Disease and spray coverage in pecan trees

Clive H. Bock

USDA-ARS-SEFTNRL, 21 Dunbar Rd., Byron, GA 31008

Overview of presentation

• Background, challenges to good fungicide coverage (particularly in relation to pecan scab)

• Describe results of some recent experiments

• Scab distribution in the tree

• Spray coverage results

• Summarize these in the context of options for control of scab

• Issues that remain to be resolved (aerial vs. ground based spraying)

Background • Mature pecan trees are tall (>15 m [>50 ft])

• Major disease is scab (Fusicladium effusum)

• Various fungicides are used to control scab

• Much of the application is by ground-based air-blast sprayers

• Good scab control in the top of the tree is perceived to be challenging (especially if wet)

• Fungal plant pathogens differ to insect pests – they are not mobile

• Many factors affect spray coverage – tractor speed, application volume, weather conditions, tree architecture and tree height

• Objective: to characterize scab distribution and the impact of scab management in the canopy of mature pecan trees

Pecan scab life cycle (Fusicladium effusum)

Overwinters as conidia

and stroma

Epidemics build up on fruit (conidia)

Autumn

Winter

Summer

Spring

Fungus becomes

dormant as ‘stroma’ and

overwintering conidia

Epidemics build up on young leaves

(conidia)

Experiment design and procedures

• Cv. Desirable 2010, 2011, cv. Wichita 2011, mature trees (>15 m [~50 ft]).

• Trees received fungicide (propiconazole, TPTH) by air-blast sprayer (Aerofan D2/40 1000), Ground speed 2 mph, 100 gallons per acre) or were non-treated

• 4 replicates of each treatment. Fully randomized design

• Leaves and fruit assessed for scab incidence and severity in Jun, early-Aug and early Oct, respectively

• Samples (10 leaves or fruit) taken at <5.0, 5.0-7.5, 7.5-10.0, 10.0-12.5 and >12.5 m [<16, 16-25, 25-32, 32-40, >40 ft]

• Data analyzed using a generalized linear mixed model with an analysis of simple effects

• Yijk = θ + αi + βj + (αβ)ij + eijk, (where θ is a constant (intercept term), αi is the main effect of fungicide treatment, βj is the main effect of height, and (αβ)ij the interaction term, and eijk the residual error)

Vertical distribution of pecan scab in mature trees

Weather and timing of fungicide sprays

• 54-y average 15 Mar-15 Oct is 739 mm (29 ins)

• 2010 was an average year with evenly distributed rainfall (766 mm [30 ins])

• 2011 was a relatively dry year (591 mm [23 ins])

2010

020406080

100120

15-Mar-10 15-Apr-10 15-May-10 15-Jun-10 15-Jul-10 15-Aug-10 15-Sep-10 15-Oct-10

Rainfall (mm)

Fungicide application

020406080

100120

15-Mar-11 15-Apr-11 15-May-11 15-Jun-11 15-Jul-11 15-Aug-11 15-Sep-11 15-Oct-11

2011

Rai

nfal

l (m

m)

Rai

nfal

l (m

m)

Vertical distribution of pecan scab on leaflets

0.0

0.5

1.0

1.5

2.0

2.5

3.0

<5 7.5 10 12.5 >12.5

Control

Fungicide

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

<5 7.5 10 12.5 >12.5

0.0

0.1

0.2

0.3

0.4

0.5

<5 7.5 10 12.5 >12.5

• Trends differed on fungicide treated and non-treated trees

• On non-treated trees more severe disease in the lower canopy 2010 (cv. Desirable) and 2011 (cv. Wichita). No difference on cv. Desirable (2011)

• Inconsistent on fungicide-treated trees. Less disease in the lower canopy on cv. Desirable (2010), similar disease on cvs. Desirable and Wichita (2011)

• Spring 2011 was very dry. Fungicide timing?

Desirable, 2010 Desirable, 2011 Wichita, 2011

ab a b b b

l lm lm lm m

a a a a a

l l l l l

a ab bc abc c

l lm m lm lm

Sca

b s

eve

rity

(%

leaf

let

area)

Sample height (m)

Within treatment, bars with different letters are significantly different (P=0.05). Whiskers are 95% confidence intervals

June/July

[<16, 16-25, 25-32, 32-40, >40 ft]

a a b b c

l lm lm l m

a bc b c c

l l l l l

a bc b c c

l l l l l

0

10

20

30

40

50

60

70

<5 7.5 10 12.5 >12.5

Control

Fungicide

0

5

10

15

20

25

30

<5 7.5 10 12.5 >12.5

0

1

2

3

4

5

<5 7.5 10 12.5 >12.5

Vertical distribution of pecan scab on fruit August

Desirable, 2010 Desirable, 2011 Wichita, 2011

Sca

b s

eve

rity

(%

fru

it a

rea)

