C3 - Powdery Mildews

Introduction

Powdery mildew diseases are common on herbs, affecting members of the Umbelliferae (e.g. parsley) and Lamiaceae (e.g. sage and mint). The disease can severely reduce plant vigour, and symptoms on the stem or foliage generally make plants unmarketable. Powdery mildew can develop on protected herbs, irrespective of production system.

Powdery mildews are usually highly host-specific, being restricted to single or closely related hosts, or to a particular plant family. Table 1 lists herbs that are commonly affected in the UK and the fungi reported to cause powdery mildew on these crops. Note that even where the same powdery mildew species is listed for more than one herb species, different pathogen races exist which may be specialised to only one host species.

Table 1. Common powdery mildew diseases of herbs in the UK

Herb crop	Powdery mildew species
Bergamot	Erysiphe biocellata, Neoerysiphe galeopsidis
Dill	Erysiphe heraclei
Mints (Mentha species)	Erysiphe biocellata
Oregano	Erysiphe species, Neoerysiphe galeopsidis
Parsley	Erysiphe heraclei
Rosemary	Neoerysiphe galeopsidis
Sage	Erysiphe biocellata, Neoerysiphe galeopsidis
Tarragon	Erysiphe artemisiae

  

Symptoms

Typical symptoms appear as sparse, powdery white growth over leaves (especially on upper surfaces) and stems, or as discrete bright white spots of fungal growth. These symptoms may be difficult to see when foliage is dense or when there is a film of water on leaf surfaces. Powdery mildew infection may also result in the development of discoloured blotches (brown, red or purple) on leaves or stems. A severe attack of powdery mildew may lead to yellowing and premature leaf fall. Figures 1-5 show typical symptoms of powdery mildew on a range of herb crops.

Disease sources and spread

During the growing season, the main sources of powdery mildew are the dispersal spores (conidia) produced in chains on the surfaces of affected plants (Figure 6). These are spread by wind and air currents, and to a lesser extent by water splash, insects and man. Seed-borne infection is unlikely to occur and disease introduction to nurseries on young plants is uncommon. Once present within a crop, disease spread can occur very rapidly.

The powdery mildews only grow on living plant tissue. Some powdery mildew species produce small black resting bodies (chasmothecia) on the host tissue when environmental conditions are unfavourable for the fungus or when the crop reaches maturity (Figure 7). These resting structures survive in crop debris and in the following season produce dispersal spores that are spread by air currents. Powdery mildews may also survive as resting fungal strands (mycelium) within dormant buds of perennial crops. In areas with mild winters, or in glasshouse crops, powdery mildew can be active throughout the year.

Conditions for infection

Infection usually starts from dispersal spores (conidia). The effect of environmental factors (temperature, moisture and light) on infection varies with powdery mildew species and host. Germination of spores is usually optimal at 15-28^oC, with a minimum temperature of 2-4^oC, and a maximum of 30-35^oC. Although germination is favoured by high relative humidity, spores of powdery mildew can also germinate at low humidities. In contrast to many fungal diseases, subsequent disease spread (development of fungal mycelium over the plant surface and abundant spore production) is favoured by warm dry weather. Young plants are particularly susceptible to infection. Other conditions that predispose plants to powdery mildew attack are high levels of nitrogen fertilisers and dry rooting conditions.

  

Integrated disease management

Cultural control

Crop management
As powdery mildews can spread very rapidly, regular crop monitoring particularly for susceptible varieties is required to detect early symptoms of the disease. Check young plants arriving on the nursery, and also the re-growth of cut herb crops. In a dense crop, mildew often starts low down in the canopy (e.g. on parsley stems), so check canopy bases and plants in the centre of beds or benches.

There is some evidence that potassium can increase plant resistance to powdery mildew, so ensure that soil potassium level is adequate and that excess nitrogen is not applied.

Environmental conditions
Manipulation of environmental conditions can be used to minimise the risk of powdery mildew, however, care is needed to ensure that this does not create conditions that may favour other diseases. Reduction of high humidity, as described for the control of grey mould (Botrytis cinerea) (Section C.2) may help to minimise infection by powdery mildew but will not necessarily limit subsequent disease spread. Some growers have observed that if an outbreak of powdery mildew has occurred, application of a fine mist of water to foliage can limit disease development, although such conditions may favour other diseases (e.g. grey mould). Ensure that the rooting environment is not too dry as this can make plants more susceptible to powdery mildew, but avoid over-watering and associated risk of root rots. Some growers observe that powdery mildew develops more under shaded compared to bright conditions.

  

Biological control

Serenade ASO (Bacillus subtilis) is a biological fungicide that has a specific Off-label Approval for use on protected herbs and has reported efficacy against powdery mildew.

