Powdery mildew of grapevines in Western Australia

Symptoms

The fungus causes ash-grey to white powdery growth on green tissue of the grapevine. In particular, the upper and lower surfaces of young leaves, shoots or clusters are highly susceptible.

The chains of conidia that develop from the powdery mildew hyphae give the infected vine tissue the characteristic powdery or dusty appearance. Severely infected vines emit a musty odour mid to late season.

Flag shoots are stunted shoots covered partly or wholly with ash-grey to white powdery growth with distorted leaves that curl upwards. These shoots become evident two to eight weeks after budburst (Figure 1).

Figure 1 A powdery mildew-infected flag shoot indicating cupped and distorted leaves (RW Emmett DEPI Victoria)

Leaves are most susceptible when they are expanding. Infections result in small yellow-green blotches 2-10mm in diameter with an irregular outline on the upper surface of leaves in spring (Figure 2). The blotches form an ash-grey to white powdery growth of hyphae which develops conidia on both sides of the leaf surface.

Web-like hyphae and chains of conidia are clearly visible with a 10x hand lens (Figure 3). In the field this fungal growth is flat, as the conidia chains are constantly broken. The blotches enlarge and may merge to cover the whole leaf (Figure 4). Smaller veins on the underside of the infected leaves may turn brown (Figure 5).

The earliest infected leaves become distorted and discoloured (Figure 6), sometimes giving the vines a wilted appearance. Severely diseased leaves blacken, dry out and fall prematurely in hot weather. Leaves become more resistant to infection with age, but are never completely resistant.

Figure 2 Yellow-green leaf blotches where early powdery mildew infection has occurred

Figure 3 Chains of powdery mildew conidia viewed under a microscope (also referred to as conidiospores)

Figure 4 Severely infected Chardonnay leaf showing ash-grey growth on the surface

Figure 5 Veins on the underside of the leaf can turn brown when infected

Figure 6 Distorted leaf with rolled margins caused by powdery mildew infection

Shoots – ash-grey to white powdery growth develops in patches until the whole shoot is covered (Figure 7). Severely diseased shoots are weakened, stunted and can die.

Figure 7 Shoot infection of powdery mildew showing colonies in a patchy distribution

Bunches of most cultivars are susceptible between flowering and up to five weeks later. Although berries develop resistance with age the bunch stalk and stems remain susceptible. Ash-grey to white powdery growth develops on immature berries and bunch stalks (Figure 8).

Severely infected berries may develop irregular shapes, crack or split and rot. Red varieties may colour unevenly. Post-veraison, berries develop a brown web-like pattern on the surface, very noticeable on white varieties (Figure 9).

Over time berries become resistant to powdery mildew infection and thus once this occurs the fungus is killed inside the berry leaving scar tissue on the berry. When the berry expands these scars can lead to berry split and a site for Botrytis cinerea infection.

Figure 8 Bunch of grapes with severe infection of powdery mildew

Figure 9 Scarring of berries where growth of powdery mildew has occurred on the skin surface

Black patches on green immature shoots develop into reddish-brown patches on mature canes. This is evidence of a powdery mildew infection earlier in the season.

Looks like

Powdery mildew is often confused with downy mildew. Downy mildew fungal growth occurs only on the underside of the leaf while powdery mildew grows on both sides of the leaf surface. Downy mildew fungal growth is white and raised while powdery mildew is ash-grey to white and flat.

Young distorted leaves and flag shoots can be confused with bud mite damage. Late in the growing season distorted and discoloured powdery mildew leaf damage can resemble rust mite damage.

Powdery mildew of grapevines is specific to grapevines and although powdery mildew of other horticultural or ornamental crops look similar they will not infect a grapevine. If powdery mildew of other plants is occurring close to your vines it indicates conditions have been suitable for grapevine powdery mildew infection.

Damage and loss

Powdery mildew is considered the most costly disease to grapevine production in Australia with the combined effect of increased costs and reduced income totalling A$76 million annually.

Powdery mildew infections around flowering and up to five weeks later pose the greatest risk of damage and loss. Early infections lead to a greater number of diseased buds and fungal resting bodies to carry the disease over to the next season. Leaf, shoot and stalk damage interferes with vine metabolism and fruit quality. Infected flowers have poor fruit set and reduced yield.

