Pecan scab managementClive Bock, USDA-ARS-SEFTNRL, Byron, GA
Structure of presentation Describe the disease and pathogen Major considerations to manage pecan scab
Cultivars Air-blast sprayers and sprayer set-up Spraying options (volutes, aerial) Tree size (tree management/mechanical hedge-pruning/spacing) Timing of sprays (pre or post pollination) Chemistry (conventional and organic)
2/3
1/3
Upper half of canopy
Lower half of canopy
Pecan scab (Venturia effusa)Lifecycle of Venturia effusa, cause of pecan scab
Autumn
Winter
Summer
Spring
Fungus becomes dormant as stroma and
overwintering conidia (twigs and shucks)
Epidemics build up on
young leaves (conidia)
Overwinters as stroma and conidia
Epidemics build up on
fruit (conidia)
Polycyclic disease (rain and wind)
The sexual stage of this fungus has been identified (Drs. Young and Charlton, Noble Research Institute).
The role of the spores produced by the sexual stage are unknown but may play an important role in disease development.
Fungicide applications to
control scab
Late August to early September
Mid-late April
Cultivars, tree size, and appropriate equipment
If planting new in the southeastern region endeavor to obtain scab resistant cultivars (Excel, Elliott, Avalon,….). Availability?
Susceptible cultivars will require fungicide sprays in most or all seasons/locations (and they are very widely grown)
It is possible to spray small trees with less powerful sprayers (<25-30 ft)
Once tree height exceeds ~25 ft, a full sized orchard sprayer is a necessity
Sprayer set-up and coverage
Sumner, P.E. 2004. Experiences with Pecan Air Blast Sprayers. Presented at The 2004 American Society of Agricultural Engineers/Canadian Society of Agricultural Engineers Annual International Meeting, Fairmont Chateau Laurier, The Westin, Government Centre, Ottawa, Ontario, Canada, 1 - 4 August 2004.
⅔ to ¾ of spray directed to upper ½ of the canopy Adjust the vanes/sprayer nozzles accordingly Even with perfectly adjusted vanes, the velocity of
the air declines with distance from the sprayer Volume can be determined based on tree size Sprayer calibration
Adapted from Sumner (2004)
2/3
1/3
Small to medium height trees (<40 ft)
= nozzle
Upper 10 nozzles: 2 x D7-45, 4 x D6-45 , 4 x D5-45Lower 7 nozzles: 4 x D5-45, 2 x D4-45, 1 x D3-45
88 GPA @ 1.5 mph for a 60 ft spacing
Upper half of canopy
Lower half of canopy
3/4
1/4
Mature, tall trees (>40 ft)
Upper half of canopy
Lower half of canopy
Upper 12 nozzles: 2 x D7-45, 6 x D6-45, 4 x D5-45Lower 6 nozzles: 2 x D5-45, 2 x D4-45, 2 x D3-45
93 GPA @ 1.5 mph for a 60 ft spacing
A
A
ABC
ABC
C
A
AB
ABC
C
BC
0 20 40 60
16
25
33
41
50Outer
Inner
Spray distribution and disease
Bock, C.H., Hotchkiss, M.W., Cottrell, T.E. and Wood, B.W. 2015. The effect of sample height on spray coverage in mature pecan trees. Plant Dis. 99: 916-925.
Height F-value =17.5, P-value = <0.0001Position F-value = 2.2, P-value = 0.2Height × Position F-value = 1.5, P-value = 0.2
Spray coverage declines with tree height
Planting distance/tree structure Air-blast sprayers provide good
protection up to ~40 ft. Protection dependent on season
Range: 0.01-73.5%
Card area sprayed (%)
Hei
ght
in c
ano
py
(ft)
B
B
B
A
0 5 10 15 20 25 30
15
25
40
55
D
C
B
A
0 20 40 60 80 100
15
25
40
55
F-value =63P-value = <0.0001
Hei
ght
in c
ano
py
(ft)
Scab severity (% fruit surface scabbed)
2013
F-value =285 P-value = <0.0001
2014
Bock, C. H., Hotchkiss, M. W., Brenneman, T. B., Stevenson, K. L., Goff, W. D., Smith, M.W., Wells, L., and Wood, B. W. Severity of scab and its effect on fruit weight inmechanically hedge-pruned and topped pecan trees. Plant Disease 101: 785-793. 2017.
