An aggressive new strain of the wheat stripe rust fungus Puccinia striiformis was discovered in 2000 that was virulent on the previously effective resistance gene Yr9. The strain spread quickly across states, forming "hot spots" and continuing to spread even as temperatures warmed. While some colleagues dismissed the importance of increased aggressiveness, the author's analysis found this to be a new, more aggressive strain adapted to warmer temperatures. Yield loss data from 1976-2005 supported this was one of the most damaging stripe rust epidemics in the US, caused by the combination of new virulence and favorable weather conditions. Fingerprinting of 60 clones found multiple strains present.

1. An Aggressive Strain of Puccinia striiformis:
Discovery and Documentation
Gene Milus
University of Arkansas

2. Observations from 2000
• Stripe rust over-wintered
and formed “hot spots”
• Disease spread quickly
• Yr9 no longer effective
• Disease did not stop
when the temperature
warmed

3. My conclusions Response from colleagues
• New race virulent on Yr9 • Forget aggressiveness; only
• More aggressive strain with virulence is important
adaptation to warmer • Forget adaptation to warm
temperatures temperatures; this was
hypothesized previously
and shown to be incorrect
• The most widespread
epidemic in US was caused
by a combination of new
virulence and favorable
weather

4. 0
5
10
15
20
25
30
35
19
7
19 6
7
19 7
7
19 8
7
19 9
8
19 0
8
19 1
8
19 2
8
19 3
8
19 4
8
19 5
8
19 6
8
19 7
8
19 8
8
19 9
9
19 0
9
19 1
9
19 2
9
19 3
9
19 4
9
Disease Laboratory website
Data from USDA ARS Cereal
19 5
9
19 6
9
losses to stripe rust
19 7
9
19 8
9
20 9
0
Number of states reporting
20 0
0
20 1
0
20 2
0
20 3
0
20 4
05

5. Stripe Rust Epidemic Years and Estimated Yield Losses in
Selected States from 1976 to 2005.
17 % Loss
10 10
25
5 5
10 0.1 0 0 0 0 0
5 7.5 5
10 0.1
19 976
0.1 3 0 0 0 0 0
5
20
19 980 8
0.1 0 0 0 0 0
1
1 7
5
6 0.1 0 0 0 0 0.1
2 1
19 983 1
3.5 0.1 25 0.1 0.1 0 5 0
5
1 8
1 1
5
0 0 0 0 02.5 15
1 1 0.5 7
19 987 4
0.1 0.1 0
2 2
1 8
0.1 0 0 0 0.3 0 7
7.3 8
20 999 0
0 0.1 0.1 3 0.1 0.1 0 0 0 10
1 9
0.1 0.1 1 0.1 10.6 0.1 0.1 10 0.5
1 26 5
0.5 0.53
20 001 0
0 0.1 0.1
2 0
0.5 0.5 1 0.1 0 0
5 3 5
0.4 1.5 8 1 4.5
20 003 2
0.13 0.1 25
5 5
2 0
4 1.5
0.13 0.1 0.1 0.14
2 1
20 04
0
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6. AR04-11SPF
AR05-1A-5
AR05-IIIA-4
AR05-IIB-3
AFLP Fingerprint Analysis AR05-IC-3
AR05-IC-5
AR03-01
AR05-IIID-5
AR05-IIID-7
AR05-IE-3
60 CLONES
AR04-18SPF
AR05-1A-1
New strain 01-69
AR05-IB-3
AR05-IIG-4
AR05-IIG-5
AR05-IE-1
AR05-IIE-1
MT81
MT83-BFMOLI
2K-127
PST-37
MT87
PST-1-3
Pst-6-3
MT94SP3
MT93-SP1
PST-35-5
AR97-01sp1
Pst-43
AR90-01sp2
Pst-29
Old strain MT83-SP1
MT85-SP1
PST-17-5
CDL-17-3
PST-45
2K-129
MT80
PST17
MT82
0.36 0.52 0.68 0.84 1.00
Coefficient

7. Characteristics of Aggressiveness
• Shorter latent period
• Larger lesions
• More spores
• Adaptation to warmer temperatures

8. New Inoculation Technique
• Quantitative
• Localized
• Quick & easy
• 0.25% Noble agar +
0.02% Tween 20
• 5 mg spores / 4 ml
• “paint” across leaf

9. Lesion area growth rate
Latent Period (days) (mm2 per day)
16 5
14 4.5
4
12
3.5
10 3
8 Old 2.5 Old
6 New 2 New
1.5
4
1
2 0.5
0 0
Low Temp High Temp Low Temp High Temp

10. Lesion width (mm) Spores per inoculation site per day
4 1800
3.5 1600
3 1400
1200
2.5
1000
2 Old Old
800
1.5 New New
600
1 400
0.5 200
0 0
Low Temp High Temp Low Temp High Temp

11. Shorter latent period x more spores per lesion
+ longer time for disease development
= faster, more severe disease