Powdery mildew:
another perspective
Kathy Evans
Tas. Institute of Agricultural Research
University of Tasmania
email: [email protected]
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
– cloudy weather
– infection frequency high at 85% RH
• Short generation times at 20-30oC
Early season in cool climates
19th Oct.
EL-12
8th Nov.
EL-15, shoot
elongating rapidly
24th Nov.
EL-17
Chardonnay – no sprays
berries 4-7 mm
caps off: 10% 80%
100
90
80
70
Mean 60
50
Incidence 40
(%) 30
10% 80%
caps off caps off
100
90
80
Mean (%)
70
60
Incidence leaves (abaxial)
Incidence - bunches
Severity - bunches
50
40
30
20
10
8-Jan
1-Jan
5-Feb
4-Dec
29-Jan
22-Jan
15-Jan
19-Feb
12-Feb
27-Nov
25-Dec
18-Dec
11-Dec
0
Date 2004/2005
20
10
Date 2004/2005
Flagshoots found Oct 22, Nov. 9
Leaves
Grape clusters
southern Tasmania
19-Feb
12-Feb
5-Feb
29-Jan
22-Jan
15-Jan
8-Jan
1-Jan
25-Dec
18-Dec
11-Dec
4-Dec
27-Nov
0
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
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
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
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
• Hobart, Tas: 21 days
Early season in cool climates
19th Oct.
EL-12
Slow rates of
leaf emergence
in month after
budburst
8th Nov.
EL-15, shoot
elongating rapidly
24th Nov.
EL-17
Spray interval & shoot growth
• Very small increase in leaf area in very cool
weeks
– Fungicide effective until it decays
• 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
flowering
imminent
4-5 leaves,
inflorescence clear
berries
4 mm
80-100%
caps off
24
Plastochron index
20
16
12
8
4
0
0
50
100
150
200
250
300
350
400
Degree days above 10 C
Distal shoot
Medial shoot
Proximal shoot
Source: Angela Smith, TIAR PhD student
450
Tissue susceptibility
Which grapevine tissues are
highly susceptible to powdery
mildew and what conditions
make tissues more susceptible?
Leaf mildew
Age-related resistance
Inoculate healthy shoot
and see which leaf has
the most powdery mildew
Leaf with most mildew
Conditions for shoot
development matter
40
25oC
Rate of leaf emergence
2 x faster at 25oC than 18oC
30
Disease severity (%)
20
10
0
0
2
4
6
8
10
12
14
16
18
40
18oC
30
Higher rate of leaf emergence:
> proportion mildewed leaves
> severity for leaf with max.
mildew
20
10
Leaf position 1 ≥ 30 mm length
0
0
2
4
6
8
10
Leaf position
12
14
16
18
Spots
per cm2
2.3
Tip end of cane
14.3
11.6
7.3
2.3
Illustration of
leaf age-related
disease resistance
Rust infecting
weedy blackberry
1.0
0.3
Base of cane
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
Age-related resistance
Berries become increasingly resistant to
mildew infection from 10 days after fruit set
Berry skin with diffuse colonisation
Riesling
Mildew-free
Photos courtesy of D. Gadoury,
Cornell University
Diffuse infection
Cap fall asynchronous
second
basal
Photo courtesy of
D. Gadoury,
Cornell University
Chardonnay 2005, flowering = 21 days
12-Dec
11-Dec
10-Dec
9-Dec
8-Dec
7-Dec
20
6-Dec
5-Dec
4-Dec
3-Dec
2-Dec
1-Dec
30-Nov
29-Nov
28-Nov
27-Nov
26-Nov
25-Nov
24-Nov
23-Nov
Capfall
(%)
22-Nov
Capfall asynchronous
100
80
60
40
12 bunches
on 1 vine
0
Chardonnay – no sprays
some softening
berries: 4 mm 7 mm
100
90
80
70
60
50
10% 80%
caps off caps off
100
90
80
Mean (%)
70
60
Incidence leaves (abaxial)
Incidence - bunches
Severity - bunches
50
40
30
20
10
8-Jan
1-Jan
5-Feb
4-Dec
29-Jan
22-Jan
15-Jan
19-Feb
12-Feb
27-Nov
25-Dec
18-Dec
11-Dec
0
Date 2004/2005
40
30
20
10
Date 2004/2005
Bunch incidence
Bunch severity
southern Tasmania
17-Feb
10-Feb
3-Feb
27-Jan
20-Jan
13-Jan
6-Jan
30-Dec
23-Dec
16-Dec
0
9-Dec
Mean
(%)
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
set
Save your best materials for applications
pre-flowering to fruit set