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Sweet Corn Disease Nursery
2001 Annual Report
Pataky, Phil Michener, Andrea Campaña
Common rust, northern leaf blight (NLB), Stewart's bacterial wilt, southern
leaf blight (SLB) and gray leaf spot (GLS) can reduce yields of susceptible
and moderately susceptible sweet corn hybrids. These diseases can be managed
more efficiently if reactions of hybrids are known.
Resistance and susceptibility are the two extremes of a continuum of host reactions to diseases. Resistance is a measure of the ability of the host to reduce the growth, reproduction, and/or disease-producing abilities of the pathogen, thus resulting in less severe symptoms of disease. Major genes for resistance, such as Rp, Ht, or rhm, can prevent or substantially limit disease development if specific virulence is not present in pathogen populations. Hybrids with major gene resistance usually have clearly distinguishable phenotypes. Major gene resistance may be ineffective when specific virulence occurs.
In the absence of effective major gene resistance, disease reactions often range from partially resistant to susceptible. Hybrids can be grouped into broad classes such as: resistant (R), moderately resistant (MR), moderate (M), moderately susceptible (MS), and susceptible (S) based on severity of disease symptoms. This procedure produces statistically "overlapping" groups without clear-cut differences between classes (e.g., the hybrid with least severe symptoms in the MR class does not differ significantly from the hybrid with the most severe symptoms in the R class). Nevertheless, a consistent response over several trials produces a reasonable estimate of the disease reaction of a hybrid relative to the response of other hybrids. These reactions can be used to assess the potential for diseases to become severe and affect yield (3).
This report summarizes the reactions of 314 sweet corn hybrids to common rust, NLB, Stewart's wilt, SLB and GLS based on performance in the 2001 University of Illinois sweet corn disease nursery.
Hybrids: Three hundred and fourteen hybrids were evaluated in 2001. This included 145 sh2 hybrids, 95 se hybrids and 74 su hybrids. Hybrids with multiple endosperm mutations or heterozygous reactions were placed in the most appropriate of these three categories. Standard hybrids with relatively consistent reactions to rust, Stewart's wilt and NLB (Table 2) were included to compare the results from the 2001 nursery to those from previous nurseries.
Experimental design and procedures: Each disease was a separate trial
with three replicates of hybrids arranged in randomized complete blocks. Each
trial was split into two main blocks of sh2 and su and se
hybrids. Each experimental unit was one or two 12-ft. rows with about 10 to
18 plants per row. The Stewart's wilt, rust (Rp1-D-avirulent race) and
NLB trials had two-row experimental units. Four trials (Stewart's wilt, rust-avirulent,
NLB and SLB) were planted May 9 at Champaign, IL. The Rp1D-virulent rust
trial was planted May 16 at Urbana, IL. The GLS trial was planted July 6 near
Table 1.  Summary statistics for Stewart's wilt, common rust, NLB, SLB, and GLS ratings.
Inoculation and disease assessment: The four trials planted May 9 were inoculated with: Erwinia stewartii (Stewart's wilt), Rp1-D-avirulent isolates of Puccinia sorghi (i.e., the "old race" of rust), Exserohilum turcicum (NLB), or Bipolaris maydis (SLB). Plants were inoculated with E. stewartii on June 11 and 21 by wounding leaves in the whorl and introducing bacteria into wounds. Suspensions of Rp1-D-avirulent P. sorghi urediniospores were sprayed into plant whorls June 11, 14, 19, 25 and 28. A mixture of conidia of races 0 and 1 of E. turcicum were sprayed into plant whorls on June 8, 13, 20, and 27. Conidial suspensions of B. maydis were sprayed into whorls on June 15, 21, 26 and 28. In the rust trial planted May 16, suspensions of Rp1-D-virulent P. sorghi urediniospores (i.e., the "new race" of rust) were sprayed into plant whorls June 18, 22, 26 and July 2. Plants were infected naturally by Cercospora zeae-maydis in the GLS trial planted July 6.
