’ growth may develop on the leaf underside due to fungal spore production, in contrast with powdery mildews where spores are more usually on the upper leaf surface. Stunting and distorted growth may also occur, together with pale foliage and profuse spore production. In some cases, resting spores (oospores) may be produced in large numbers in internal tissues.

On sage, brown, angular lesions (delimited by veins) are seen on the upper leaf surface and at leaf margins (Figure 1). When infection is severe, the lesions may enlarge and merge to give complete leaf death (Figure 2). Grey spore production may be visible on the lower leaf surface (Figure 2). On parsley, symptoms may first be seen as irregular pale yellow or light brown lesions on the upper leaf surface (Figure 3), which later enlarge and become grey/brown in colour (Figure 4); white spore production may be visible on the leaf underside (Figure 5). Downy mildew symptoms on imported coriander were observed as brown/black water-soaked lesions on the upper leaf surface with abundant white spore production on the lower leaf surface (Figure 6). On chives, downy mildew symptoms are similar to those on onion (Figure 7). Yellow patches develop at the tip and down the length of affected leaves. As the disease progresses, entire leaves become light brown in colour with patches of brown/purple spore production.

With basil downy mildew (Peronospora belbahrii), yellowing of the upper surface of leaves often occurs in sections delineated by veins. Purplish grey spores develop on the lower surface of leaves, coinciding with yellowing on the upper surface (Figure 8). Affected leaf tissue eventually turns dark brown. See symptom photos and more information on this disease in HDC Factsheet 08/11 and Tuttle McGrath, 2009 (see Further Information).

Disease sources and spread

Infected plants are the main source of disease and air-borne spores produced on leaves are the most important means of disease spread. These spores are short-lived, surviving just a few days. Downy mildew fungi can also survive between seasons as dormant fungal strands or resting spores (oospores) in crop debris, or woody tissue. P. destructor is reported to survive on onion seed, so infested seed could also potentially be a source of inoculum for downy mildew on chives. While it has been documented that basil seed can be infested with downy mildew, the extent to which this occurs worldwide is not known. Seed transmission of downy mildews has not been reported for other herb species.

Conditions for infection

Development of downy mildews is favoured by conditions of high humidity and prolonged leaf wetness. Free moisture is required for infection to occur. Spores are normally produced during the night if relative humidity is high, then released into the air in the morning as the temperature rises and humidity falls. Temperature requirements vary with species and are not generally well described for downy mildew fungi affecting herbs. However, for P. destructor (downy mildew on chives), spore production can occur in the range 4-25^oC, with an optimum of 13^oC. The spores can remain viable for about 4 days, and require free water and temperatures of 7-16^oC to germinate. Studies are ongoing in HDC project FV 390 to gather information on high and low risk environmental conditions for downy mildews on sage, parsley, mint and basil (see Further Information). For example, in September 2011 an outbreak of downy mildew on outdoor parsley occurred when leaf wetness periods exceeded 30 hours and temperatures were between 5 and15oC.

Integrated disease management

Cultural control

In general, herb varieties resistant to downy mildew are not available. However, evaluations are being conducted to determine if there are differences in susceptibility among varieties and species of basil (Tuttle McGrath, 2009).

Ensure that new plants introduced to the nursery are symptom free. After an outbreak of downy mildew, carefully remove infected crop debris to prevent disease spread and to reduce carry-over to the next season. Consider treating structures, benches and equipment with a disinfectant after a severe outbreak.

Due to the potential for basil downy mildew to be introduced via seed, ensure basil seed is obtained from a reputable supplier.

Disease risk can be reduced by limiting leaf wetness duration to less than 4 hours. Where possible, crops should be well ventilated with adequate spacing. Avoid overhead irrigation and watering late in the evening. Crops covered with fleece or mesh may be more susceptible to downy mildew development.

Biological control

There are currently no biological fungicide products approved for control of downy mildew diseases in the UK. There is continuing interest in the use of alternatives to conventional fungicides for downy mildew control. In HDC Project HNS 135, a range of natural products was evaluated for efficacy against rose downy mildew (see Further Information).

Chemical control (protected herbs)

Approvals for downy mildew fungicides on herbs have increased recently due to more frequent and severe disease outbreaks in field-grown herbs (parsley, mint and sage) and on protected basil. Fungicides should only be used for downy mildew control in conjunction with cultural control practices. Seed treatment with Apron XL (Metalaxyl-M) may give early protection against downy mildew. Possible products that could be used preventatively if there is a high risk of disease development are listed in Table 1. Treat a few plants initially to check crop safety. The development of fungicide resistance is a real risk in downy mildews although not reported for the species that affect herbs. For example, some downy mildews (e.g. Bremia lactucae on lettuce) have developed resistance to phenylamide fungicides such as metalaxyl-M (in SL 567A and Fubol Gold). To minimise resistance risk, this fungicide should always be used in a mixture with other fungicides that have a different mode of action. There is also a risk of fungicide resistance when using CAA fungicides such as dimethomorph and mandipropamid. See Section A – Principles of IPM for further details on strategies to minimise the risk of selecting resistant strains.

Table 1. Fungicides with activity against downy mildew fungi and permitted on protected herbs (March 2013)

Fungicide group and active ingredient(s)	Example products	Compatibility with pest biological control
1. CAA Dimethomorph Mandipropamid	Paraat Revus	Safe to Phytoseiulus persimilis, moderately harmful to Orius Not yet known
2. Dithiocarbamate +phenylamide Mancozeb + metalaxyl-M	Fubol Gold WG	Moderately harmful*
3. Phenylamide Metalaxyl-M	SL567A Apron XL	Safe to many biocontrols; moderately harmful to Amblyseius cucumeris and Phytoseiulus.
4. Phosphonate + carbamate Fosetyl-aluminium + propamocarb hydrochloride	Previcur Energy	Safe to many biocontrols; moderately harmful to Aphidoletes and Amblyseius swirskii.

*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

HDC Factsheet 04/04. Control of downy mildew diseases on hardy nursery stock and herbaceous perennials.

HDC Factsheet 08/11. Downy mildew of basil.

HDC Project HNS 135. Container grown rose: evaluation of natural products for prevention and control of downy mildew and improved shelf-life. Final Report, 2008.

HDC Project FV 390. Outdoor herbs: epidemiology and control of downy mildew in sage, parsley, mint and basil under protection. Annual Report, March 2012.

Koike, S.T., Gladders, P. & Paulus, A.O. 2007. Vegetable Diseases. A Colour Handbook. London, UK: Manson Publishing Ltd.

Tuttle McGrath, M. 2013. Expect and prepare for downy mildew in basil. http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html