Berries with cracks or splits predispose them to attack by other fungi. Infected bunches can cause off flavours and may be down graded or rejected by wineries. Severe powdery mildew infections in subsequent seasons can reduce the vigour and productivity of the vine.

Varietal susceptibility

All Vitis vinifera varieties are susceptible, in varying degrees. The more susceptible wine varieties include chardonnay, chenin blanc, riesling, semillon, verdelho and cabernet sauvignon. Shiraz and grenache are less susceptible. The most susceptible table grape varieties include flame seedless, red globe, cardinal, italia and crimson seedless.

Life cycle

Powdery mildew is a disease of young tissue and only grows on green parts of the vine. However, not all stages of development of the powdery mildew fungus are found on green material.

Sexual and asexual structures

Asexual spores are called conidiospores. These form on specialised hyphae on the surface of the tissue. The hyphae grow vertically from the plant surface and bear chains of conidiospores, also called conidia (refer to Figure 3). The conidia produced during this process are clones of one another.

Sexual spores are called ascospores. These are produced from sexual fruiting bodies called chasmothecia (previously cleistothecia) that develop when the hyphae of two compatible isolates make contact. Chasmothecia are 0.1mm in diameter, just visible with the naked eye and form mid to late summer on leaves, shoots and bunches.

They are white when young and change to yellow, orange, brown then black as they mature (Figure 10). Chasmothecia turn yellow after about seven days from initiation, begin to form the bristle like appendages after three weeks and after 4-5 weeks are disconnected from the surviving colony. Sexual variation in the ascospores may lead to strains of the powdery mildew fungus that are more resistant to fungicides or that are more virulent.

Figure 10 Chasmothecia on the underside of the leaf show different maturity: yellow (young), brown and black with bristles (mature)

Overwintering

The fungus survives the winter months in two ways:

• Infected buds – the fungus grows down between the bud scales on infected shoots in early spring and remains in the buds through the winter. It remains dormant, with the bud scales providing protection from the elements, particularly in regions such as WA that experience mild winters compared to other growing regions in the northern hemisphere.
• Chasmothecia – mature chasmothecia are washed into bark crevices and other sheltered places such as leaf litter and remain over the winter. Chasmothecia located in the bark have a higher rate of survival than those in the soil or leaf litter.

Infection and spread

Powdery mildew primary infections occur by either flag shoots or chasmothecia.

Infected buds produce shoots called flag shoots (described earlier under ‘symptoms’) in spring. Depending on how much powdery mildew was present early the previous season to cause infected buds there will generally be one flag shoot per 1000 shoots. Flag shoots produce conidiospores that spread the disease early in the season and are thought to be the main source of carryover in Australia.

Chasmothecia produce ascospores after a minimum of 2.5mm of rain and when temperatures are 10-30°C. This occurs mostly between budburst and flowering (late winter and early spring). Ascospores infect the lowest leaves and shoots as these are closest to the over wintering chasmothecia, as a result it is usually the underside of these leaves that are infected first. The ascospores germinate and produce powdery mildew colonies that then produce conidiospores.

Conidiospores are spread by wind dispersal. Conidiospores landing on the green parts of the vine germinate and infect the vine by sending haustoria (root-like appendages) into the epidermal (surface) cells. The fungus absorbs nutrients from the grapevine for its development. The absorption of nutrients by the fungus eventually leads to death of the infected tissue.

Both conidiospores and ascospores can infect the vine within 24 hours of dispersal. Germination, infection and development of ascospores to conidiospores and of conidiospores take around 5-12 days depending on temperatures. Several infection cycles can occur through the growing season and the incidence of infection increases rapidly if controls are not applied or are ineffective after infection.

Weather conditions

Powdery mildew is favoured by:

• mild cloudy weather;
• low to moderate light such as sheltered parts of the canopy or vineyard;
• optimum temperatures 22-28°C with a range of 6-33°C; and
• humid conditions (enhances sporulation).