Disease symptoms were rated on a plot basis. Stewart's wilt was rated on July
26-27 using a scale from 1 (E. stewartii within 2 cm of inoculation wounds)
to 9 (severe systemic infection or dead plants). The percentage of the leaf
area infected by common rust was rated from 0 to 100% on July 30-31 (Rp1-D-avirulent
trial) and August 6 (Rp1-D-virulent trial). Leaf area infected by NLB
was rated from 0 to 100% on July 31 and August 1. Leaf area infected by SLB
was rated on August 2-3. Leaf area infected by GLS was rated on September 20.
Data analysis: Disease ratings were analyzed by ANOVA. Hybrid reactions were classified according to standard deviations from the mean (z-scores), Bayesian least significant difference (BLSD) separations (k=100), and the FASTCLUS procedure of SAS using various groupings of 6 to12 clusters.
Hybrid reactions ranged from very little disease to severe symptoms
(Table 1). Reactions of standard hybrids to Stewart's wilt, rust, NLB, SLB,
and GLS were generally within expected ranges (Table 2). The criteria for classifying
hybrid reactions are listed in Table 3. Table 4 includes reactions and actual
ratings of the 314 hybrids based solely on the 2001 trial.
This is the only data we have for some of these hybrids. For hybrids that have
been evaluated in previous years, a more complete assessment of disease reactions
is presented in another report, "Reactions of sweet corn hybrids to prevalent
diseases - 2001".
Stewart's wilt: Stewart's wilt ratings ranged from 1.9 to 7.3. Yield is affected minimally if infection is non-systemic, i.e., ratings £ 3 (1). Twenty-nine hybrids with ratings 2.5 or below were not different from four hybrids (Green Giant 27, Green Giant 62, HMX 0356 WS and HMX 8343 BS) with the lowest rating (1.9). These hybrids were classified as resistant. Forty hybrids with ratings from 2.5 to 3 were classified between resistant and moderately resistant, i.e., R/MR. Fifty-eight hybrids with ratings from 3 to 3.6 were classified as moderately resistant. Stewart's wilt was rated 5 or above for 63 hybrids classified from moderately susceptible to susceptible.
Table 2. Reactions of sweet corn hybrids included as standards in the 2001 disease nursery
Table 3. Criteria for classifying hybrid reactions to diseases in the 2001 nursery
|Table 4. Reactions of sweet corn hybrids in the University
disease nursery in 2001
|Sugary Hybrids||Sugary Enhancer Hybrids|
|Shrunken-2 Hybrids||Statistical Analysis for all Hybrids|
Common rust: Rust severity ranged from about 0 to 60% in
both trials (Rp1-D-avirulent and Rp1-D-virulent trials). Yield
usually decreases about 0.5% for each 1% leaf area infected by rust, i.e., rust
severity (2). In the trial inoculated with Rp1-D-avirulent isolates,
139 hybrids were Rp-resistant. Rust severity was 1% or less on 120 Rp-resistant
hybrids and between 1% and 5% on 19 Rp-resistant hybrids. In the trial inoculated
with Rp1-D-virulent isolates, severity was 0% (pustules were not observed)
on 9 hybrids (BSS 0977, GH 1829, GH 2042, GH 2298, GH 2303, GH 2385, GH 5703,
GSS 0966 VP and GSS 0978 VP). Apparently, rust resistance in these nine hybrids
is conveyed by an Rp gene other than Rp1-D. Only two hybrids (BSS 0977
and GH 5703) were Rp-resistant in both trials. For 137 of the 139 hybrids that
were Rp-resistant in the Rp1-D-avirulent trial, rust severity ranged
from 17% to 56% in the Rp1-D-virulent trial. Five of these hybrids (Green
Giant 62, Green Giant 82, Green Giant 94, SVR 08302418 and SVR 08705760) had
less than 20% rust in the Rp1-D-virulent trial. For 7 of the 9 hybrids
that were Rp-resistant in the Rp1-D-virulent trial, rust severity ranged
from 10% to 23% in the Rp1-D-avirulent trial. Four hybrids (Green Giant
27, Green Giant 74, Green Giant 75, and PX 9314639) had less than 18% rust in
both trials but did not have an Rp-reaction in either trial. These hybrids have
partial resistance to rust. Nine hybrids (BSS 8142, Encore, Esteem, EX 8414247,
GH 0934, Green Giant 6, Lancelot, Merlin, and Tuxedo) with 10% to 23% rust in
both trials were classified as moderately resistant. In each trial, hybrids
with more than 38% rust were classified as moderately susceptible or worse.
