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Sweet Corn Disease Nursery

Reactions of Sweet Corn Hybrids
to Prevalent Diseases

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Jerald K. Pataky
Department of Crop Sciences, University of Illinois, Urbana, IL 61801
Revised October 2001

Common rust (Puccinia sorghi), northern leaf blight - NLB (Exserohilum turcicum), Stewart's bacterial wilt (Erwinia stewartii), maize dwarf mosaic - MDM (MDM viruses), southern leaf blight - SLB (Bipolaris maydis), anthracnose leaf blight - ALB (Colletotrichum graminicola), southern rust (Puccinia polysora) and gray leaf spot - GLS (Cercospora zeae-maydis) are endemic diseases of sweet corn grown in North America. Reactions to these diseases vary among commercial sweet corn hybrids. Since 1984, nearly 2,300 hybrids have been evaluated for disease reactions in nurseries at the University of Illinois at Urbana-Champaign.. Hybrids were classified as resistant (R), moderately resistant (MR), moderate (M), moderately susceptible (MS), and susceptible (S) based on standard deviations from trial means (z-scores), separations based on multiple comparison tests (BLSD), and multivariate clustering procedures. This type of classification produces statistically "overlapping" groups without clear-cut differences between classes (e.g., the hybrid with the 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 of a hybrid over several trials gives a reasonable estimate of the disease reaction of that hybrid relative to all other sweet corn hybrids. These reactions can be used to plan disease management strategies by assessing the potential for diseases to become severe and affect yield of a specific hybrid.

Results of the annual disease nurseries have been reported each year in the Midwestern Vegetable Variety Trial Report. They are also available on the web at This article summarizes the disease reactions of 660 commercially-available hybrids that that have been evaluated in UIUC nurseries since 1984. The summary includes reactions of 124 sugary hybrids (114 yellow, 4 bi-color and 6 white), 195 sugary enhancer hybrids (76 yellow, 80 bi-color, 37 white, and 2 red), 334 shrunken-2 hybrids (173 yellow, 105 bi-color, 55 white and 1 multi-color), and 7 brittle hybrids (5 yellow, 1 bi-color and 1 white).

Reactions of Sweet Corn Hybrids in the University of Illinois Disease Nursery
Table 1: Yellow Sugary Table 6: Yellow Shrunken
Table 2: Bi-Color/White Sugary Table 7: Bi-Color Shrunken
Table 3: Yellow Sugary Enhancer Table 8: White and Multi-Colored Shrunken
Table 4: Bi-Color Sugary Enhancer Table 9: Yellow/Bi-Colored/White Brittle
Table 5: White and Red Sugary Enhancer  

General Information

Details about specific trials are reported annually in the Midwestern Vegetable Variety Trial Report. Plants usually were inoculated with P. sorghi, E. turcicum, E. stewartii, C. graminicola, B. maydis, P. polysora, or C. zeae-maydis to insure uniform disease pressure. Evaluations of reactions to MDM and systemic Stewart's wilt were based on natural infection when disease pressure was relatively uniform.

In this summary, hybrids are grouped by: i) endosperm phenotype (sugary, sugary enhancer, shrunken, and brittle); ii) kernel color (yellow, bi-color, white, and other), and iii) alphabetically by hybrid name. Hybrids with multiple endosperm mutations (e.g., sweet breeds, triples sweets, extra tenders) are placed under the most appropriate of the four categories of endosperm phenotypes. The table includes: seed source (SC - company which entered the hybrid in the nursery), endosperm type (ET), kernel color (KC), relative maturity (RM) from 1 to 5 as reported by each seed company (where 1 = first early, 2 = second early, 3 = mid-season, 4 = main season, and 5 = full season), hybrid name, and reactions to diseases: common rust, NLB, Stewart's wilt, MDM, SLB, ALB, southern rust, and GLS.