Powdery mildew is reduced on exposed leaf surfaces by:

• air temperatures of 35°C or higher; and
• direct sunlight (UV radiation reduces colony expansion and germination of conidia).

Unlike most other grapevine diseases, powdery mildew does not require free moisture for infection (except for the production of ascospores from chasmothecia as discussed above). Free water from rain, dew, irrigation or high volume spraying can cause poor or abnormal germination of conidiospores or wash them from the vine surface. However, established colonies repel water and those that are sheltered by the vine canopy will probably survive. Water on vines may also reduce canopy temperature and increase humidity, thus encouraging sporulation and more infection.

Monitoring

Early detection is important to reduce disease development.

Where to monitor

• vineyard areas where the disease has been present in previous seasons;
• vines that have had flag shoots the previous year tend to produce more flag shoots due to the level of infection in the past;
• sheltered areas or densely shaded vines;
• most susceptible varieties; and
• ends of rows that may have been unsprayed.

When to monitor

Budburst onwards at two weekly intervals, bearing in mind that:

• flag shoots are most readily evident two to eight weeks after budburst before the canopy becomes too large;
• flag shoots are generally delayed in budburst compared to healthy shoots; and
• ascospore infections occur mostly on lower leaves of the shoots.

How to monitor

Some things to consider when monitoring include:

• inspect 200 vines from both sides of the row, examining leaves and later bunches;
• powdery mildew is easier to see when leaves are orientated at an angle to the sun;
• use a 10x hand lens to check suspect vine material for hyphae and conidiospores early in the season and chasmothecia later in the season; and
• mark flag shoot locations and infection sites with flag tape to enable later assessments of disease spread and effectiveness of management options.

If you are unusure whether you have powdery mildew samples can be submitted to a pathology lab for confirmation from DDLS - Plant pathology services.

Management

If powdery mildew was a problem last season, it is most likely that high levels of overwintering infected buds and chasmothecia will be present in the vineyard. Early season management is essential.

If powdery mildew was not a problem last season, monitoring and appropriate management options should be considered. Weather conditions early in the season will affect management decisions to prevent epidemics occurring later in the season.

Vineyard establishment

Consider orientation of rows in the direction of prevailing winds. Select varieties and clones that have open bunches. For wine grapes consider planting on rootstocks that reduce vegetative growth. Avoid overcrowding. Select trellis types that open up the canopy.

Cultural

Canopy management practices that permit good air circulation, spray penetration and filtered sunlight exposure are highly beneficial. Some of these include pruning methods, shoot training, shoot thinning, leaf plucking, vine trimming and hedging. Nitrogen fertilisers should also be used with caution to avoid excessive vegetative growth.

Biological

Currently there are no commercially available biological control agents registered for powdery mildew control in Australia. Ampelomyces quisqualis (a parasitic fungus of powdery mildew) has been reported to control the chasmothecia stage of Erysiphe necator, and has been reported in some vineyards. Fungus-eating mites such as the Tydeid mite and beetles have been reported to reduce powdery mildew colonies on vines.

Chemical

There are several chemical groups available for powdery mildew control in Western Australia. The chemicals registered for use on powdery mildew for the production of wine and table grapes are listed in the Viticulture spray guide for Western Australia. Further application requirements for the control of grapevine powdery mildew are listed in the Australian Wine Research Institutes (AWRI) 'Dog Book'.

Early season control is the key. The majority of chemicals currently registered for this disease are registered for use as a preventative option before infection has occurred. A preventative spray program reduces the risk of disease development and damage but increases the number of sprays needed.

Key aspects of preventative spraying

• In periods of rapid vine growth spray intervals of 7-10 days may be required to protect new growth.
• If temperatures of 35°C or greater occur disease development is slowed and spray intervals of more than 14 days can be used.
• Fungicide application just before flowering and during the five weeks after are the most important as these protect the berries during the period when they are most susceptible to powdery mildew.
• In most seasons four to six applications of fungicides per season will be adequate to control powdery mildew.
• Sulphur should be used as an early spray to prevent mite damage. Excluding sulphur applications from spray programs may give rise to mite problems.
• After veraison additional sprays are only required if build up of disease on foliage and bunch stalks is present. For table grapes it is essential to maintain fresh, green, disease free foliage and bunch stalks until harvest commences.

