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Sweet Corn Disease Nursery
1998 Annual Report
Common rust, northern leaf blight - NLB, Stewart's bacterial wilt, maize dwarf mosaic - MDM, southern leaf blight - SLB, and southern rust can significantly reduce yields of susceptible and moderately susceptible sweet corn hybrids. Resistance and susceptibility to these diseases differs among commercial hybrids (2). The level and type of resistance must be known in order to select hybrids for disease management.
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, HtN, Mdm1, or Rpp) prevent or substantially limit disease development. In the absence of major gene resistance, disease reactions can range from partially resistant to susceptible.
Hybrids with major gene resistance usually can be identified from specific phenotypes, e.g., RP-reactions to rust. Other 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 grouping procedure produces statistically "overlapping" classes of hybrids 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), but consistent responses of a hybrid over several trials gives a good indication of the relative performance of that hybrid with regard to its reaction to a certain disease. This reaction is "relative" because a hybrid is classified from R to S based on its response in comparison to all other hybrids being evaluated.
This report summarizes the responses of 262 commercial sweet corn hybrids to common rust, NLB, Stewart's wilt, MDM, SLB, and southern rust based on reactions in disease nurseries in 1998.
Materials and Methods
Hybrids: Two-hundred-and-sixty-two hybrids and 15 other lines were evaluated in 1998. Hybrids evaluated in 1998 ranged from 60 to 100 days in maturity. Standard hybrids with consistent reactions to rust, Stewart's wilt, and NLB (Table 2) were included to compare the results from trials in 1998 to those from previous nurseries.
Experimental design and procedures: Each disease was a separate trial with three replicates of each hybrid arranged in randomized complete blocks. Each hybrid was planted in a single 10 ft. row with approximately 12 plants in each replicate. Each trial was split into two main blocks of sh2 or su and se hybrids. Four trials (common rust, NLB, Stewart's wilt and SLB) were planted May 17 at Champaign, IL. Late trials (southern rust and MDM) were planted June 30.Table 1. Summary statistics for common rust, NLB, Stewart's wilt, MDM, SLB, and southern rust ratings
Inoculation and disease assessment: Five of the six trials were inoculated with one of five pathogens: Puccinia sorghi (common rust), Exserohilum turcicum (NLB), Erwinia stewartii (Stewart's wilt), Bipolaris maydis (SLB), or Puccinia polysora (southern rust). Multiple inoculations were necessary to insure sufficient spread of pathogens in June and July. Urediniospore suspensions of P. sorghi were sprayed into plant whorls on June 15, 19, 24, and 29. Plants were inoculated with a mixture of races 0 and 1 of E. turcicum on June 15 and 23 by spraying a conidial suspension into whorls. Plants were inoculated with E. stewartii on June 16, 23, and 29 by wounding leaves in the whorl and introducing bacteria into wounds. Conidial suspensions of B. maydis were sprayed into whorls on June 18 and 24, and July 1 and 8. Urediniospores of P. polysora were sprayed into whorls on July 16 and 22. Plants in the MDM trial were infected naturally by aphids vectoring the MDM virus.
Standard diagrams were used to rate diseases (Figure 1). Stewart's wilt was rated on July 6-8 using a 1 to 9 scale. The percentage of the leaf area infected by common rust was rated from 0 to 100% on July 28-29. The percentage of the leaf area infected by NLB was rated from 0 to 100% on July 29-31. The number of plants per plot with MDM symptoms was counted on August 3-4 and converted to a percentage. SLB was rated from 1 (small, chlorotic flecks; no secondary spread) to 9(large, necrotic lesions; abundant secondary spread) on August 4-5. Leaf area infected by southern rust was rated from 0 to 9 on August 25-28.
Data analysis: Disease ratings were analyzed by ANOVA. Hybrid reactions to each pathogen were classified according to standard deviations from the mean (z-scores) and by Bayesian least significant difference (BLSD) separations (k=100).
Results and Discussion
Hybrid reactions ranged from very little disease to severe symptoms (Table 1). Reactions of standard hybrids to rust, Stewart's wilt and NLB were generally within the expected range (Table 2). The criteria for classifying hybrid reactions are listed in Table 3. Table 4 gives the reactions, actual ratings, and ranks of the 262 hybrids based solely on the 1998 trial. This is the only data we have for some hybrids. A better assessment of reactions of hybrids which have been evaluated in previous years is presented in the 1998 report of "Reactions of sweet corn hybrids to prevalent diseases" (1).
Rust: Rust ratings ranged from 0 to 38% leaf area infected. Rust pustules were not observed on about 30% of the materials in the trial, including 78 hybrids and 6 other lines which had RP-resistant reactions. Most of the RP-hybrids carry the gene Rp1D, but some may have other effective RP genes, such as Rp1I, Rp3C, or compound RP-genes Of the hybrids that were not RP-resistant, those with less than 18% leaf area infected were grouped in the R-MR class (2), including: Early Cogent, Fleet, Green Giant Code 6, GG Code 24, GG Code 61, GG Code 63, and XPH 3123.
