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Sweet Corn Disease Nursery
2000 Annual Report
J.K. Pataky, P.M. Michener, N.D. Freeman, and M.C. Pate
Common rust, northern leaf blight (NLB), Stewart's bacterial wilt, maize dwarf mosaic (MDM), and southern leaf blight (SLB) can reduce yields of susceptible and moderately susceptible sweet corn hybrids. Disease management can be improved 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 Mdm1) can prevent or substantially limit disease development if specific virulence is not present in pathogen populations. Hybrids with major gene resistance usually are identified from specific 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 (2).
This report summarizes the reactions of 247 sweet corn hybrids to common rust, NLB, Stewart's wilt, MDM and SLB based on performance in the 2000 University of Illinois sweet corn disease nursery.
Materials and Methods
Hybrids: Two-hundred-and-forty-seven hybrids were evaluated in 2000. This includes 115 sh2 hybrids, 126 se or su hybrids and 6 bt1 hybrids. Maturity ranged from about 60 to 110 days. Standard hybrids with relatively consistent reactions to rust, Stewart's wilt, and NLB (Table 2) were included to compare the results from the 2000 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 bt1 or su and se hybrids. Each experimental unit was a single 12-ft. row with about 8 to 20 plants. Three trials (NLB, Stewart's wilt, and SLB) were planted May 15 at Champaign, IL. Four additional trials were planted in Champaign or Urbana on June 6 (common rust and Stewart's wilt), June 19 (Stewart's wilt and MDM), June 29 (Stewart's wilt and MDM), and July 6 (common rust, Stewart's wilt and MDM).
Table 1.   Summary statistics for common rust, NLB, SLB, Stewart's wilt, and MDM ratings
Inoculation and disease assessment: The three trials planted May 15 were inoculated with one of three pathogens: Exserohilum turcicum (NLB), Erwinia stewartii (Stewart's wilt), or Bipolaris maydis (SLB). A mixture of conidia of races 0 and 1 of E. turcicum were sprayed into plant whorls on June 13, 19, 22, 27 and 30. Plants were inoculated with E. stewartii on June 12, 15, and 27 by wounding leaves in the whorl and introducing bacteria into wounds. Conidial suspensions of B. maydis were sprayed into whorls on June 14, 21, and 28. Plants in the other four trials were infected naturally. Common rust was prevalent in the trials planted June 6 and July 6. Plants in the trials planted June 19 and 29 and July 6 were infected with MDM viruses. Stewart's wilt occurred naturally in all four late-planted trials.
Disease symptoms were rated on a row basis. The percentage of the leaf area infected by common rust was rated from 0 to 100% on August 4-5 (June 6 trial) and August 30-31 (July 6 trial). Leaf area infected by NLB was rated from 0 to 100% on July 30-31. Stewart's wilt was rated in the inoculated trial on July 10-14 (seedling wilt phase) using scale from 1 (E. stewartii within 2 cm of inoculation wounds) to 9 (severe systemic infection or dead plants). SLB was rated on August 1-2 using a scale from 1 (small, chlorotic lesions; little secondary spread) to 9 (large, necrotic lesions; abundant secondary spread). The number of plants with MDM symptoms was counted in the naturally-infected MDM trials on July 24-25 (June 19 trial), August 1-2 (June 29 trial) and August 2-3 (July 6 trial). The number of plants systemically infected with E. stewartii was counted on July 10-11 (June 6 trial); July 11-12 (June 19 trial - early seedling stage); July 20-21 (June 19 trial - 6-to 8-leaf stages); July 30-31 (June 29 trial); and August 4-5 (July 6 trial). Incidence (%) of plants with MDM or systemic Stewart's wilt symptoms was calculated as: (number of symptomatic plants / stand counts) *100.
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 4 to 12 clusters.
Results and Discussion
Hybrid reactions ranged from very little disease to severe symptoms (Table 1). Reactions of standard hybrids to rust, NLB, Stewart's wilt, SLB, and MDM were generally within expected ranges except for the occurrence of rust on hybrids with Rp-resistance (Table 2). The criteria for classifying hybrid reactions are listed in Table 3. Table 4 includes reactions and actual ratings of the 247 hybrids based solely on the 2000 trial. (This table has been divided into 2 sections to attempt to make it quicker to download, Table 4-1 includes all data for the su/se trial, and Table 4-2 includes all data for the sh2 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 reactions is presented in another report, "Reactions of sweet corn hybrids to prevalent diseases - 2000" (1).