Rust severity on hybrids that did not have Rp-resistance was similar between
the two trials, r = 0.90 (Figure 1).
Northern leaf blight: NLB severity ranged from 2 to 64%. Usually, NLB does not affect yield substantially unless severity is 20% or higher (4). About 60% of the NLB lesions were due to race 0 and about 40% were due to race 1. Infection of hybrids with the Ht1 gene (i.e., chlorotic-lesion resistance to race 0) probably was less severe than if race 1 was more frequent. Severity of NLB averaged 19% for 85 hybrids with the Ht1 or HtN genes and 34% for 229 hybrids without Ht-resistance (Figure 2). Seven hybrids with less than 6% leaf area infected were considered resistant. These included: Day Star, GH 3054, HMX 0356 WS, HMX 0394 S, HMX 8343 BS, Morning Star and Twin Star. Fifteen hybrids with Ht-1-reactions had less than 12% leaf area infected. These hybrids were classified between resistant and moderately resistant (R/MR), and included: BSS 0977, BSS 8142, Crisp n Sweet 710 A, Crisp n Sweet 710 A RR, Crisp n Sweet 710 RR, Green Giant 62, GSS 0966 VP, GSS 3381, HB 1800 NG, Prime Plus, Summer Sweet 7630, Summer Sweet 7631, WSS 9870, Xtra Tender 282A, and Xtra Tender 382A. Thirty-one hybrids with less than 19% leaf area infected were classified moderately resistant. Twenty-two of these 31 hybrids had Ht1-resistance. Nine moderately resistant hybrids that were not Ht-resistant included: 97-6449, BSS 1690, Mystique, PX 9362439, Seneca Nation, Silver Dollar, Summer Sweet 781 Ultra, SVR 8492909, and Tuxedo. Sixty-four hybrids with more than 35% leaf area infected were classified from moderately susceptible to susceptible. Only three of these hybrids, ACX 909, ACX 934 and Lumina, had Ht-1 chlorotic-lesion reactions
Southern leaf blight: SLB ratings ranged from 4% to 59% with a mean
of 23%. Overall, SLB was slightly less severe on sh2-hybrids than on su or se-hybrids.
SLB severity averaged 19% for sh2-hybrids and 26% for su/se-hybrids (Figure
3). Twenty-eight hybrids with SLB ratings 10% or below were classified as
resistant. Twenty-three of these 28 resistant hybrids were shrunken-2. The five
SLB-resistant su/se-hybrids included: Climax, EX 8415137, Green Giant 82, Kandy
Plus, and Merlin. Fifty-nine hybrids with ratings from 10% to 16% were classified
as R/MR of which 43 were sh2-hybrids. Thirty-six hybrids with more than 35%
leaf area infected were classified as moderately susceptible to susceptible.
These included 9 sh2-hybrids: ACX 908, GSS 5771, GSS 0966 VP, Mirai 003, Mirai
005, Prime Plus, Primetime, Xtra Tender 175A, and Xtra Tender 273A.
Gray leaf spot: Naturally-occurring gray leaf spot was very severe in the trial planted July 6 near Havana, IL. Severity of GLS ranged from 30 to 100% with a mean of 72%. Four hybrids with less than 40% leaf area infected were classified as moderately resistant, including: BSS 8142, GH 3054, GSS 3381, and HMX 0356 WS. Twenty-seven hybrids with GLS severity between 40% and 60% GLS were classified as moderate. Forty-two hybrids with GLS severity > 88% were classified as susceptible.
Freeman, N. D., and J. K. Pataky. 2001. Levels of Stewart's wilt resistance necessary to prevent reductions in yield of sweet corn hybrids. Plant Dis. 85:(in press).
Pataky, J. K. 1987. Quantitative relationships between sweet corn yield and common rust, Puccinia sorghi. Phytopathology 77:1066-1071.
Pataky, J. K., and D. M. Eastburn. 1993. Using hybrid disease nurseries and yield loss studies to evaluate levels of resistance in sweet corn. Plant Disease 77:760-765.