Disease reactions are averaged over all trials in which a hybrid was evaluated and are presented on a 0 to 9 scale, where: 0 = no disease, 1 = resistant, 3 = moderately resistant, 5 = moderate, 7 = moderately susceptible, and 9 = susceptible. Reactions listed as 2, 4, 6, 8, are between these categories (e.g., 2 indicates a reaction between resistant and moderately resistant, R-MR). Hybrids with Rp-reactions to rust are noted with an asterisk or the letter 'D' or 'X'. The number of trials in which a hybrid was evaluated for each disease is noted with a superscript. For example, the reaction of the yellow sugary hybrid Bold to northern leaf blight is listed as 4 3. This indicates that the NLB reaction of Bold relative to all sweet corn hybrids is between moderately resistant and moderate and this average is based on 3 trials at UIUC in which Bold was evaluated.

Interpretation of Disease Reactions

Resistance and susceptibility are the two extremes of a continuum of host reactions to diseases. Resistance measures the ability of the host to reduce the growth, reproduction, or disease-producing abilities of the pathogen, thus resulting in less severe symptoms. 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 have clearly distinguishable phenotypes. Major gene resistance may be ineffective when specific virulence occurs. For example, a biotype of the P. sorghi virulent on hybrids with the Rp1-D resistance gene has occurred throughout North America since 1999. Therefore, rust pustules were observed in 2000 and 2001 on hybrids with Rp1-D-resistance.

If hybrids do not have effective major gene resistance, disease reactions often range from partially resistant to susceptible. Since our nurseries have included most of the sweet corn hybrids available commercially, our ratings reflect the disease reactions of a hybrid relative to all sweet corn.

Hybrids with resistant reactions (1) were among the best hybrids evaluated in our trials in the past 18 years. Those rated 9 were susceptible (S) and were among the worst. The moderately resistant ratings (3) included hybrids with disease reactions that were better than average. The moderately susceptible ratings (7) include hybrids that were worse than average. Hybrids with moderate ratings (5) had average reactions. Classification of hybrid reactions can vary among years (e.g., some hybrids may have been classified M in one trial and MR or MS in another trial due to random variation). Average reactions based on at least three years of data are probably more accurate estimates than those based on one or two years.

Symptoms will occur on hybrids rated R, but the amount of the disease on these hybrids is less than the amount on hybrids rated MR ,M, MS, or S. Similarly, the effects of diseases on yield are related to this scale of hybrid reactions.

Common rust. Rust reactions are complex now because of the new race that is virulent against Rp1-D and a few other Rp-resistance genes. If a hybrid does not have Rp-resistance, the rust rating is given in the first column. For example, the reaction of Bonanza to common rust is listed moderately susceptible and susceptible (8) and this is based on 3 trials. Hybrids that do not have Rp-resistance have the same reaction to the old and new races of rust.

Rp-resistance is noted in the first column with a letter or an asterisk. The letter 'D' designates hybrids with resistance conveyed by the Rp1-D gene or another Rp gene that is ineffective against the new race. Hence these hybrids have a rating of 0 (i.e., no rust) when the old race is present; but they are infected by the new race. For example, Bold has Rp1-D resistance (based on 5 trials). It was not infected by the old race of rust in 3 trials (03 for the old race) but it was susceptible in 2 trials when the new race was present (02 for the new race). Rp-resistant hybrids with an asterisk (*) probably have the Rp1-D gene, but they have not been evaluated in trials when the new race is present. Hybrids with an Rp-reaction designated 'X' have Rp-resistance that is effective against the new race. Many of these hybrids may have resistance conveyed by the Rp1-E, Rp1-I, Rp1-K or Rp-G genes of by compound rust resistance genes (e.g., Rp-GI or Rp1-JFC), but it is impossible from these trials to determine exactly which Rp gene conveys resistance to these hybrids. Some populations of rust may infect some of these other Rp genes. For example, the hybrid GH 2042 was Rp-resistant in one trial when the new race was present, but it was infected by rust and had a moderately resistant (3) reaction in one trial when the new race was not present.

