Downy mildew of grapevines

Symptoms

Downy mildew attacks all green parts of the grapevine.

Leaves

The symptoms vary with leaf age. On young leaves (in spring), the disease will appear on the upper surface as small yellow spots referred to as oil spots. They are about 10mm diameter, often with a chocolate halo (Figures 1 and 2). These spots tend to grow to about 50mm diameter as they mature and the halo fades. As they enlarge they may appear to cover most of the leaf, especially if there is more than one spot on the leaf (Figure 3). On red varieties the oil spot can appear red (Figure 4).

Figure 1 A single oil spot on a grapevine leaf

Figure 2 Downy mildew oilspots on a leaf with a chocolate covered halo that fades as it ages

Figure 3 Multiple infections can lead to several oil spots on a single leaf which can merge

Figure 4 A red oil spot. On occasion these appear on red varieties

After warm humid nights, a dense, raised, white cottony growth develops on the underside of the yellow oil spots (Figure 5). This is commonly referred to as 'white down'. As the spots age naturally, or after a sporulation event (see later) or hot weather, their centres dry out and become a reddish brown leaving an outer ring of yellow (Figure 6).

The fungus in this yellow ring remains active and given favourable conditions at night, can produce a ring of 'white down' on this outer active edge (Figure 7).

Figure 5 White down (spores) develop beneath oil spots after warm, humid nights

Figure 6 An old oil spot that has died in the centre but remained active with an outer ring of yellow

Figure 7 After favourable conditions the active yellow ring surrounding the dead tissue is still capable of producing new spores

Later in the growing season (late summer and autumn) on mature leaves, leaf infections will appear as small, angular, yellow spots that are limited in growth by veins (Figure 8). These form a tapestry-like (mosaic) pattern that soon turn reddish brown. Defoliation can occur in severely affected vines.

Figure 8 Infection on mature leaves will be yellow-brown and remain small, confined by the leaf veins forming a mosaic pattern

Shoots

Infection on young shoots, stems and tendrils is seen as oily brown areas (Figure 9). These oily patches may spread into leaf stalks, which turn brown and may die. After warm humid nights these oily patches may also sporulate and be covered with white down.

Figure 9 Downy mildew leaf infection has spread to the shoot causing the oily black appearance

Inflorescences, bunches and berries

Infection on inflorescences, young berries and bunches are seen as oily brown areas. After suitable warm humid nights they may be covered with white down (Figure 10). Infected inflorescences and young bunches rapidly turn brown and wither (Figure 11). Infected young berries stop growing, harden and may later develop a purple hue. They turn dark brown, shrivel and fall from bunches.

Figure 10 Downy mildew infection (white spores) of a grapevine inflorescence

Figure 11 Infection near flowering can result in the death of the bunches which then turn brown

Berries become resistant to infection when they are pea size (5-6mm diameter). However, they may still be killed if the berry or bunch stems become infected. They may also sunburn and fail to ripen if defoliation occurs from leaf infection (Figure 12).

Figure 12 Downy mildew infection of a grape bunch showing infected berries with white down and resistant healthy green berries

Looks like

Young downy mildew infections may be confused with powdery mildew, caused by Erysiphe necator. Downy mildew spots are oily with a chocolate halo and develop 'white down' on the underside of the leaf.

Powdery mildew spots are often smaller and yellow-green which then develop a thin layer of ash-grey powdery spores that may eventually cover both sides of the leaf (see Powdery mildew of grapevines in Western Australia).

Yellow spots on leaves may also be due to spray drift damage from herbicides such as paraquat (e.g. Spray.Seed®) or sucking insect damage. Paraquat damage is distinguishable from downy mildew as it does not grow and tends to develop a small brown spot in the centre of the yellow area (Figure 13).

Figure 13 Herbicide damage can be confused with downy mildew

Often downy mildew infection occurs at the bottom of leaf tips or at the junction of the petiole, where water has collected, and when the infection dies or is killed through management techniques this can be confused with botrytis leaf infection (Figure 14).

Frost damage to leaves can cause mosaic symptoms similar to those seen in mature leaf infection of downy mildew (Figure 15). Confirmation that it is downy mildew and not another form of damage requires a bag test where the downy mildew will produce white down on the underside of the oilspot whilst the others will not (see ‘Monitoring’).

