DOwny Mildew
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Beginner
Scientific Name
Plasmopara viticola
Identification
Leaves:
- Roughly circular yellowish discolourations, called “oil spots”
- White fluffy growth primarily on the lower leaf surface
- As lesions age, they turn brown from the centre outward
- Severely infected leaves may drop
- Infected shoot tips curl (“shepherd’s crook”)
- Covered with white fluffy sporulation
- Severe infections will cause the rachis and cluster to twist like a cork-screw
- Entire surface can be covered with sporulation
- White fluffy sporulation when shot-size
- May shrivel and drop off
- Berries of red cultivars infected between 3 and 5 mm in diameter size will turn colour prematurely
- Those of white cultivars acquire a mottled appearance
- Stay hard when healthy berries start to soften at veraison
Often Confused With
Powdery Mildew – dusty appearance on upper and lower surfaces of leaves and on berries
Erineum mite – raised blisters on the upper surface of the leaf, no oil spots, dense hairs on the underside below raised blisters
Early herbicide injury – tissue quickly turns brown and no sporulation on underside
Black rot- tan spots on leaves with black pynicdia
Uneven greening of leaves during periods of rapid growth in spring – indistinct margins and no sporulation on underside
Grape berry moth – infested berries turn colour prematurely but are hollowed out and filled with frass
Botrytis bunch rot on young berries or injured tissues – infected tissues produce tan-coloured conidia borne on dark conidiophores
Veraison: berries change colour but also get softer
Period of Activity
Some oospores are mature and ready to release sporangia by bud break but maturation of others continues throughout the growing season. The first infections are usually not detected until some time after bloom unless there are frequent warm rains before bloom and a lot of inoculum from previous years. New infections can occur throughout the growing season as long there is new green tissue being produced and conditions are favourable. Leaves are most susceptible to infection until they are fully expanded; infections that occur after this time tend not to develop as rapidly or extensively. Berries are susceptible to infection from bloom through 4 weeks post-bloom.
Scouting Notes
Give priority to areas of the vineyard that remain wet: those with heavy soil, poor drainage, low spots or dense foliage. Downy mildew tends to develop on the shadiest (east or north) side of the canopy first. Monitor for the appearance of oil spots on the leaves with a white, fluffy sporulation that can be removed with the fingers on the lower leaf surface. Check for infected shoot tips distorted in a “shepherd’s crook” that become white with sporulation. To confirm downy mildew infection if sporulation is not present, enclose the tissue in a plastic bag containing a damp paper towel overnight. White fluffy growth will be evident if symptoms are caused by downy mildew.
Threshold
There is no threshold for initiating downy mildew control. Fungicides should be applied before infection occurs.
Advanced
Scientific Name
Plasmopara viticola
Identification
Leaves:
- Roughly circular yellowish discolourations, called “oil spots”
- White fluffy growth primarily on the lower leaf surface
- As lesions age, they turn brown from the centre outward
- Severely infected leaves may drop
- Infected shoot tips curl (“shepherd’s crook”)
- Covered with white fluffy sporulation
- Severe infections will cause the rachis and cluster to twist like a cork-screw
- Entire surface can be covered with sporulation
- White fluffy sporulation when shot-size
- May shrivel and drop off
- Berries of red cultivars infected between 3 and 5 mm in diameter size will turn colour prematurely
- Those of white cultivars acquire a mottled appearance
- Stay hard when healthy berries start to soften at veraison
Often Confused With
Powdery Mildew – dusty appearance on upper and lower surfaces of leaves and on berries
Erineum mite – raised blisters on the upper surface of the leaf, no oil spots, dense hairs on the underside below raised blisters
Early herbicide injury – tissue quickly turns brown and no sporulation on underside
Black rot- tan spots on leaves with black pynicdia
Uneven greening of leaves during periods of rapid growth in spring – indistinct margins and no sporulation on underside
Grape berry moth – infested berries turn colour prematurely but are hollowed out and filled with frass
Botrytis bunch rot on young berries or injured tissues – infected tissues produce tan-coloured conidia borne on dark conidiophores
Veraison: berries change colour but also get softer
Biology
The causal fungus, Plasmopara viticola overwinters as thick-walled oospores in infected tissues and in the soil. In the spring, the minimum temperature for the development of the fungus is 11°C. A minimum of 10 mm of rain is required for oospores to produce sporangia that are dispersed by rain splash to young green tissue. The sporangia release motile zoospores that swim to the stomates, germinate and infect tissues. The incubation period (the time from infection to appearance of symptoms) can be from 5 to 21 days depending on temperature with the fastest development at 20-25ºC. As tissues reach maturity, the incubation is extended. Under high humidity, new sporangia are produced on lesions on the underside of the leaf or on berries. Each foliar lesion has the potential to produce up to 3 cycles of sporangia before it starts to die off and becomes inactive. Disease development is most rapid between 20 º C and 25 ° C with frequent rain or dew.
Period of Activity
Some oospores are mature and ready to release sporangia by bud break but maturation of others continues throughout the growing season. The first infections are usually not detected until some time after bloom unless there are frequent warm rains before bloom and a lot of inoculum from previous years. New infections can occur throughout the growing season as long there is new green tissue being produced and conditions are favourable. Leaves are most susceptible to infection until they are fully expanded; infections that occur after this time tend not to develop as rapidly or extensively. Berries are susceptible to infection from bloom through 4 weeks post-bloom.
Scouting Notes
Give priority to areas of the vineyard that remain wet: those with heavy soil, poor drainage, low spots or dense foliage. Downy mildew tends to develop on the shadiest (east or north) side of the canopy first. Monitor for the appearance of oil spots on the leaves with a white, fluffy sporulation that can be removed with the fingers on the lower leaf surface. Check for infected shoot tips distorted in a “shepherd’s crook” that become white with sporulation. To confirm downy mildew infection if sporulation is not present, enclose the tissue in a plastic bag containing a damp paper towel overnight. White fluffy growth will be evident if symptoms are caused by downy mildew.
Threshold
There is no threshold for initiating downy mildew control. Fungicides should be applied before infection occurs.
Management Notes
Selection of a site with good air drainage, north-south orientation of the rows to maximize sun exposure, and practices that open the canopy so that air circulation and spray coverage are improved can help control of downy mildew.
Pruning, training and leaf-removal to reduce shading and allow rapid air movement can reduce the frequency and duration of periods of high humidity which favour the disease.
Good weed control and proper soil drainage will reduce relative humidity and the amount of free moisture which may increase the spread of the fungus.
Management with fungicides – Fungicides are used to control downy mildew in most commercial vineyards. See OMAFRA Publication 360, Guide to Fruit Production - Chapter 5 Grapes (PDF): Recommendations for downy mildew at Shoot length 20-25 cm, Trace bloom, Immediate post-bloom to early fruit set, Berries pea-sized, Berry touch to cluster closure and Beginning of ripening (veraison) through harvest. If downy mildew was a problem the previous year, an application of fungicides at 10-15 cm shoot length is advisable.