Table 2. Reactions of sweet corn hybrids included as standards in the 2000 disease nursery
Table 3. Criteria for classifying hybrid reactions to diseases in the 2000 nursery
Table 4-1. Reactions of sweet corn hybrids in the University of Illiinois disease nursery in 2000. (su/se trials)
Table 4-2. Reactions of sweet corn hybrids in the University of Illiinois disease nursery in 2000. (sh2 trials)
Common Rust: Rust severity ranged from 2 to 57% leaf area infected with a mean of 24%. Rust pustules were observed on all hybrids including those with Rp-resistance although many Rp-resistant hybrids had fewer and smaller pustules than non-Rp hybrids. Rust severity was less than 5% for three hybrids (BSS 0977 VPA, GSS 0966 A, and GSS 0978 A) which have Rp-resistance based on the Rp1-I gene. Twenty of 21 hybrids with rust severity between 5 and 10% had Rp-resistance based on the Rp1-D gene. Apparently, P. sorghi biotypes with virulence against Rp1-D (3) comprised a relatively low percentage of the rust population thereby allowing Rp1-D to provide some control. Rust severity was about half as much on the Rp1-D version of a hybrid as on the non-Rp version of the same hybrid (e.g., 15% vs 25% for Bodacious, 15% vs 36% for Crisp n Sweet 710A, 10% vs 24% for Incredible, 16% vs 32% for Primetime/Prime Plus, and 14% vs 33% for Summer Sweet 8102). Although these differences were observed in both trials, they may not occur if P. sorghi biotypes with virulence against Rp1-D comprise a larger portion of the rust population. Sixty-four hybrids with more than 33% leaf area infected were classified as moderately susceptible to susceptible. Rust severity was greater than 40% on 17 of these hybrids.
Northern leaf blight: NLB severity ranged from 6 to 53% with a mean of 24%. For many hybrids, NLB was rated from two replicates due to flooding in the third replicate. Therefore, hybrid reactions to NLB may be less accurate than in previous nurseries. Also, about 80% of the NLB lesions were due to race 0 and about 20% were due to race 1 This resulted in less severe infection of hybrids with the Ht1 gene (i.e., chlorotic-lesion resistance to race 0) than would have occurred if race 1 was more frequent. Severity of NLB averaged 19% for 82 hybrids with the Ht1 or HtN genes and 31% for 163 hybrids without Ht-resistance. Ten hybrids with less than 10% leaf area infected were considered to be resistant. These include: BSS 8142, Crisp n Sweet 710A, Crisp n Sweet 710A-RR, Day Star, GSS 3381, Hi37 x Hi36c, HMX 8343 BS, Summer Sweet 7630, Summer Sweet 7710, and Xtra Tender 182A. Fifteen hybrids with 10 to 15% NLB infection were considered to be resistant/moderately resistant. These include: ba11 x 190, Big Time, Boreal, BSS 0977 VP A, Envy, GH 3054, Green Giant Code 62, GSS 0966 A, GSS 5771, Prime Plus, Seneca PX 8201, Seneca PX 9364169, Summer Sweet 7631, Twin Star, and Xtra Tender 282A. Nineteen of these 25 hybrids were rated resistant or moderately resistant (1 to 3) in previous nurseries; two were rated MR/M (4) previously; and four had not been evaluated before this trial. Thirty-seven hybrids with 15 to 20% leaf area infected were considered to be moderately resistant in the 2000 trial. Forty-eight of the 62 hybrids classified from resistant to moderately resistant in the 2000 trial had either the Ht1 or HtN gene.