Two-hundred and twenty of the 648 hybrids evaluated for common rust have Rp-resistance. Rp-resistance in 171 of these hybrids is conveyed by a gene (probably Rp1-D) that is not effective against biotypes of P. sorghi that are virulent against Rp1-D. These are designated with the letter "D". Hence they have different reactions depending on whether or not virulence is present in the population of P. sorghi. Thirty-nine hybrids were Rp-resistant against the old race but have not been evaluated against the new race (designated with an asterisk). Ten hybrids were resistant against the new race (designated with an 'X'). Of the 428 hybrids that did not have Rp-resistance, 44 had R-MR or MR reactions and 129 had reactions that ranged from MS to S. Among the 181 Rp-resistant hybrids that have been evaluated against the new race, 10 are Rp-resistant, 28 have MR or R-MR reactions, and 31 have reactions that range from MS to S.

Northern leaf blight. Of 649 hybrid evaluated for NLB, 126 have reactions that range from R to MR. Many of these 126 hybrids have NLB resistance conveyed by the Ht genes, Ht1 or HtN. Yield of hybrids with R to MR reactions will be affected very little by NLB unless environmental conditions are extremely favorable for disease development. NLB reactions of 170 hybrids range from MS to S. Yield of these hybrids can be reduced substantially by NLB.

Stewart's wilt. Of 654 hybrids evaluated for Stewart's wilt, 165 have R to MR reactions that should limit systemic infection under most situations although some systemic infection and yield reduction may occur on these hybrids if flea beetles feed on emerging seedlings. This usually occurs under extremely dry conditions when populations of flea beetles are very large. Stewart's wilt reactions of 376 hybrids range from M to S. Stewart's wilt infection can be systemic on these hybrids with a higher incidence and severity of systemic infection for hybrids that are more susceptible.

Maize dwarf mosaic. Only 40 of 419 hybrids evaluated for MDM have reactions that are MR or better. Another 29 hybrids have reactions that range from MR-M to M. The majority (268 of 419) of the hybrids evaluated are susceptible to this viral disease. Although reactions of hybrids to various strains of MDM have not been differentiated, MDM resistance in some hybrids may differ between MDMV-A and SCMV-MB (i.e., MDMV-B).

Southern leaf blight. Over a third (248 of 634) of the hybrids evaluated have R to MR reactions to SLB. Only 68 hybrids have reactions that range from MS to S.

Anthracnose leaf blight, southern rust, and gray leaf spot. Fewer hybrids have been evaluated for reactions to ALB, southern rust and GLS than for other diseases. With the exception of one hybrid (Sure Gold) that has Rpp-resistance to southern rust, none of the hybrids are highly resistant to southern rust or GLS. In fact, the reactions of most hybrids to these two diseases range from M-MS and S. Of 235 hybrids evaluated for ALB, 48 have reactions from MS to S and 71 have reactions from R to MR.

Table 1. Reactions of commercially-available hybrids to diseases in the
UI disease nurseries from 1984 to 2001.

  Number of hybrids in each reaction category
RP Resistant Moderately resistant Moderate Moderately susceptible Susceptible
0 1 2 3 4 5 6 7 8 9
Common rust 220 0 15 29 51 98 106 73 25 31
  Rp1-D-virulent * 10 0 3 25 35 34 43 20 5 6
NLB   9 42 75 95 154 104 74 63 33
Stewart's wilt   19 49 97 119 125 92 67 48 44
MDM   11 9 20 9 20 4 33 45 268
SLB   41 70 137 129 108 71 42 17 19
AlB   16 16 39 26 68 22 26 13 9
Southern rust 1 0 0 0 1 8 56 53 54 82
GLS   0 1 10 53 62 171 45 37 34
Classification: 0 - Rp-resistance, 1 - resistant, 3 - moderately resistant, 5 - moderate, 7 - moderately susceptible, 9 - susceptible.
* Reactions of Rp-resistant hybrids to biotypes of P. sorghi virulent against the Rp1-D gene.

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