Grape Powdery Mildew

Dan O’Gorman: Agriculture and Agri-Food Canada, Summerland Research and Development Centre (SuRDC)

• Biology

• Symptoms

• Disease Cycle

• Weather condition

• Control

Grape Powdery Mildew

P. Sholberg

• Most common and easily recognized plant diseases

• Powdery mildews are usually very host specific

• Requires living tissue to grow

• Colonizes the surface of plant tissue and invade top layers of epidermis cells

• Powdery mildews cause extensive losses in plant growth and crop yield

Grape Powdery Mildew

Two stages to powdery mildew infections: Primary and Secondary infections

Grape Powdery Mildew

https://www.agric.wa.gov.au/spring/powdery-mildew-grapevines-western-australia?page=0%2C2

https://www.agric.wa.gov.au/spring/powdery-mildew-grapevines-western-australia?page=0%2C2

P. Sholberg

P. Sholberg

Primary infection: caused by ascospores released in spring by overwintering structures called cleistothecia

Secondary infection: caused by conidia spores which can infect, develop new conidia and re-infect in a continuous cycle through the summer

Secondary infection cycles approx. every 5 to 30 days depending on the weather

Cleistothecia

Photo: Peter Sholberg AAFC

Photo: Peter Sholberg AAFC

Genera of Powdery Mildew Fungi (Drawing by C.B. Kenaga, E.B. Williams, and R.J. Green)

• Powdery Mildew infection (primary and secondary)

• Haustoria are specialized hyphae for nutrient absorption from the plant cells. • The plant is damaged by the loss of nutrients to the fungus, disruption of photosynthesis, and premature death of leaves or other infected plant tissues

https://www.agric.wa.gov.au/spring/powdery-mildew-grapevines-western-australia?page=0%2C2

Symptoms

• Observed in all “succulent parts” of the grapevine

Peter Sholberg AAFC

• Symptoms can also be observed in late-summer, fall and winter as dark blotchy areas on dormant canes

http://seresinestate.blogspot.ca/2010_10_01_archive.html

Peter Sholberg AAFC

Symptoms

• Powdery mildew affects fruit causing russeting, splitting and rotting

Peter Sholberg AAFC Peter. Sholberg, AAFC

Ascospores will colonize any fresh green tissue

Grape Powdery Mildew - Primary Infection (spring)

First signs of the disease can be observed after bud-break (early spring).

http://seresinestate.blogspot.ca/2010_10_01_archive.html

Grape Powdery Mildew- Secondary Infections (summer)

Peter Sholberg, AAFC

Mildew Disease Cycle Primary infection

period

Secondary Infection period Grapevine powdery mildew life cycle. Drawing by R. Sticht. Reproduced with permission from

Compendium of Grape Diseases, 1988, American Phytopathological Society, St. Paul, MN. USA.

Effect of Temperature on the Development of Grape Powdery Mildew

Temperature of leaf*

(oC)

Days for spores to develop and infect vine parts and

produce new spores

6 32

9 25

12 18

15 11

17 7

23 6

26 5

30 6

33 oC for at least 3 days X - but 10% of spores can

recover in 5 days

40.5 oC for at least 6 hrs X - kills the fungus

• Primary infection in the spring requires leaf wetness and temperature

• Secondary infection in mid/late season does not require free moisture BC Ministry of Agriculture (http://www.agf.gov.bc.ca/cropprot/grapeipm)