Figure 14: Downy mildew infection at the end of a leaf tip that has died and looks similar to a botrytis leaf infection.

Figure 15 Frost damage to a vine leaf can be confused with downy mildew infection

White growth on the underside of a leaf may also be due to grape leaf blister mite damage. This white growth is distinguishable from downy mildew as it forms within blister-like green galls that bulge on the upper side of the leaf.

White fungal growth on a leaf may also be due to other fungi such as Penicillium, Aspergillus or Rhizopus. These infections will eventually change to green, blue, black or brown. Many can also cause grapevine bunch rots.

Plasmopara viticola is specific to grapevines (for example, Vitis vinifera), although not all Vitis spp. are susceptible. The American rootstock species and hybrids are less susceptible or resistant (refer to 'Varietal susceptibility').

Other species of downy mildew, such as those found on cucurbits and roses, do not attack grapevines. Although, weather conditions that favour the development and spread of grapevine downy mildew also may encourage the development and spread of cucurbit and rose downy mildews.

Damage and loss

Severe infection will cause leaves to fall prematurely, reducing yield and berry sugar content and will expose remaining bunches to sunburn. Total crop loss may occur if severe infection is not managed, especially near flowering. If partial bunch infection occurs this can cause significant aesthetic issues for table grape growers. Severe leaf fall also can cause yield loss in the following season due to the inability of the vine to store reserves.

Varietal susceptibility

All varieties of Vitis vinifera are highly susceptible to downy mildew infection. The strain of the disease in Western Australia does not infect or show symptoms on V. rupestris, V. cordifolia and V.rotundifolia and V. riparia vines. These are often used as rootstocks in WA.

Disease cycle

Downy mildew is an obligate parasite (meaning it requires a living host) and therefore it grows on all green parts of the vine. However, there is one overwintering stage of downy mildew development that is not found on green tissue.

Overwintering

Oospores (resting bodies) are formed in late summer or autumn from the mycelium within leaves, shoots or berries (Figure 16). These resting bodies fall to the ground when leaves and bunch parts fall in autumn. There they overwinter in infected leaves and litter in the soil for 3-5 years (possibly up to 10 years).

Oospores are the sexual structures of downy mildew. An oospore has a thick wall that makes it less susceptible to fungicides and adverse weather conditions, such as exposure to the sun, than zoospores or sporangia (see below) (Figure 17).

Figure 16 Downy mildew oospores develop within grape leaves in Autumn. The leaf veins highlight the size of the oospores.

Figure 17 Close up of downy mildew oospores showing the double wall, making oospores resistant to fungicides and the weather.

Primary infection (soil to vine)

The 10:10:24 ‘rule of thumb’ refers to the conditions required for primary (first) infection to occur. At least 10mm rainfall (and irrigation) is required while the temperature is 10°C or more over a 24-hour period. Not all 10:10:24 conditions are suitable for a primary infection but this 'rule of thumb' provides a guide to monitor for favourable primary infection conditions when no other options are available.

More specifically the conditions required for oospores to germinate are:

• soil needs to be wet for at least 16 hours;
• usually achieved by 3-5mm rainfall (and/or irrigation); and
• temperature also needs to remain above 10°C.

The germinated oospores then release zoospores (that swim in free water) which then need to be splashed by rain or irrigation to the vine canopy before the end of the 24 hour period. This process usually requires another 3-5mm of rain (and/or irrigation) to ensure sufficient splash and leaf wetness for infection on the underside of the leaves. For this, the foliage must remain wet for at least 2-3 hours at 20°C (or 4-5 hours at 10°C) for the spores to infect the leaf and complete the primary infection cycle.

Soil moisture levels are important for the germination of oospores. A combination of rainfall and irrigation together can be sufficient for germination to occur.

Oil spots

The zoospores released during primary infection that establish on the underside of the leaf begin to grow hyphae. These hyphae grow inside the leaves to form oilspots that appear 5-17 days (but more often 5-10 days) after infection has occurred. The development of oilspots is quickest in warm weather (18-27°C). At warmer or cooler temperatures the incubation period is longer.

Primary infection levels are usually low with only 1-3 oilspots developing per 50m of vine row per germination event. Hence, primary infections are very difficult to find if only a few germination events have occurred at the beginning of the season.