• Trends differed on fungicide treated and non-treated trees

• On non-treated trees most severe disease was in the lower canopy

• Fungicide-treated trees had either less severe disease in the lower canopy (cv. Desirable, 2010) or similar severity at all heights (cvs. Desirable and Wichita, 2011 )

Sample height (m)

[<16, 16-25, 25-32, 32-40, >40 ft]

Within treatment, bars with different letters are significantly different (P=0.05). Whiskers are 95% confidence intervals

a b b b b

l l l l l

a b b b b

l l l l l

bc ab a c c

l l l l l

0

20

40

60

80

100

<5 7.5 10 12.5 >12.5

Control

Fungicide

0

5

10

15

20

25

30

35

40

<5 7.5 10 12.5 >12.5

0

5

10

15

20

25

30

35

<5 7.5 10 12.5 >12.5

Vertical distribution of pecan scab on fruit October

Desirable, 2010 Desirable, 2011 Wichita, 2011

Sca

b s

eve

rity

(%

fru

it a

rea)

• Trends differed on fungicide treated and non-treated trees

• On non-treated trees most severe disease was in the lower to mid- canopy

• Fungicide-treated trees had similar scab severity at all heights

[<16, 16-25, 25-32, 32-40, >40 ft]

Sample height (m)

Within treatment, bars with different letters are significantly different (P=0.05). Whiskers are 95% confidence intervals

Desirable, August 2010

Sca

b s

eve

rity

(%

shuc

k ar

ea

dis

eas

ed) • Severity declined with tree

height in all seasons

• Fungicide treatment has a significant effect reducing scab in the low-mid canopy (<10 m [32 ft])

• Above 12.5 m [40 ft], there was no significant effect of fungicide on scab severity

Treatment

<5.0 m [<16 ft]

Tre

e h

eig

ht

Vertical distribution of scab in the pecan canopy

a

b

0

10

20

30

40

50

60

70

Control Fungicide

Control

Fungicide

a

b

0

10

20

30

40

50

60

70

Control Fungicide

a

b

0

10

20

30

40

50

60

70

Control Fungicide

a

a

0

10

20

30

40

50

60

70

Control Fungicide

a

b

0

10

20

30

40

50

60

70

Control Fungicide

5.0-7.5 m [16-25 ft]

7.5-10.0 m [25-32 ft]

10.0-12.5 m [32-40 ft]

>12.5 m [>40 ft]

a

b

0

5

10

15

20

25

30

Control Fungicide

a

b

0

5

10

15

20

25

30

Control Fungicide

a

b

0

5

10

15

20

25

30

Control Fungicide

a a

0

5

10

15

20

25

30

Control Fungicide

a a

0

5

10

15

20

25

30

Control Fungicide

a

b

0.0

1.0

2.0

3.0

4.0

5.0

Control Fungicide

a a

0.0

1.0

2.0

3.0

4.0

5.0

Control Fungicide

a

b

0.0

1.0

2.0

3.0

4.0

5.0

Control Fungicide

a a

0.0

1.0

2.0

3.0

4.0

5.0

Control Fungicide

a a

0.0

1.0

2.0

3.0

4.0

5.0

Control Fungicide

Desirable, August 2011

Wichita, August 2011

August

Within each column of charts, bars with the different letters are significantly different (P=0.05). Whiskers are 95% confidence intervals

a

b

0

15

30

45

60

75

90

Control Fungicide

Control

Fungicide

a

b

0

15

30

45

60

75

90

Control Fungicide

a

b

0

15

30

45

60

75

90

Control Fungicide

aa

0

15

30

45

60

75

90

Control Fungicide

a

b

0

15

30

45

60

75

90

Control Fungicide

a

b

0

15

30

45

Control Fungicide

a

b

0

15

30

45

Control Fungicide

a

b

0

15

30

45

Control Fungicide

a

b

0

15

30

45

Control Fungicide

aa

0

15

30

45

Control Fungicide

a

b

0

5

10

15

20

25

30

35

Control Fungicide

a

b

0

5

10

15

20

25

30

35

Control Fungicide

a

a

0

5

10

15

20

25

30

35

Control Fungicide

a

a

0

5

10

15

20

25

30

35

Control Fungicide

aa

0

5

10

15

20

25

30

35

Control Fungicide

Desirable, August 2010

Desirable, August 2011

Wichita, August 2011

Sca

b s

eve

rity

(%

shuc

k ar

ea

dis

eas

ed)

• Severity declined with tree height in all seasons

• Fungicide treatment most often had a significant effect reducing scab in the low-mid canopy (<10 m [32 ft])

