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

With recent approvals, growers now have the option to include biological fungicides as part of integrated management for powdery mildew diseases. Serenade ASO (Bacillus subtilis) is a biological fungicide that has an Extension of Authorisation for minor use (EAMU, previously SOLA) for use on protected herbs, and has reported efficacy against powdery mildew. AQ10 (Ampelomyces quisqualis) is also available for powdery mildew control but since it acts by parasitism of powdery mildew fungi, it may not be sufficiently fast acting for crops of short duration where there is zero tolerance of blemishes. It may be more suitable for longer season herb crops. Both of these biological fungicides have a zero day harvest interval.

Certain products containing extract of giant knotweed (Reynoutria sachalinensis) are reported to be effective against powdery mildew, working by strengthening the plant’s defences against pathogen attack. Previous trials (Bennison & Green, 2007; see Further Information) showed that weekly applications of a product containing giant knotweed extract checked powdery mildew development on protected mint, even under high disease pressure. Applications of a similar product to protected parsley were effective for powdery mildew control when applied 2 days before infection (Defra, 2012; see Further Information). These products are not currently commercially available or approved as biofungicides in the UK.

Chemical control (protected herbs)

Cultural controls together with biological fungicides should be used where possible to minimise the risk of disease outbreaks. For situations where risk of powdery mildew infection is very high, or where it is necessary to prevent secondary spread from a disease outbreak, Table 2 includes conventional fungicides 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. A Defra literature review describes use of inorganic salts (including potassium bicarbonate) for powdery mildew control and this was further investigated in Defra project PS 2125 (see Further Information). 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. Potassium bicarbonate (at 10 g/L) plus wetter was also found to be effective for control of parsley powdery mildew (Defra, 2012). 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).

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 infection or even 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.

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. Biological fungicides and products such as sulphur fungicides 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 2013)

Fungicide group and active ingredient(s)	Example products	Compatibility with pest biological control
1. Carboxamide + QoI Boscalid + pyraclostrobin	Signum	Not yet known
2. Commodity substance Potassium hydrogen carbonate	Agricarb Omex K50 Potassium hydrogen carbonate	Not yet known*
3. DMI Prochloraz	Scotts Octave	Moderately harmful**
4. Microbial Ampelomyces quisqualis Bacillus subtilis	AQ10 Serenade ASO	Safe Safe
5. QoI Azoxystrobin	Amistar	Safe
6. Sulphur Sulphur	Microthiol Special Solfa WG Thiovit Jet	Slightly harmful to a wide range. More harmful to e.g. Encarsia, Phytoseiulus and Trichogramma

*Safe to Neoseiulus (Amblyseius) cucumeris and Aphidius colemani. Not yet known for other biocontrols.

**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

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

Defra project PS 2125. Novel strategies for optimising powdery mildew management on outdoor cucurbits and protected herbs. Final report, 2012. http://randd.defra.gov.uk

Pope, T., Maulden, K., Bennison, J. & Green, K. 2011. Side-effects testing of novel powdery mildew fungicides against biological control agents. Proceedings of the IOBC meeting on “Integrated Control in Protected Crops, Temperate Climate”, Sutton Scotney, UK 19-22 Sep 2011.