Managing Stewart’s wilt and

common rust in sweet corn

Jerald K. Pataky Department of Crop Sciences Professor of Plant PathologyUniversity of Illinois

j-pataky@uiuc.edu

www.sweetcorn.uiuc.edu

Stewart’s wiltErwinia stewartii

Seedling wilt phase

Symptoms following leaf veins

Ooze test

bacterial ooze

Corn flea beetle

Flea beetle feeding

Two important generationsof flea beetles

• overwintering generation - peak numbers at first plantings

- decreases until ~ Memorial Day • second generation - peaks between June 20 to July 10 - size affected by weather ( larger when dry)

Flea beetle winter survival

> 32 F survive*27 - 32 F % survive*< 27 F killed

Average daily air temperature Dec., Jan., Feb.

*size of previous population

Ten Warmest Winterssince 1895

10. 1930-1931 36.2 9. 1952-1953 36.2 8. 1999-2000 36.2 7. 1908-1909 36.3 6. 1998-1999 36.4 5. 1953-1954 36.8 4. 1920-1921 36.9 3. 1997-1998 36.9 2. 1991-1992 37.9 1. 1931-1932 40.0

Laura Sweets, University of Missouri, Columbia

Flea beetles and Stewart’s wiltAvg. temp (F) > 28

December, January & February

1995

1996

1994

1997

1999

19982000

1994-1998 Dekalb Genetics 1999-2001 University of Illinois

2001

December 2000 16.7 F

Dec Jan Dec JanLocation 01 02 00 01Janesville, WI 31 28 12 21 Rockford, IL 31 29 12 21Mendota, IL 32 29 13 21Urbana, IL 35 34 17 26Brownstown, IL 37 35 21 29Dixon Springs, IL 41 38 28 34

Will flea beetles survive 2001/2002? Average daily temperature

Control Stewart’s wilt

• resistant hybrids

• control flea beetles

susceptible moderate resistant

Scale for rating Stewart’s wilt

• flea beetle feeding wounds• limit movement of E. stewartii

Resistance to Stewart’s wilt

moderately susceptible

moderately resistant

susceptible

Information on hybrid reactions to Stewart’s wilt

• Midwestern Vegetable Variety Trial Report

• www.sweetcorn.uiuc.edu

• extension publications

• literature from seed companies

R

S

Effect of Stewart’s wilton sweet corn yield

• growth stage • level of resistance

Stewart’s wilt and sweet corn yieldeffect of growth stage at time of infection

Suparyono & Pataky, 1987

Y = 80.9 + 13.7X - 2.9X2

r2

= 0.75

Stewart's wilt rating

2 3 4 5 6 7

Perc

ent yie

ld (

ea

r w

eig

ht)

0

20

40

60

80

100

120

20001999

Fig. 6. 1999 and 2000 combined regression, relationship between percent yield (ear weight)and Stewart's wilt rating

Stewart’s wilt and yield

effect of host reaction (resistant to susceptible) Freeman & Pataky, 2000

Marketable ears and incidence of systemic infection

Y = 93.0 - 0.93X

r2 = 0.65

Systemically infected plants (%)

0 20 40 60 80 100

Pe

rce

nt

ma

rke

tab

le e

ars

0

20

40

60

80

100

120

Effects of Stewart’s wilt on yield

Hybrid 3- to 5- 5- to 7- 7- to 9-reaction leaf leaf leaf

Growth stage

R 0% 0 % 0 %MR 0-30% 0 % 0 %MS 10-40% 0-20 % 0 %S 40-100% 15-35% 3-15%

• prior to 3-leaf stage -- main stalk death

systemic infection main stalk death

Growing point is killed

Tillers from main stalk death

Bold 10% MRGH 2628 11% R/MRBonus 11% RMore 17% R/MRGH 2757 30% MSEmpire 33% M/MSJubilee 60% S

Main stalk Hybrid Hybrid death (%) Rxn

Stewart’s wilt and main stalk death

July 1995, Manito, IL

Controlling flea beetles

• seed treatments • in-furrow applications• foliar applications

Insecticides

Seed treatment insecticides

• imidacloprid (Gaucho)• thiamethoxam (Adage/Cruiser)• TI-435 - clothianidin

60% to 80% control of Stewart’s wilt

TreatedNon-treatedFlea beetle feeding wounds

Urbana early trial (Planted: 27 April)

0

5

10

15

20

25

30

35

40

Check Adage 50

Adage100

Adage200

Adage250

Gaucho3.2

Gaucho4.8

Gaucho6.2

G2 4

G2 8

Inci

denc

e (%

) of

sys

tem

ic S

ewar

t's w

ilt

1 June

15 June

Stewart’s wilt control on Jubilee

Manito early trial (Planted: 26 April)