• At 7.5 m [25 ft] and below, there was a consistent effect of fungicide on scab severity

Treatment

<5.0 m [<16 ft]

Tre

e h

eig

ht

5.0-7.5 m [16-25 ft]

7.5-10.0 m [25-32 ft]

10.0-12.5 m [32-40 ft]

>12.5 m [>40 ft]

October

Within each column of charts, bars with the different letters are significantly different (P=0.05). Whiskers are 95% confidence intervals

Vertical distribution of scab in the pecan canopy

Fungicide treatment and height

-200

-150

-100

-50

0

50

100

150

<5 7.5 10 12.5 >12.5

-200

-150

-100

-50

0

50

100

150

<5 7.5 10 12.5 >12.5

-200

-150

-100

-50

0

50

100

150

<5 7.5 10 12.5 >12.5

Desirable 2010

Desirable 2011

Wichita 2011

• On leaves and fruit in August there was a consistent fungicide effect on scab at heights ≤10 m [32 ft] [(Control-Treated)/Control]*100

• On leaves and fruit in August at >10 m [32 ft] there was an inconsistent effect of fungicide

• In October on fruit fungicide reduced scab at all heights

• Is this due to a direct fungicide effect, or a cumulative effect on the epidemic in the tree?

The difference in pecan scab severity between treated and non treated trees

June: severity per infected leaflet (% area)

August: severity per fruit (% area)

October: severity per fruit (% area)

Reduc

tion

in

scab

se

veri

ty (

%)

Sample height (m)

[<16, 16-25, 25-32, 32-40, >40 ft]

Water sensitive cards placed in trees

• Used water sensitive cards (Syngenta) at different height in the canopy to measure spray distribution

• Placed two cards at each of 6 heights in the canopy of three trees (0, 1.5, 5, 10, 14 and 16 m [0, 5, 16, 32, 45, 52 ft])

• Moneymaker trees up to ~25 m (80 ft)

• Durand-Wayland m3210

• Replicated three times (3 trees)

• Analyzed using a general linear model

Spray coverage in mature trees

Water sensitive cards

2012

Water sensitive cards • A decrease with spray coverage with height

• Up to 10 m [32 ft], spray coverage appears good

• Performed image analysis cards to measure area covered and the number of droplets

• Compared coverage to height in the tree

Fungicide spray coverage in mature trees

0 m [0 ft]

Tre

e heig

ht

1.5 m [5 ft]

5 m [16 ft]

10 m [32 ft]

14 m [45 ft]

16 m [52 ft]

1. Card is photographed and digitized

2. Image analysis is used to separate spray area from background

3. The area covered by spray is measured

4. The number of spray droplets are counted

ab

a

a a

bc

c

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 1.5 5 10 14 16

a

aa

a

bb

0

5

10

15

20

25

30

35

40

45

50

0 1.5 5 10 14 16

Are

a co

vere

d b

y sp

ray

(%)

Height (m)

• Percent area coverage is significantly less at heights >10 m [32 ft]

• But up to 10 m [32 ft], spray coverage appears comparable at all heights tested

• Number of droplets followed a similar trend

• The height to which scab control was observed in trees described in earlier experiments

Fungicide spray coverage in mature trees

Percent card area covered by spray

Number of droplets per card

Num

ber

of d

ropl

ets

pe

r ca

rd

Height (m)

Data analyzed using general linear modeling. Letters indicate significant differences using the Student-Newman-Keuls test (P=0.05). 95% Confidence Intervals are indicated.

[0, 5, 16, 32, 45, 52 ft]

Summary • Non-treated and fungicide treated trees differed in scab distribution

• In treated trees there was less disease in the lower canopy

• Fungicide reduced the overall epidemic within treated trees in Oct

• Ground-based spray coverage is effective to at least 10 m [32 ft]

• Which was the height to which disease was consistently reduced (≤10m)

So…

• Ground based spraying is likely inadequate for mature trees when/where scab is an issue (particularly if much taller than 10 m [32 ft])

Questions remain….

• How effectively does aerial application fill this gap?

• Can we adjust ground-based spray volume/speed for better coverage in tall trees?

• What about pruning appropriately or hedging to keep tree height below that for which air blast sprayers are efficacious?

• Fungicide resistance….

We thank the GA Pecan Commodity Commission for financial support to aid the research

Dr Bruce Wood

Dr Mike Hotchkiss

Also Shad Stormant, Emma Cutchens, Keith Hough, Bridget Rawls, Stephanie de Vos, Wanda Evans, Shirley Anderson, Ginger Moreland and Sam Njoroge

Acknowledgements

Thank you, and any questions?