Stewart's wilt: Stewart's wilt ratings at the 6- to 8-leaf
stage in the inoculated trial ranged from 1.7 to 5.4 with a mean of 3. Incidence
of systemically infected plants calculated from all four naturally-infected,
late-planted trials averaged 13% and ranged from 0 to 56% among hybrids. Incidence
of systemic Stewart's wilt averaged 5%, 10%, 12%, 15% and 25% from five ratings
of the four trials. Severity of Stewart's wilt (seedling ratings) and incidence
of systemically infected plants were correlated, r=0.68. Hybrids were classified
for Stewart's wilt reactions based on a cluster analysis of severity and mean
incidence ratings. Nine hybrids were classified as resistant based on severity
rated 2 or below and incidence of 5% or below. These included: Ambrosia, Bonus,
Millennium, Miracle, Seneca Nation, SVR 8492239, Ultimate, Xtra Tender 179A,
and Xtra Tender 271A. Forty hybrids with ratings below 2.5 and incidence below
11% were classified as R/MR. Thirty-eight of the 49 hybrids rated resistant
or R/MR had been evaluated in previous nurseries, and 35 had been classified
from resistant to moderately resistant. Thirty-seven hybrids with Stewart's
wilt ratings below 2.9 and incidence of systemic infection below 16% were classified
as moderately resistant. Stewart's wilt severity usually was above 4 and incidence
of systemic infection was above 20% for 41 hybrids classified as moderately
susceptible to susceptible.
Southern leaf blight: SLB ratings ranged from 1 to 8 with a mean of 3. Thirteen hybrids with SLB ratings below 2 were classified as resistant. These included: 217 x ba11, ba11 x 190, Climax, Crisp n Sweet 710A, Crisp n Sweet 710A-RR, EX 8410057, Millennium, Seneca PS 7404, Starship II, Summer Sweet 8100, Summer Sweet 8102, Summer Sweet 8102R, and WSS 1921. Seventy hybrids with ratings from 2 to 2.5 were classified as R/MR. Thirty-nine hybrids rated between 2.5 and 3 were classified as moderately resistant. Sixteen hybrids rated 5 and above were classified as moderately susceptible to susceptible.
Maize dwarf mosaic: Incidence of MDM-infected plants averaged 30% for the trial planted June 19 in which plants were infected predominantly by MDMV-A. Incidence averaged 82% for the two trials planted June 29 and July 6 in which plants were infected predominantly by SCMV-MB (i.e., MDMV-B). Incidence ranged from 0 to 75% among hybrids in the MDMV-A trial and from 17 to 100% in the SCMV-MB trials. Viruses other than MDMV and SCVM-MB may have caused symptoms similar to those of MDM especially in the two later-planted trials (June 29 and July 6). Thirty-one hybrids with less than 5% incidence of infected plants in the MDM trial were classified as resistant. Thirteen of these hybrids also were classified from moderately resistant to resistant in the SCM trials based on an average incidence below 50%. These include: Bonus, Eliminator, GH 3054, GH 4809, Green Giant Code 75, HMX 8343 BS, Millennium, Rustler, SVR 08705760, SVR 08705774, Topacio, ba11 x KbtL13, and ba11 x 190. Ten hybrids classified as resistant to MDM were classified as MR/M or moderate for SCM with an average incidence between 50 and 60%. These include: 217 x ba11, El Toro, GH 0934 A, GH 2783, HMX 8392 S, Morning Star, SVR 08302389, SVR 8482598, Snow White, and Twin Star. Eight hybrids with less than 5% incidence in the MDM trial were rated from M/MS to susceptible in the SCM trials based on incidence above 60%. Seven hybrids with 5 to 10% incidence in the MDM trial were classified as moderately resistant if incidence in the SCM trial was below 80%. Five hybrids with 10 to 20% incidence in the MDM trial were classified as MR/M or moderate if incidence in the SCM trials was less than 60%. Twenty-three hybrids that were classified from resistant to MR/M in the SCM trials also were classified as resistant to MR/M in the MDM trial. Four hybrids that were a moderate in the SCM trials were resistant in the MDM trial; and five that were moderate in the SMC trials were susceptible in the MDM trial. Most of the hybrids that were classified as resistant to moderate in the MDM trial probably have the Mdm1 gene or other genes for MDM resistance. Hybrids that also were classified as resistant to moderate in the SCM trials probably have additional genes for resistance to viruses that cause MDM symptoms.
Pataky, J. K., 2000. Reactions of sweet corn hybrids to prevalent diseases - revised October 2000. In: Midwestern Vegetable Variety Trial Report for 2000. Purdue Univ. Agric. Exp. Sta. Bull. No.
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.