Reliance on monitoring for powdery mildew symptoms can reduce the amount of chemicals applied but involves a higher level of risk of disease development and damage if early symptoms are missed

Fungicide resistance

Due to the ability of Erysiphe necator to sexually reproduce within Australian vineyards and the potential for multiple life cycles over a single growing season, it is considered medium risk for the development of resistance to fungicides used to manage it.

The continual use of one fungicide or one group of fungicides increases the risk of resistance developing to that fungicide or that group of fungicides. To reduce the risk of resistance developing within your vineyard always read the chemical label and regularly consult the resistance management guidelines for grapevine powdery mildew produced by Croplife Australia.

Powdery mildew outbreaks

In southern regions late December to early January is the likely time for outbreaks to occur. In northern regions (Gascoyne) late September to early October outbreaks may occur. To achieve better spray coverage and prevent further disease development the following practices are recommended:

• trim shoot growth to allow shoots to become more erect and expose bunches;
• lift wires to expose bunches;
• leaf pluck two to three leaves immediately above and below bunches;
• adjust spray nozzles and direct air flow of ducted spray machines to ensure most spray is deposited into fruit zone;
• use high spray volumes and highest rate of wetting agent (as leaves and bunches infected with powdery mildew are difficult to wet); and
• apply two to three sprays each seven to ten days apart to ensure maximum coverage of leaves and bunches. Spray in the opposite direction with the final spray.

Caution: Exposed bunches are susceptible to sunburn. Application of fungicides late in the season may give rise to chemical residues in the fruit or visual residue problems on berries. Visual residue on berries at harvest is undesirable for table grapes. Refer to product labels, AWRI 'Dog Book' and the viticulture spray guide for general comments and restrictions on usage.

Postharvest sprays are of limited value except for young vines. Buds will already be infected and most chasmothecia will have already lodged into bark crevices and other sheltered places. The cost, amount of chemical and the chemical resistance consideration to control the survival structures of the fungus usually outways any benefit achieved by the spray.

Young vines may require postharvest sprays to ensure continued shoot growth and to prevent premature defoliation so that the young vine can establish its vine framework and can lay down its carbohydrate reserves for the following season.

References and further reading

Coombe, BG and Dry, PR (eds) 1992, Viticulture - Volume 2: Practices, Winetitles, Adelaide.

Flaherty, DL, Christensen, LP, Lanini, WT, Marois, JJ, Phillips, PA and Wilson, LT (eds) 1992, Grape Pest Management – Second Edition, The Regents of the University of California, Los Angeles.

Gadoury, DM, Cadle-Davidson, L, Wilcox, WF, Dry, IB, Seem, RC and Milgroom, MG 2012, Grapevine powdery mildew (Erysiphe necator): a fascinating system for the study of biology, ecology and epidemiology of an obligate biotroph. Molecular Plant Pathology 13(1): 1-16.

Glenn, D, Aitken, D and Braybrook, D (eds) 1998, IPM Viticulture: Research to Practiceâ Training Workshop Manual, Department of Natural Resources and Environment, Knoxfield.

Nicholas, P, Magarey, P and Wachtel, M 1994, Diseases and Pests – Grape Production Series Number 1, Winetitles, Adelaide.

Scott, E 1988, Mildews: What they are and how they survive in the vineyard. Australian Viticulture Vol 2, No. 6, pp 5-16.

Wicks, T, Emmett, B, Hitch, C and Magarey, P 2002, Post-harvest fungicide applications – are they worth the effort?  The Australian and New Zealand Grapegrower & Winemaker - 30^th Annual Technical Issue, Ryan Publications, Adelaide No. 461 p. 156.

Wicks, T, Hall, B, Hitch, C and Malic, B 1998, Control of established powdery mildew infections. The Australian Grapegrower & Winemaker – 26^th Annual Technical Issue, Ryan Publications, Adelaide No. 414a pp 138-139.

Winkler, AJ, Cook, JA, Kliewer, WM and Lider, LA 1974, General Viticulture, The Regents of the University of California, Los Angeles.