Northern leaf blight: NLB ratings ranged from 3 to 81% with a mean of 34%. Four sh2 hybrids (Day Star, Morning Star, HMX 6364 BS, and UFB 4439) had less than 10% of the leaf area infected and appeared to have HtN-resistance to NLB. Development of NLB lesions on HtN-hybrids usually is delayed until after pollination. Fourteen hybrids had less than 20% NLB severity and were classified as R-MR (2), including ten sh2 hybrids (BSS 1605, BSS 8142, BSS 9686, GSS 4606, Prime Plus, Summer Sweet 7620, Summer Sweet 7630, Summer Sweet 7631, Summer Sweet 7710, and Ultimate), and two se hybrids (Mystique and Seneca SV 7403), and a bt hybrid, Waimanalo Supersweet which is adapted for the subtropics. Sixty-seven hybrids had chlorotic lesions indicative of Ht1-reactions to Exserohilum turcicum race 0. Some hybrids with Ht1-reactions were classified from moderate to moderately susceptible (5 to 9) because of the severity of NLB caused by E. turcicum race 1.
Table 2.   Reactions of sweet corn hybrids included as standards in the 1998 disease nursery
Stewart's wilt: Stewart's wilt ratings ranged from 1.1 to 8.3 with a mean of 3.5. Sixteen hybrids were rated 2.0 or below and were given a Stewart's wilt reaction of 1 (R), including eight su hybrids (Bonus, Eliminator, Green Giant Code 59, GG Code 63, GH 0934-A, GH 0937-A, GH 2783, and GH 7419), six se hybrids (Ambrosia, Merlin, Miracle, Pristine, Seneca SV 7403 B, and Seneca SX 6502 W), one heterozygous se hybrid, Terminator, and one sh2 hybrid, GSS 9299. Twenty-five hybrids rated between 2 and 2.75 were given a disease reaction of 3 (MR).
Maize dwarf mosaic: Incidence of MDM-infected plants averaged 57% for the entire trial and ranged from 2 to 97% among hybrids. Hybrids that have the gene Mdm1 or other major genes for MDM-resistance had a low percentage of infected plants. Some hybrids that were susceptible to MDM in previous years also had low percentages of infected plants probably because they escaped infection. Twenty-three hybrids with fewer than 20% infected plants were given an MDM reaction of 1 (R) although some probably were "escapes". Ten hybrids with less than 20% infected plants in 1998 were classified R to MR for MDM in previous trials, including four su hybrids (Bonus, Eliminator, HMX 5371, and XP 8410357), four sh2 hybrids (Bandit, HMX 5375, HMX 6383 S, and Swifty), and two se hybrids (HMX 5349 WES-R and Topacio). Thirty-four hybrids with 20 to 35% MDM-infected plants were given an MDM reaction of 3 (MR). Ten of those thirty-four hybrids were rated R to MR for MDM in previous years, including five sh2 hybrids (Candy Corner, HMX 0381, HMX 3392 S, HMX 5376 S, and Peter 235), two sweet-breed hybrids (HMX 6357 SB and Sweet Chorus), one se hybrid (Vanguard), one heterozygous se hybrid (Terminator), and one su hybrid (HMX 5372). These 20 hybrids rated R or MR in this and previous trials probably have major genes for MDM-resistance. Hybrids that probably "escaped" infection in 1998 based on susceptible reactions in previous years included: Criterion, Frosty, GG Code 6, and Quickie. Hybrids with more than 50% MDM-infected plants were given an MDM reaction of 9 (S).
Table 3. Criteria for classifying hybrid reactions to diseases in the 1998 nursery
Southern leaf blight: SLB ratings ranged from 1.0 to 6.3 with a mean of 3.2. Forty-nine hybrids with SLB ratings 2 or below were classified as R, including 47 sh2 hybrids, one se hybrid (Merlin), and one su hybrid (GG Code 24). One-hundred-four hybrids rated between 2 and 3.2 were classified MR.
Southern rust: Southern rust ratings ranged from 0 to 8.8 with a mean
of 6.9. Two sh2 hybrids, HMX 5375 S and HMX 5376 S, had no southern rust
pustules and appeared to carry an Rpp gene for resistance. These hybrids also
were RP-resistant to common rust. Most of the other 260 hybrids in the trial
were severely infected with southern rust. Two hybrids (BSS 8142 and Waimanalo
Supersweet) were rated lower than 5 and classified MR. Eight hybrids rated from
5 to 6 were given a southern rust reaction of 5 (M), including 6 sh2
hybrids (EX 8414667, EX 8414717, HMX 6383 S, Morning Star, Shimmer, and Trigger)
and 2 se hybrids (EX 8410017 and Tablemaster).
Pataky, J. K., L. J. du Toit, and D. M. Eastburn. 1997. Reactions of sweet corn hybrids to common rust, northern leaf blight, Stewart's wilt, anthracnose leaf blight, southern leaf blight, maize dwarf mosaic, and southern rust. Pg. 112-128, In: Midwestern Vegetable Variety Trial Report for 1997. Purdue Univ. Agric. Exp. Sta. Bull. No. 758.
We thank Rachel Goldstein, Jamie Howard, Jenn McAlister, Kim Nicol, Matt Poppe, Michelle Short, and Randy Zimmerman for their assistance with these trials.
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