15.0 m
12.5 m
10.0 m
7.5 m
5.0 m
Fruit weight with tree height
The nuts will be larger and of higher quality where spray coverage (scab control) is better
Hedge-pruning reduces tree height and ensures more spray reaches the foliage and fruit
The relationship between scab and fruit weight at different heights
y = -0.24x + 41.7R² = 0.09
0
10
20
30
40
50
60
0 20 40 60 80 100
y = -0.20x + 42.3R² = 0.410
10
20
30
40
50
60
0 20 40 60 80 100
y = -0.20x + 42.2R² = 0.40
0
10
20
30
40
50
60
0 20 40 60 80 100
y = -0.34x + 51.2R² = 0.54
0
10
20
30
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0 20 40 60 80 100
Results from an experiment in 2013 (a wet, scab
conducive year) in non-hedged trees of cv Desirable
Fru
it w
eigh
t (g
)
Severity per fruit (% area diseased)
<21 ft
21-33 ft
33-46 ft
>46 ft
Bock, C. H., Hotchkiss, M. W., Brenneman, T. B., Stevenson, K. L., Goff, W. D., Smith, M. W., Wells, L., andWood, B. W. Severity of scab and its effect on fruit weight in mechanically hedge-pruned and topped pecantrees. Plant Disease 101: 785-793. 2017.
Tree size - hedging
A
B B
0
10
20
30
40
50
No hedging One sidehedged
Both sideshedged
A
B B
0
2
4
6
8
No hedging One sidehedged
Both sideshedged
F-value =26.6P-value = <0.0001
Scab
sev
erit
y (%
fru
it s
urf
ace
scab
bed
) 2013F-value =6.2P-value = 0.002
2014
Scab
sev
erit
y (%
fru
it s
urf
ace
scab
bed
)
Hedge pruning reduces tree height
More spray reaches the foliage and fruit
Tall, non-hedged trees have significantly more severe scab (and hence yield loss)
A 2 to 4 y cycle of hedging? In regard to scab control, shorter trees will always be better
Hedge-pruning ~60 ft
trees to ~40 ft
Bock, C. H., Hotchkiss, M. W., Brenneman, T. B., Stevenson, K. L., Goff, W. D., Smith, M.W., Wells, L., and Wood, B. W. Severity of scab and its effect on fruit weight inmechanically hedge-pruned and topped pecan trees. Plant Disease 101: 785-793. 2017.
Sprayer set-up - volutes
Increases spray volume going to the upper canopy of tall trees
Do not have much data on the spray coverage or efficacy of volutes applied spray
Sprayed (no volute) Sprayed (with volute)
Demonstration spray results at USDA-ARS-SEFTNRL pecan field day, September 2015. Spray applied using a regular orchard sprayer (no volute) or spray applied using a volute
Aerial application
0
20
40
60
80
5.0 7.5 10.0 12.5 15.0
Control Air-blast Aerial Air-blast+Aerial
0
10
20
30
5.0 7.5 10.0 12.5 15.0
5.0 7.5 10.0 12.5 15.0
Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air
ab e bcde e a e abc e a e de e ab cde de e bcd bcde de e
5.0 7.5 10.0 12.5 15.0
Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air Control Ground Aerial Gnd_Air
a c c c b c c c b c c c b c c c b b c c
The same number/timing of aerial sprays as ground may provides a similar level of control
Both combined minimize disease throughout canopy If limited then apply as many aerial sprays as possible to
complement ground sprays during nut development Economics?
2013
2014
3 September
10 October
6 sprays applied
5 sprays applied
Height (m)
Treatment × heightF = 16.0 (P<0.0001)
Treatment × heightF = 8.7 (P<0.0001)
Ground = 100 GPAAerial = ~10 GPA
Scab
sev
erit
y
(% f
ruit
su
rfac
e sc
abb
ed)
Scab
sev
erit
y
(% f
ruit
su
rfac
e sc
abb
ed)
Bock et al. (unpublished)
Chemistry and timing of sprays (pre/post pollination)
UGA spray guide outlines well the chemistries and timing of sprays available for managing pecan scab
Two periods: Pre pollination (10-14 day intervals) and post pollination (10 to 21 day intervals)
Some fungicides are known to be inherently efficacious on leaf or nut scab
Tin and Elast on fruit (also combinations and Quadris top) Phosphites and other products on foliage Fungicide resistance is an issue and should be considered in
the program
Conventional fungicdes
No. Fungicide group FRAC code
Common name Trade names Risk of resistance
1 MBCs (benzimidazoles) 1 thiophanate-methyl Topsin-M High
2 DMIs (sterol inhibitors) 3 Propiconazole Orbit, Propimax,Bumper, Quilt, Quilt Excel
Medium
Febuconazole Enable
Tebuconazole Folicur, Tebuzole, Monsoon, Orius, Toledo, Absolute
Metaconazole Quash
Difenconazole Quadris Top
3 QoIs (strobilurins) 11 Kresoxim-methyl Sovran Medium-low
Azoxystrobin Abound, Quilt, Quilt Excel, Quadris Top
Pyraclostrobin Headline
Trifloxystrobin Absolute
4 Guanadines U12 Dodine Elast Medium-low
5 Organotins 30 Fentin hydroxide SuperTin Medium-low
AgriTin
