Managing Stewart

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Managing Stewarts wilt and common rust in sweet
corn
Jerald K. Pataky Department of Crop Sciences
Professor of Plant Pathology University of
Illinois
j-pataky_at_uiuc.edu www.sweetcorn.uiuc.edu
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Stewarts wilt
Erwinia stewartii
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Seedling wilt phase
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Symptoms following leaf veins
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Ooze test
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bacterial ooze
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Corn flea beetle
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Flea beetle feeding
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Two important generations of 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)

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Flea beetle winter survival
Average daily air temperature Dec., Jan., Feb.
gt 32 F survive 27 - 32 F survive lt 27
F killed
size of previous population
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Ten Warmest Winters since 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
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December 2000 16.7 F
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Will flea beetles survive 2001/2002? Average
daily temperature
Dec Jan Dec Jan Location 01 02
00 01 Janesville, WI 31 28 12
21 Rockford, IL 31 29 12
21 Mendota, IL 32 29 13
21 Urbana, IL 35 34 17
26 Brownstown, IL 37 35 21 29 Dixon
Springs, IL 41 38 28 34
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Control Stewarts wilt

• resistant hybrids
• control flea beetles

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susceptible moderate
resistant
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Scale for rating Stewarts wilt
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Resistance to Stewarts wilt

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

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moderately resistant
moderately susceptible
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susceptible
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Information on hybrid reactions to Stewarts
wilt

• Midwestern Vegetable
• Variety Trial Report
• www.sweetcorn.uiuc.edu
• extension publications
• literature from seed companies

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R
S
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Effect of Stewarts wilt on sweet corn yield

• growth stage
• level of resistance

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Stewarts wilt and sweet corn yield effect of
growth stage at time of infection
Suparyono Pataky, 1987
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Stewarts wilt and yield effect of host
reaction (resistant to susceptible) Freeman
Pataky, 2000
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Marketable ears and incidence of systemic
infection
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Effects of Stewarts wilt on yield
Growth stage
Hybrid 3- to 5- 5- to 7- 7- to 9- reaction
leaf leaf leaf
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

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systemic infection
main stalk death
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Growing point is killed
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Tillers from main stalk death
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Stewarts wilt and main stalk death
Main stalk Hybrid Hybrid death () Rxn
Bold 10 MR GH 2628 11 R/MR Bonus
11 R More 17 R/MR GH 2757
30 MS Empire 33 M/MS Jubilee 60 S
July 1995, Manito, IL
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Controlling flea beetles
Insecticides

• seed treatments
• in-furrow applications
• foliar applications

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Seed treatment insecticides
60 to 80 control of Stewarts wilt

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

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Non-treated
Treated
Flea beetle feeding wounds
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Stewarts wilt control on Jubilee
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Stewarts wilt control on Jubilee
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Stewarts wilt control on Jubilee
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Stewarts wilt control on Jubilee
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treated
non-treated
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Treated
Non-treated
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imidacloprid
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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
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Probability of 5 systemic infection in central
Illinois Planted after 1 June 1998 to 2000
1998 to 2000 represents worse case scenario
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Common rust
Puccinia sorghi
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Effect of rust on sweet corn yield
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Rust severity is affected by

• abundance of urediniospores
• host growth stage
• weather
• host resistance

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Urediniospores
Teliospores
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Telia
Uredinia
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Initial inoculum (urediniospores) does not
overwinter in the Corn Belt
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Puccinia pathways (urediniospores)
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Secondary inoculum (urediniospores) comes from
infected corn
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5,000 urediniospores per pustule 200
urediniospores per day for 4 wks
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50 pustules 10,000 spores per day
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Rust severity is affected by

• abundance of urediniospores
• host growth stage
• weather
• host resistance

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Juvenile tissue is more susceptible than
adult-plant tissue
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Epidermal Cell Differentiation in Juvenile and
Adult Leaves

• 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)

Adult
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• 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)

late juvenile
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early juvenile
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Whorls - primary site of infection
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Infection in the leaf whorl
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Rust severity is affected by

• abundance of urediniospores
• host growth stage
• weather
• host resistance

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• 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

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Rust develops rapidly on late-season crops

• urediniospores are abundant
• air is humid
• low night temperature creates dew

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Infection on wet leaf tissue (heavy dews)
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Control common rust

• plant resistant hybrids
• apply fungicides

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Rust resistance

• general (partial) resistance
• Rp-resistance

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partial resistance
susceptible
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Information on hybrid reactions to common rust

• Midwestern Vegetable
• Variety Trial Report
• www.sweetcorn.uiuc.edu
• extension publications
• literature from companies

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Probability of severe rust
Rust severity and (estimated
Reactions of sweet corn hybrids yield
reductions) R MR M MS S 0-10
(3) 0.63 0.26 0.11 0.11 0.11 10-20
(9) 0.37 0.47 0.16 0.11 0 20-30 (15)
0 0.26 0.63 0.26 0.16 30-40
(21) 0 0 0.11 0.32 0.26 40-50
(27) 0 0 0 0.21 0.32 gt50
(gt30) 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
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Common rust on sweet corn matures August 5 in
central Illinois

• 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

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Rp-resistance
Bands of chlorotic flecks (qualitative reaction)
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Single Rp genes in the Rp1 region
Rp1-D Rp-G Rp1-E (Rp1-I, Rp1-K) Rp1-M Rp1-C
(Rp1-N) Rp1-A (Rp1-F)
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Susceptible Rp-resistant
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Rp1D-resistant hybrid
Rock Falls, IL September 1999
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Rp1D-resistance greenhouse Sept. 1999
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virulent
avirulent
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Rp1-D resistance Los Mochis, Mexico March 2000
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Future of Rp resistance
more confusing - variable

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

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Future of Rp resistance
more confusing - variable

• 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

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Future of Rp resistance
more confusing - variable

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

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Future of Rp resistance
more confusing - variable

• 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

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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

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Fungicides

• EBDCs
• TILT
• strobilurins
• (QUADRIS, F-500)

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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)
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Strobilurins may change use of fungicides on
sweet corn

• more efficacious
• may have different thresholds ?

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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
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Rust fungicide trial - 2001
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Non-treated
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strobilurin-treated
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Sterling
Rust fungicide trial - 2001
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Tentative conclusions

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

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• Jerald Pataky
• Department of Crop Sciences
• University of Illinois

j-pataky_at_uiuc.edu www.sweetcorn.uiuc.edu