0

5

10

15

20

25

30

Check Adage 50

Adage100

Adage200

Adage250

Gaucho3.2

Gaucho4.8

Gaucho6.2

G2 4

G2 8

Inci

denc

e (%

) of

sys

tem

ic S

ewar

t's w

ilt

2 June

22 June

Stewart’s wilt control on Jubilee

Manito late trial (Planted: 22 June)

0

5

10

15

20

25

30

Check Adage 50

Adage100

Adage200

Adage250

Gaucho3.2

Gaucho4.8

Gaucho6.2

G2 4

G2 8

Inci

denc

e (%

) of

sys

tem

ic S

ewar

t's w

ilt

25 July

9 August

Stewart’s wilt control on Jubilee

Urbana late trial (Planted: 19 June)

0

10

20

30

40

50

60

70

80

90

100

Check Adage 50

Adage100

Adage200

Adage250

Gaucho3.2

Gaucho4.8

Gaucho6.2

G2 4

G2 8

Inci

denc

e (%

) of

sys

tem

ic S

tew

art's

wilt

10 July

20 July

4 August

Stewart’s wilt control on Jubilee

non-treated

treated

Non-treated

Treated

imidacloprid

Economic value of seed treatment insecticides

Value = value of the crop ($/A) x incidence (%) x 0.7 (70% control)

Example: Crop value estimated at $800/A x 5% systemic infection (w/o control) x 0.7 (70% control) = $28 / A estimated value of seed treatment

1.00.870.750.440.19>10%

00.130.190.310.315-10%

000.060.250.5<5%

S

(9)

MS

(7-8)

M

(5-6)

MR

(3-4)

R

(1-2)

Incidence of systemic infection

Hybrid reaction to Stewart’s wilt

Probability of 5% systemic infection in central Illinois

Planted after 1 June 1998 to 2000

1998 to 2000 represents worse case scenario

Common rust

Puccinia sorghi

Effect of rust on sweet corn yield

Y = 99.4 - 0.58 Xr = 0.81 n = 232

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70

Rust severity (%) one week before harvest

Pe

rce

nt

ma

xim

um

yie

ld (

ea

r w

eig

ht)

Rust severity is affected by:

• abundance of urediniospores

• host growth stage

• weather

• host resistance

Urediniospores Teliospores

Telia

Uredinia

Initial inoculum (urediniospores)

does not overwinterin the Corn Belt

Puccinia pathways (urediniospores)

Secondary inoculum (urediniospores)

comes from infected corn

~ 5,000 urediniospores per pustule200 urediniospores per day for ~ 4 wks

50 pustules = 10,000 spores per day

Rust severity is affected by:

• abundance of urediniospores

• host growth stage

• weather

• host resistance

Juvenile tissue is more susceptible than adult-plant tissue

Adult

late juvenile

early juvenile--

Adult Leaf Epidermal Traits

• bulliform cells, macrohairs, prickle hairs• alkane-rich shorter chain leaf waxes• neutral (purple) reaction with toluidine blue-O histochemical stain• highly-crenulated cell walls• rectangular cell shape (cross-section) • thick cuticle (~ 3 micron cross-section)

Juvenile Leaf Epidermal Traits

• crystalline waxes rich in primary alcohols• acidic (aqua) reaction with toluidine blue-O histochemical stain• weakly-crenulated cell walls• rounded cell shape (cross-section)• thin cuticle (~1 micron cross section)

3

8

Epidermal Cell Differentiation in Juvenile and Adult Leaves

Whorls - primary site of infection

Infection in the leaf whorl

Rust severity is affected by:

• abundance of urediniospores

• host growth stage

• weather

• host resistance

Moisture and temperature affect: • urediniospore germination• rate of infection• sporulation

• 6 hours of moisture• 60 to 75 F optimal• ~38 to 95 F minimum and maximum

Rust develops rapidly on late-season crops

• urediniospores are abundant

• air is humid

• low night temperature creates dew

Infection on wet leaf tissue (heavy dews)