6 Phosphites 33 Phosphorous acid Phostrol, ProphytFungiphite, Reliant
Medium-low
7 Ziram M3 Zinc dimethyldithiocarbamate
Ziram Low
Fungicides available to manage pecan scab (GA Pest Management Handbook, http://www.ent.uga.edu/pmh/)
a
ab
cd
d
bc
d
bc
d
d
d
0
2
4
6
8
10
12
0
4
8
12
Control(untreated)
Phosphite TPTH
7-Jul-10
Scab
sev
erit
y (p
erce
nt
area
infe
cted
)
Scab
sev
erit
y (p
erce
nt
area
infe
cted
)Phosphites – updates
0
10
20
30
40
50
60
70
80
90
100Rep 1 Rep 2
y = 17.1x - 4.6R² = 0.95
0
50
100
0 1 2 3 4 5
Fungicide resistance is an issue in scab
Phosphites are efficacious – particularly on foliage
We have demonstrated that higher rates effective on fruit
Worked with the pecan industry to generate data on phosphite usage in light of recent phosphiteMRLs in the EU
MRL ruling in Dec 2017 = 500 ppm
Ph
osp
hit
e(p
pm
)
Spray dates (no. of sprays)
Phosphites are efficacious – fruit and foliage
Higher rates are more efficacious – especially on fruitResidues were an issue 2013-2018 – now resolved
Phosphiteapplied at 3 pt/100 g/acre
Rate of phosphite (in 100 g/acre)
Phosphiteapplied at 3 pt/100 g/acre
Bock CH, Brenneman TB, Hotchkiss MW, Wood BW. 2012. Evaluation of a phosphite fungicide to control pecan scab in the southeastern USA. Crop Protection 36: 58-64.
Organic chemistry to manage scab
aa
a
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a
b
b a
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a
b a
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2011 2012 2014 2015 2016 2014 2015
Control Bordeaux mixture
Byron, GA Colquitt, GA
Historically Bordeaux mixture (hydrated lime + copper sulfate) was used to manage scab (~1960)
We have tested Bordeaux mixture in multiple trials over several years and at two sites. Number of applications:
Byron, GA 2011 (6), 2012 (5), 2014 (5), 2015 (6) and 2016 (6)
Colquitt, GA 2014 (5) and 2015 (5)
Efficacy is not reliable, and level of control is not particularly impressive
Although some historical reports indicated it could be effective
Scab
sev
erit
y
(% f
ruit
su
rfac
e sc
abb
ed)
Location and year
cv. Stuart (80 y old trees)
Fruit disease assessments: Byron, GA2011 (12 Oct), 2012 (8 Oct), 2014 (18 Aug), 2015 (12 Aug) and 2016 (22 Aug)Colquitt, GA 2014 (20 Aug) and 2015 (1 Sep)
Means within the same year with different letters are significantly different (α = 0.05)
Bock, C.H., Hotchkiss, M.W., Shapiro-Ilan, D.I., Wells, L., Brock, J., Brenneman., and Mizell., R. 2018. Efficacy of Bordeaux mixture in reducing pecan scab in the southeastern USA. Organic Agriculture. https://doi.org/10.1007/s13165-018-0218-x
New organic chemistry?
a
cd cd
bcab
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a aa a
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a a
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Explored a range of OMRI approved alternatives to Bordeaux mixture
A total of 5 sprays applied in both 2012 and 2014
Assessments made on 19 October 2012 and 18 August 2014
Regalia consistently, and to a lesser extent Serenade reduced severity of pecan scab
Efficacy variable between years with different products (inoculum pressure?)
Scab
sev
erit
y (%
fru
it s
urf
ace
scab
bed
)
Product
Scab
sev
erit
y (%
fru
it s
urf
ace
scab
bed
)
2012 2014
Bock, C.H., Hotchkiss, M.W., Shapiro-Ilan, D.I., Wilkins, B., Wells., D.E., Brock, J., Brenneman., Wells, L., and Mizell., R. 2018. A comparison of organic fungicides: alternatives for reducing scab on pecan. Organic Agriculture. (Accepted)
Summary Scab is a difficult disease to manage
Fortunately there are several angles form which we can approach management of scab
Cultivars (if planting a new orchard select a scab resistant cultivar if possible)
Tree spacing (wider the better to reduce scab)
Tree size (hedge-pruning?)
Chemistry (alternate modes of action). Organic options available
Ground and aerial application (ensure calibrated appropriately)
Acknowledgemnts
Funding:Georgia Commodity Commission for Pecans
USDA-ARS project 6606-21220-011–00D
USDA-National Institute of Food and Agriculture award #2013-51106-21234 (Organic Transitions Program)
Collaborators and technical assistance at the USDA-ARS, Byron :Dr. Mike Hotchkiss, Dr. Bruce Wood (retired), Wanda Evans, Minling Zhang, Unicka Stokes, Sue Burrell, Jason Shipp, Caylee Carson, Sarah Morrel
Collaborators at the University of Georgia, Tifton: Dr. Tim Brenneman, Dr. Lenny Wells, Dr. Katy Stevenson
Thank you
Questions?