Secondary infection (leaf to: leaf, shoot, inflorescence, berries, stalk)

Active oilspots need to be present before secondary infections can occur. These oilspots (and surfaces of other diseased tissue) produce sporangia (seen as white down) on suitable warm wet nights. Oilspots are known to survive the high summer temperatures in WA. During these periods the oil spot may remain inactive, by not producing sporangia, but when conditions become suitable again sporulation will resume.

Sporulation requires at least four hours of darkness to develop, during which time the temperature is 13°C or more and humidity is 98% or more.

To then cause infection the foliage must also be wet for at least 2-3 hours once sporulation has occurred. The wet foliage can be the result of rainfall, overhead irrigation or occasionally from heavy dew.

Secondary infections can occur any time during the growing season whenever oil spots are present and conditions are favourable.

Sporangia are the asexual structures of downy mildew. Sporangia are produced on tiny treelike structures known as sporangiophores. A single sporangium can in turn produce between 1-10 zoospores. Zoospores are able to move (swim) through water but are spread mainly by wind and rainsplash.

Spread

Primary infections begin the disease cycle by providing a source of oil spots. Overseas experience has shown that primary infection events occur throughout the growing season and are important for disease development and spread.

Secondary infections produce spores that can be spread by wind and rain to establish new infection sites within the local vicinity of the primary oil spot. Secondary infections can drive the disease to epidemic levels rapidly if conditions are favourable.

Monitoring

Good management of downy mildew is dependent on good monitoring of favourable weather conditions for primary and secondary infection events and of disease progress in the vineyard.

Automatic weather stations can be used for monitoring and predicting weather events. Weather stations collect information on temperature, rainfall, leaf wetness and humidity (suitable for downy mildew) and process the data for the likelihood of a primary or secondary infection event. Alternatively a maximum/minimum thermometer and a rain gauge can be used as a guide.

Monitoring in the vineyard is important to confirm possible infections from weather events. Physical examination can confirm whether a pre-infection or post-infection fungicide is required and will also indicate if spray coverage was effective after an event.

Monitoring in the vineyard should occur every 7-14 days when weather conditions are favourable from 3-4 weeks after budburst (that is, when shoots are approximately 10cm long).

Some general guidelines on vineyard monitoring include:

• Vines to be monitored should be representative of the block being assessed.
• Monitor for possible source areas of downy mildew infection such as wetter more sheltered parts of the vineyard (for example, near windbreaks and sheds), vines with dense canopies or areas that have previously been infected.
• Inspect both sides of 200 vines by scanning the foliage between mid-morning and mid-afternoon.
• Spend about 30 seconds per vine. Less time will be required early in the season when vine canopies are small.
• Focus on the canopy near the ground in lower lying areas where the soil may remain wet for extended periods.
  • The disease is difficult to detect in dense shaded canopies. The foliage may need to be parted to scan the inner leaves.
  • Use a hand lens to check suspect spots on leaves for evidence of the fungus.
  • Take samples of suspect leaves and bunches and conduct a bag test (The bag test for downy mildew of grapes).
  • Tag suspect sample sites so they may become the reference points if the sample is confirmed positive.

Monitoring for downy mildew can be combined with monitoring for other pests (for example, diseases and insects).

Note: Other fungi may infect leaves and bunches. Downy mildew spores are white and raised. Powdery mildew spores are ash-grey and are a uniform thin layer. Botrytis spores are cream to grey-brown. Penicillium, Aspergillus or Rhizopus are white and raised but change to green, blue, black or brown. If there is any uncertainty suspect samples may be submitted to a pathology laboratory for confirmation DDLS - Plant pathology services.

Oilspots that have already developed 'white down' on their underside (whether this fungal growth is fresh or old) may not develop any more 'white down' even after a bag test is conducted.

Management options

Cultural

Oospores may spread from property to property and region to region by the movement of infected leaves and litter in the soil or on vines in late summer and autumn. Avoid distribution of infected soil and plant matter by equipment and machinery (for example, mechanical harvesters, leaf pluckers, trimmers and utilities), by soil still adhered to rootlings, or by potted vines from nurseries.

Sporangia may spread across property boundaries by wind. It is very difficult to prevent the spread of these spores. It is believed that sporangia rarely spread more than 200m by wind.

