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Powdery mildew:

another perspective

Kathy Evans

Tas. Institute of Agricultural Research

University of Tasmania

email: Katherine.Evans@utas.edu.au

International workshop

The research issues (powdery mildew)

• breeding vines to resist disease

• identifying susceptible tissues

– environment/viticulture influencing susceptibility

• basic pathogen biology

– How are spores produced? Inhibit spores?

• predicting disease risk

• novel control methods

This presentation

1. Observations from cool climates

2. Understanding grapevine susceptibility

3. Improving spray timing

Conducive conditions

• Young, green tissues

– new leaves, laterals, green floral parts, berries

• Low UV and high humidity

– dense and/or shaded canopies – dense and/or shaded canopies

– cloudy weather

– infection frequency high at 85% RH

• Short generation times at 20-30oC

Early season in cool climates

19th Oct.19th Oct.

EL-12

24th Nov.

EL-17

8th Nov.

EL-15, shoot

elongating rapidly

5 06 0

7 08 0

9 010 0

an(%) Inc idenceleaves(aInc idence-bunches

50

60

70

80

90

100

10%caps off: 80%

Mean

Chardonnay – no sprays

berries 4-7 mm

0

1 02 0

3 04 0

27-Nov4-Dec11-Dec 18-Dec25-Dec1-Jan 8-Jan15-Jan 22-Jan29-Jan5-Feb 12-Feb19-Feb

Dat e2004/2005

Mean(%)

Se verity-bunches

0

10

20

30

40

50

27

-No

v

4-D

ec

11

-De

c

18

-De

c

25

-De

c

1-J

an

8-J

an

15

-Ja

n

22

-Ja

n

29

-Ja

n

5-F

eb

12

-Fe

b

19

-Fe

b

Date 2004/2005

Leaves Grape clusters

Incidence

(%)

southern TasmaniaFlagshoots found Oct 22, Nov. 9

Leaf mildew

Latent period longer when cool

According to Delp (1954):

Time between spore germination and spore production.

Constant Temperature (oC)

Latent Period (days)

7 32

9 259 25

12 12

15 11

17 7

23 6

26 5

30 6

Latent period longer in vineyard

Research in New York State (Cornell Uni):• Chardonnay leaves inoculated 3 weeks after budburst

• Latent periods longer than expected

Year Average Latent Period (days)

Predicted Observed

2008 7.5 13

2009 7 9

Effect of low temperatures

• Leaves exposed for ≥ 2 h to 2-8oC had

smaller mildew colonies when inoculated

after ‘cold’ treatment

• Mortality of hyphae in young mildew • Mortality of hyphae in young mildew

colonies exposed to 2oC for 8 h

• Longer latent periods explained by

negative impact of low temperatures

Cool night temperatures

• Cool night temperatures may be lowering

mildew risk

• Radiational cooling of the leaf surface can

lead to a drastic difference from ambient air lead to a drastic difference from ambient air

temperature

Cool night temperatures

In 2005-2007, the number of days between

budburst and flowering with a minimum

temperature of 6oC was:

• Loxton, SA: 17 days• Loxton, SA: 17 days

• Hobart, Tas: 21 days

Early season in cool climates

19th Oct.19th Oct.

EL-12

24th Nov.

EL-17

8th Nov.

EL-15, shoot

elongating rapidly

Slow rates of

leaf emergence

in month after

budburst

Spray interval & shoot growth

• Very small increase in leaf area in very cool

weeks

– Fungicide effective until it decays

• Rapid leaf emergence late spring • Rapid leaf emergence late spring

– Leaves unprotected by fungicide until next spray

• Use number of leaves emerged since the

last spray to time sprays?

Chardonnay on VSP

16

20

24

ron

ind

ex

80-100%caps off

flowering imminent

4-5 leaves, inflorescence clear

berries4 mm

0

4

8

12

0 50 100 150 200 250 300 350 400 450

Degree days above 10 C

Pla

sto

ch

r

Distal shoot Medial shoot Proximal shoot

Source: Angela Smith, TIAR PhD student

Which grapevine tissues are highly susceptible to powdery mildew and what conditions

make tissues more susceptible?

Tissue susceptibility

Leaf mildew

Age-related resistance

Inoculate healthy shoot

and see which leaf has and see which leaf has

the most powdery mildew

Leaf with most mildew

Conditions for shoot

development matter

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18

Dis

ease s

everity

(%

)

25oCRate of leaf emergence

2 x faster at 25oC than 18oC

Higher rate of leaf emergence:

> proportion mildewed leaves

> severity for leaf with max. 0 2 4 6 8 10 12 14 16 18

0

10

20

30

40

0 2 4 6 8 10 12 14 16 18

18oC

Dis

ease s

everity

(%

)

Leaf position

> severity for leaf with max.

mildew

Leaf position 1 ≥ 30 mm length

2.3

14.3

11.6

Spotsper cm2

Tip end of cane

Illustration of

leaf age-related

disease resistance7.3

2.3

1.0

0.3 Base of cane

disease resistance

Rust infecting

weedy blackberry

Key concept

Environment during shoot development will

affect the proportion & position of leaves with

powdery mildew

Timing important too

French researchers suggest:

Bunch mildew more severe with early leaf

infection as the leaf with most mildew will

be closer to the bunch

Leaf with most mildew

Bunch

Resistance in berries

• Berries become increasingly resistant to mildew infection from 10 days after fruit set

RieslingBerry skin with diffuse colonisation

Age-related resistance

Mildew-free Diffuse infection

Photos courtesy of D. Gadoury,

Cornell University

basalsecond

Cap fall asynchronous

basal

Photo courtesy of

D. Gadoury,

Cornell University

Capfall asynchronous

60

80

100

Capfall(%)

Chardonnay 2005, flowering = 21 days

0

20

40

22

-No

v

23

-No

v

24

-No

v

25

-No

v

26

-No

v

27

-No

v

28

-No

v

29

-No

v

30

-No

v

1-D

ec

2-D

ec

3-D

ec

4-D

ec

5-D

ec

6-D

ec

7-D

ec

8-D

ec

9-D

ec

10

-De

c

11

-De

c

12

-De

c

12 buncheson 1 vine

5 06 0

7 08 0

9 010 0

an(%) Inc idenceleaves(aInc idence-bunches

Chardonnay – no sprays

50

60

70

80

90

100

Mean

(%)

some softening7 mm4 mmberries:

0

1 02 0

3 04 0

27-Nov4-Dec11-Dec 18-Dec25-Dec1-Jan 8-Jan15-Jan 22-Jan29-Jan5-Feb 12-Feb19-Feb

Dat e2004/2005

Mean(%)

Se verity-bunches

southern Tasmania

0

10

20

30

40

9-D

ec

16

-De

c

23

-De

c

30

-De

c

6-J

an

13

-Ja

n

20

-Ja

n

27

-Ja

n

3-F

eb

10

-Fe

b

17

-Fe

b

Date 2004/2005

Bunch incidence Bunch severity

(%)

Risk index from Germany

Risk index = climate index x ‘berry resistance’ index

(W. Kast et al.)

Take home message

Spray intervals should be kept tight

• During rapid shoot growth and weather

conducive to disease

• Pre-flowering, during flowering, early fruit • Pre-flowering, during flowering, early fruit

set

Save your best materials for applications

pre-flowering to fruit set