In Germany, a product Milsana® (extract of giant knotweed, Reynoutria sachalinensis) marketed as a ‘plant strengthener' was reported to be effective against powdery mildew. Trials in this project (Bennison & Green, 2007; see Further Information) showed that weekly applications of Milsana® plus wetter checked powdery mildew development on mint, even under high disease pressure. This product is not currently available in the UK.

  

Chemical control (protected herbs)

Cultural controls should be used to minimise the risk of disease outbreaks. For situations where risk of powdery mildew infection is high, or where it is necessary to prevent secondary spread from a disease outbreak, Table 2 lists products that have activity against powdery mildew and are currently permitted for use on protected herbs. In addition to conventional fungicides, potassium hydrogen carbonate (also known as potassium bicarbonate) is available as a Commodity Substance Approval for powdery mildew control. One advantage of this product is that it can be used close to harvest. Detailed information on use of potassium bicarbonate on horticultural crops is available as an HDC report (see Further Information).  A Defra literature review also describes use of inorganic salts for powdery mildew control and this is being further investigated in Defra project PS 2125.  The product SB Plant Invigorator is a formulation of surfactants and nutrients that works by physical action only, and is approved for use against aphids and whitefly on all edible crops. Product literature indicates that this product can provide useful control of powdery mildew.

In project trials (Bennison & Green, 2007; see Further Information), a single application of Amistar (azoxystrobin) or Thiovit Jet (sulphur) plus wetter, followed by weekly applications of potassium bicarbonate (plus wetter) provided effective control of powdery mildew on protected mint, even under high disease pressure. Similar levels of disease control were maintained with potassium bicarbonate (plus wetter) only. 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. Sulphur has no stipulated harvest interval, although late applications can result in visible deposits on the product. Also note that sulphur products can have deleterious effects on certain biological products used for pest control (see Table 2).

Before selecting products to use for powdery mildew control on protected herbs, test treat a small batch of plants before widespread application if using a product for the first time, to check crop safety. Potassium bicarbonate is known to occasionally cause scorch on herb crops, particularly when used at the maximum approved rate (2% solution). In project trials, a rate of 0.25% plus wetter was found to be safe on mint under protection. 

Powdery mildew control is likely to be most effective when the fungicides listed in Table 2 are applied as preventative treatments. The exception to this is potassium bicarbonate, which can delay or suppress disease development after symptoms are visible. However, repeated applications are usually required because it is not persistent on the plant and because disease development is potentially rapid.

It should be noted that products containing potassium bicarbonate at less than 99% are not classified as food grade, and are not covered by the Commodity Substance Approval. Such products are marketed for the horticultural industry as ‘foliar supplements' or ‘foliar feeds'. Experience from UK growers and consultants suggests that under some circumstances, foliar feeds and supplements containing potassium bicarbonate may give incidental control of powdery mildew, but they are not marketed for this purpose. Trials in this project indicated that weekly applications of a non food-grade potassium bicarbonate product (K50) plus wetter (SW7) from Omex, checked powdery mildew development on mint, even under high disease pressure.

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. Resistance to powdery mildew fungi is common and in particular, care should be taken when using products containing prochloraz, or strobilurins (Amistar and Signum). If more than one fungicide treatment is applied to a crop, alternate products from different groups. Products such as sulphur fungicides, Serenade ASO and potassium bicarbonate are less likely to result in problems of fungicide resistance. 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 powdery mildew diseases and permitted on protected herbs (March 2009)

Fungicide group and active ingredient(s)	Example products	Compatibility with pest biological control
1.  Biological control Bacillus subtilis	Serenade ASO	Safe
2. Commodity substance Potassium hydrogen carbonate (Potassium bicarbonate)	Potassium bicarbonate	Not yet known
3. DMI Prochloraz	Scotts Octave	Moderately harmful*
4.  QoI Azoxystrobin Carboxamide + QoI Boscalid + pyraclostrobin	Amistar Signum	Safe                                     Not yet known
5. Sulphur Sulphur	Microthiol Special Solfa WG Thiovit Jet	Slightly harmful to a wide range. More harmful to e.g. Encarsia, Phytoseiulus and Trichogramma

*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 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

Bennison, J. & Green, K. 2007.  Protected herbs: Best Practice Guidelines for integrated pest and disease management.  Final report for Defra project HH3118TPC and HDC project PC 210. (available from the HDC).

Defra project PS 2117.  Potential of simple salts to partially substitute for conventional foliar fungicides.  Final report, 2008.  http://randd.defra.gov.uk

HDC Factsheet 11/03. Control of powdery mildew diseases on cut flowers.

HDC Project CP48. Use of potassium hydrogen carbonate for powdery mildew control. Final Report, June 2005.