Canopy management practices that encourage air movement will help to dry out leaves and improve sunlight and spray penetration. This will help to prevent infection. Such practices include:

• lower planting density;
• trellising and pruning to open the canopy;
• shoot training to open the canopy;
• vine trimming and hedging;
• lateral shoot thinning; and
• leaf plucking.

Vegetative growth may also be managed by the selection of appropriate rootstocks prior to planting and by careful application of fertilisers (for example, nitrogen). Excessive growth leads to dense shaded canopies that may encourage the development of downy mildew.

Chemical

All chemicals registered for use against downy mildew of grapes in WA are listed in the department's Viticulture Spray Guide. Consultation with the Australian Wine Research Institute (AWRI) 'Dog Book' should occur prior to use of chemicals for any further application requirements.

Pre-infection fungicides (applied as close as possible but prior to an infection event)

Pre-infection (protectant) fungicides help to prevent downy mildew zoospores from entering the green vine tissue. Spray coverage needs to be excellent to adequately protect all of this green tissue. In particular they need to be applied to the underside of leaves and the back of bunches. It is important to time their application as close as possible but prior to the possible infection event (for example, when possible primary or secondary weather events are forecast).

A pre-infection spray program tends to be used where downy mildew is well established in a region or vineyard and occurs frequently. Growers who are unable to conduct careful monitoring tend to use a pre-infection spray program. Large vineyards and table grape growers tend also to use pre-infection spray programs or those with soils which make access limited after a rain event, delaying post-infection spray application.

Pre-infection fungicides have limited movement from the areas where they are deposited and any new growth after the spray has been applied will not be protected. Rain and overhead irrigation will dilute or may wash the protectant sprays off the vine. Hence, further applications will be required before the next possible infection event.

A pre-infection spray program often requires application on a 7-14 day schedule. This may be expanded to a 21 day program later in the season as shoot growth slows and possible infection events are less. As flowering is the critical period to prevent crop loss, the spray program may need to be tightened to every 5-7 days to coincide with possible infection events.

(Note: In table grapes pre-infection fungicides should only be used provided it is prior to 10mm berry size as spray residues on berries may occur).

Post-infection fungicides (applied as soon as possible after an infection event)

Post-infection (eradicant) fungicides are systemic and penetrate the vine tissue killing the downy mildew fungus from within the vine tissue. Use of these fungicides involves withholding sprays until an infection event has occurred. Relying on post-infection fungicides requires careful monitoring and has a greater risk of downy mildew becoming established. However, if downy mildew is not established in the region or vineyard and few possible infection events occur it has the advantage of using fewer sprays that have greater effectiveness.

Post-infection fungicides work best when applied as soon as possible after an infection event - within five days of infection and before oilspots appear. No additional spraying should be required until weather conditions favour another possible infection event. In this situation, pre-infection fungicides may be used once again.

Once the fungus is visible it is difficult to kill. A single post-infection spray is usually not effective, although it may reduce the number of spores and limit spread of the disease. Hence, follow up sprays of post-infection fungicides may be required after the initial post-infection fungicide spray.

(Note: In table grapes not all post-infection fungicides can be used after 10mm berry size for the control of downy mildew. Contact your nearest Department of Primary Industries and Regional Development office or consultant for further information).

Did the post-infection fungicide work? Use The bag test for downy mildew of grapes to check whether the post-infection fungicide was effective. Conduct the tests 24 hours and then, if required, another three days after spraying with samples of healthy bunches, limp and browning bunches and leaves with oilspots. Keep each sample in a separate moist bag.

Consider immediate repeat spraying with a post-infection fungicide if fresh white down is evident on the underside of the leaf or on bunches of these samples when inspected the next morning.

Post-infection products should be used after an infection event if there is any concern that the pre-infection fungicides applied were not adequate. Browning bunches may be too badly infected for the post-infection fungicide to prevent crop loss and only normal looking bunches will benefit from the spray.

Fungicide resistance

Due to the ability of Plasmopara viticola to sexually reproduce within Australian vineyards and the use of eradicant fungicides for control it is considered high risk for the development of resistance. Fungicide resistance has been detected in WA for this disease. 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 downy mildew produced by Croplife Australia. Always read the chemical label thoroughly prior to use.