Control common rust

• plant resistant hybrids

• apply fungicides

Rust resistance• general (partial) resistance • Rp-resistance

partialresistance

susceptible

Information on hybrid reactions to common rust

• Midwestern Vegetable Variety Trial Report

• www.sweetcorn.uiuc.edu

• extension publications • literature from companies

Rust severity %and (estimated Reactions of sweet corn hybridsyield reductions)** R MR M MS S 0-10% (3%) 0.63 0.26 0.11 0.11 0.1110-20% (9%) 0.37 0.47 0.16 0.11 020-30% (15%) 0 0.26 0.63 0.26 0.1630-40% (21%) 0 0 0.11 0.32 0.2640-50% (27%) 0 0 0 0.21 0.32>50% (>30%) 0 0 0 0 0.16 * probability based on 18 years of evaluations in University of Illinois sweet corn hybrid disease nurseries

** yield loss estimated by multiplying rust severity by 0.06

Probability* of severe rust

• MR - 75% chance of less than 20% rust • M to MS - ~75% chance of more than 20% rust • S - 75% chance of more than 30% rust

Common rust on sweet cornmatures ~ August 5 in central Illinois

Rp-resistance

Bands of chlorotic flecks (qualitative reaction)

Single Rp genes in the Rp1 region

Rp1-DRp-GRp1-E (Rp1-I, Rp1-K) Rp1-MRp1-C (Rp1-N)Rp1-A (Rp1-F)

Susceptible Rp-resistant

Rp1D-resistant hybrid

Rock Falls, IL September 1999

Rp1D-resistancegreenhouse Sept. 1999

avirulent

virulent

Rp1-D resistance Los Mochis, Mexico

March 2000

Future of Rp resistance

• one Rp hybrid may be resistant while another Rp hybrid may be completely susceptible • different Rp genes

more confusing - variable

Future of Rp resistance

• one Rp hybrid may be slightly infected while another Rp hybrid may be severely infected • new race present and hybrids have different levels of partial resistance

more confusing - variable

Future of Rp resistance

• an Rp hybrid may be resistant at one location but susceptible in another area • different isolates of rust

more confusing - variable

Future of Rp resistance

• an Rp hybrid may be resistant at an early planting but susceptible at a later planting • a virulent isolates of rust is introduced during the season

more confusing - variable

Know more about hybrid reactions to rust

• Rp-resistant hybrids• Rp gene• background reaction (R, MR, M, MS, S) • non-Rp hybrids• reaction (R, MR, M, MS, S)

• scout for rust• any pustules on Rp hybrids • thresholds on MR, M, MS, and S hybrids

Fungicides• EBDCs

• TILT

• strobilurins (QUADRIS, F-500)

Fungicides*• fungicides are preventative• fungicides ARE NOT curative

Rule of thumb: one or two early applications are superior to multiple late applications • juvenile tissue is more susceptible• infection occurs in the whorl • pustules on lower leaves = inocula for 2o infection (5,000 urediniospores per pustule)

* EBDCs, Tilt (1 to 2% action threshold)

Strobilurins

may change use of fungicides on sweet corn

• more efficacious • may have different thresholds ?

Rust & NLB - 'Florida XP-7' Leaf area necrotic (%) Rick Raid - UF-Belle Glade Planted: 17 Feb 1997

0

10

20

30

40

50

60

70

Non-treated

Manzate Tilt Folicur Quadris Tilt +Manzate

Folicur +Manzate

Quadris+Manzate

Quaddris+ Tilt

Lea

f ar

ea n

ecro

tic

(%)

Common rust - 'Silver Queen' Leaf area infected (%)

Helene Dillard, NYAES- Geneva Planted: 21 June 2000

0

5

10

15

20

25

30

Non-treated Quadris 0.38 28 July 09 August

Quadris 0.77 28 July 09 August

Tilt 0.25 28 July 09 August

Quadris 0.38 09 August 21 August

Quadris 0.77 09 August 21 August

Tilt 0.25 09 August 21 August

Per

cen

t le

af a

rea

infe

cted

Location: Urbana, IL - May 29, 2001

Hybrids: Snow White, Sterling

Compounds: Tilt, BASF F-500, Quadris (low, high rate)

Application: July 5 (2 to 4-leaf) 5% July 11 (4 to 6-leaf) 15%

July 17 (6 to 8-leaf) 30% July 24 (row tassel) 40%

21 treatments

Rust fungicide trial - 2001

Non-treated

strobilurin-treated

5.5

6

6.5

7

7.5

8

5% 15% 30% 40% Tilt weekly Check

Yie

ld (

ton

s/A

Tilt Strobilurin

Rust fungicide trial - 2001

Sterling

• Thresholds for strobilurins probably will be in the 5% to 15% range (compared to 1 to 2% for Tilt or EBDCs)

Tentative conclusions

Jerald Pataky Department of Crop Sciences

University of Illinois

j-pataky@uiuc.eduwww.sweetcorn.uiuc.edu