powdery mildew
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Beginner
Scientific Name
Erysiphe necator (previously Uncinula necator)
Identification
On all tissues, powdery mildew looks like a greyish-white powder.
Leaves
- Frequently first found on the undersides of leaves
- Lesions become apparent on the upper sides of leaves as well
- Increase in size and number if the disease is left unchecked
- On rapidly growing leaves, infections on the underside may cause the leaves to appear puckered on top
- Severely infected leaves may become brittle and drop off
- Starting as early as late July, very small orange then brown and eventually black spherical structures (cleistothecia) develop on the upper and lower surfaces of infected leaves
Shoots:
- Initially greyish-white, develop into brown irregular blotches
- Up to a few cm
- Indistinct margins and remain visible after shoot hardening
- Severe infections make the rachis brittle
- Can result in clusters being dropped, especially if mechanical harvesting is done
- Symptoms on the rachis are similar to those on shoots
Berries:
- Become covered in conidia
- An initial floury appearance that later becomes dark and grey
- Dry out and may drop off
- Later infections (3-4 weeks post bloom) will have superficial greyish scarring but not a lot of mycelial growth or sporulation
Often Confused With
Downy mildew – usually associated with a yellow oil spot and sporulation on the underside of the leaf. On berries, the sporulation is more raised and in clumps.
Spray residue– easily visible, more dense than powdery
Hairs – uniformly distributed primarily on underside of leaves and shoot tips where present.
Period of Activity
Ascospores are mature and start to be released by bud break and are usually exhausted by bloom. Secondary infections can occur throughout the growing season as new leaves develop.
Scouting Notes
Give priority to sections of the vineyard that remain wet: heavy soil, poor drainage, low spots, dense foliage. Powdery mildew tends to develop on the shadiest (east) side of the canopy first. Most vinifera cultivars are very susceptible to powdery mildew. Refer to Publication 360 Table Relative Susceptiblity of Grape Cultivars to Diseases for more information regarding cultivar susceptibility. Monitor for the appearance of greyish-white spots on the upper and lower leaf surfaces starting at about 3-5 leaf growth stage, although lesions frequently aren’t found until after bloom. The spots are particularly visible when the leaf is held at approximately a 30° angle. Be sure to sample leaves from the interior of the canopy since disease development is favoured by shade.
Threshold
There is no threshold for initiating powdery mildew control. Fungicides should be applied before infection occurs, although some fungicides can eradicate superficial growth. See Table 6-6 for the activity of fungicides on grape diseases.
Advanced
Scientific Name
Erysiphe necator (previously Uncinula necator)
Identification
On all tissues, powdery mildew looks like a greyish-white powder.
Leaves
- Frequently first found on the undersides of leaves
- Lesions become apparent on the upper sides of leaves as well
- Increase in size and number if the disease is left unchecked
- On rapidly growing leaves, infections on the underside may cause the leaves to appear puckered on top
- Severely infected leaves may become brittle and drop off
- Starting as early as late July, very small orange then brown and eventually black spherical structures (cleistothecia) develop on the upper and lower surfaces of infected leaves
Shoots:
- Initially greyish-white, develop into brown irregular blotches
- Up to a few cm
- Indistinct margins and remain visible after shoot hardening
- Severe infections make the rachis brittle
- Can result in clusters being dropped, especially if mechanical harvesting is done
- Symptoms on the rachis are similar to those on shoots
Berries:
- Become covered in conidia
- An initial floury appearance that later becomes dark and grey
- Dry out and may drop off
- Later infections (3-4 weeks post bloom) will have superficial greyish scarring but not a lot of mycelial growth or sporulation
Often Confused With
Downy mildew – usually associated with a yellow oil spot and sporulation on the underside of the leaf. On berries, the sporulation is more raised and in clumps.
Spray residue – easily visible, more dense than powdery
Hairs – uniformly distributed primarily on underside of leaves and shoot tips where present.
Biology
In Ontario, E. necator overwinters as cleistothecia lodged in the bark of trunks. In the spring, the ascospores within the cleistothecia are focibly ejected with rain of 2-3 mm or more and temperatures of at least 10° C (optimum 20-25° C). These airborne ascospores infect the leaves and clusters growing close to the trunk and the fungus produces a superficial mat of mycelium and conidia. Conidia are produced within 5 -7 days under optimum temperatures (20-30° C). These wind-blown conidia germinate without free water to infect leaves, berries, shoots and rachises and produce another generation of sporulating lesions. This cycle repeats throughout the season as long as conditions are favourable. Leaves are most susceptible to infection until they are fully expanded; infections that occur on older leaves tend not to develop as extensively and are associated with small (1-2 mm diameter) necrotic areas where infection was limited. Berries are susceptible to infection from bloom through 4 weeks post-bloom in V. vinifera and about 2 weeks after fruit set in Concord. Berries infected during their final stage of susceptibility (near cluster closure) may develop injuries not visible to the naked eye which can allow infection by bunch-rotting organisms later in the season.
Since the majority of the fungus is outside host tissue, it is vulnerable to ultraviolet radiation. The cloudy conditions and high humidity that are associated with rainy periods are favourable to disease development. Shady canopies provide excellent protection for the fungus.
Period of Activity
Ascospores are mature and start to be released by bud break and are usually exhausted by bloom. Secondary infections can occur throughout the growing season as new leaves develop.
Scouting Notes
Give priority to sections of the vineyard that remain wet: heavy soil, poor drainage, low spots, dense foliage. Powdery mildew tends to develop on the shadiest (east) side of the canopy first. Most vinifera cultivars are very susceptible to powdery mildew. Refer to Publication 360 Table Relative Susceptiblity of Grape Cultivars to Diseases for more information regarding cultivar susceptibility. Monitor for the appearance of greyish-white spots on the upper and lower leaf surfaces starting at about 3-5 leaf growth stage, although lesions frequently aren’t found until after bloom. The spots are particularly visible when the leaf is held at approximately a 30° angle. Be sure to sample leaves from the interior of the canopy since disease development is favoured by shade.
Threshold
There is no threshold for initiating powdery mildew control. Fungicides should be applied before infection occurs, although some fungicides may eradicate superficial growth. See Table 5-6 for the activity of fungicdes on grape diseases.
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 powdery 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 powdery mildew in commercial vineyards. See OMAFRA Publication 360, Guide to Fruit Production - Chapter 5 Grapes (PDF): Recommendations for powdery mildew at 3-5 leaves unfolded, 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.
Different cultivars all have different susceptibilities to diseases. See Table 5-5 Relative Susceptibility of Grape Cultivars to Diseases.
Fungicides have different modes of action, and differ in their activity on grape diseases. See Table 5-6 Activity of fungicides on grape diseases.