20-30°C Optimum

> 33°C Stops germination

> 40°C Spores die

Secondary Infection

> 10°C 12-15 hrs continuous wetness

Primary Infection

temp & rain

Okanagan Weather 2016

Primary infection in the spring requires leaf wetness and temperature

1. Ascospores are released in the spring after a rainfall of at least 2.5 mm

2. Infection requires at least 12-15 hours of continuous wetness at 10-15 °C

> 10°C 12-15 hrs continuous wetness

0

10

20

30

40

50

60

70

80

March April May June July

ave (⁰C)

rain (mm)

https://weather.gc.ca/city/pages/bc-84_metric_e.html

Primary Infection

temp & rain

Okanagan Weather: March 2016

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7 8

Mean Temp Definition °C

Total Rain Definition mm

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7 8

Mean Temp Definition °C

Total Rain Definition mm

> 10°C 12-15 hrs continuous wetness

http://climate.weather.gc.ca/climate_data/generate_chart_e.html

Primary Infection

temp & rain

Okanagan Weather: April 2016

> 10°C 12-15 hrs continuous wetness

http://climate.weather.gc.ca/climate_data/generate_chart_e.html

Primary Infection

temp & rain

Okanagan Weather: May 2016

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7

Mean Temp Definition °C

Total Rain Definition mm

> 10°C 12-15 hrs continuous wetness

Primary Infection

temp & rain

http://climate.weather.gc.ca/climate_data/generate_chart_e.html

Okanagan Weather 2016

Secondary infection in mid/late season does not require free moisture

• 20-29°C – optimum fungal growth (infection cycle ~5-6 days) • > 33°C –(3 days) stops spore germination (~10% of spores can recover) • > 40°C – (for at least 6 hrs) fungus is killed

Secondary Infection

20-30°C Optimum

> 33°C Stops germination

> 40°C Spores die

0

5

10

15

20

25

30

35

40

March April May June July

high (⁰C)

ave (⁰C)

http://climate.weather.gc.ca/climate_data/generate_chart_e.html

Disease Management

Cultural Control

• Remove lateral shoots in dense canopies/remove leaves in fruit zone: low light intensity favours powdery mildew development.

• Use drip or micro-jet irrigation system

• Control vigour: excessive vigour can lead to higher disease pressure

• Variety selection: select varieties that are less susceptible to

mildew

Susceptibility of Grape Cultivars to Powdery Mildew

Susceptible Intermediate Least Susceptible

Bacchus Cabernet Franc

Cabernet Sauvignon Chancellor Chardonnay Chasselas

Gamay Gewurztraminer

Grenache Himrod

Madeleine Angevine Madeleine Sylvaner

Malbec Petit Verdot Rkatzeteli

Riesling Sauvignon blanc

Schonburger Siegerebe

Syrah Viognier

Chelois Chenin Blanc

Concord Foch

Pinot blanc Malbec Merlot Ortega

Pinot Noir Perlett

Sheridan Vidal Blanc

Weissburgunder

Auxerrois Malvoisie

Melon Pinot Gris Semillon

BC Ministry of Agriculture (http://www.agf.gov.bc.ca/cropprot/grapeipm)

Chemical control

• Sulfur can be used exclusively, or in combination with DMI, strobilurin, or other classes of chemicals

• Alternate between different fungicide groups to avoid fungicide resistance problems

• Products like summer oils have also been shown to be good at suppressing mildew

Disease Management

://www.pinterest.com/anteekvintage/farm-collectibles/& http://ernestartist.org/June17_2011_02.htm

• Protect grape foliage from primary infection by application of lime sulfur (dormant spray will kill the over-wintering structures) and fungicides from early shoot growth until after bloom.

• Good control early in the season to prevent establishment of the disease is key to preventing a powdery mildew epidemic later in the summer.

Key timing for Powdery Mildew infection (and spray applications):

Disease Management

Dormant – New growth (5-10 cm) – Bloom (pre or post) –> –> –>

For more information on the UC Davis powdery mildew forecasting models: http://www.ipm.ucdavis.edu/DISEASE/DATABASE/grapepowderymildew.html

Powdery Mildew Disease Forecasting

• University of California, Davis model is the most widely available mildew model

• Can be used to help time fungicide applications

• Works well in California and has helped reduce the number of fungicide applications on grapes

• Unfortunately the UC Davis model has limitations in British Columbia

- The model only works for predicting the secondary infections in BC

• There are two parts to the model to align with the two different infection periods of the pathogen (primary and secondary infections)