Oakes Irrigation Research Site
Carrington Research Extension Center * North Dakota State University
P.O. Box 531, Oakes, ND 58474-0531, Phone: (701) 742-2744, FAX: (701) 742-2700,

E-mail: Blaine.Schatz@ndsu.edu Leonard.Besemann@ndsu.edu

Field Evaluation of Sunflower Hybrids and Breeding Lines for Susceptibility to Sclerotinia Head Rot Oakes, ND (2014)

Leonard Besemann and Michael Wunsch

Methods

General agronomics: The study was on a Hecla sandy loam/Maddock sandy loam soil type. The soil fertility results from the fall soil test: pH = 6.8; 2.0% organic matter; soil N 21 lbs/acre; soil P = 20 ppm, soil K = 181 ppm, soil and S 32 lb/acre. The previous crop was field corn and the tillage operation consisted of disking twice followed by two passes with a multiweeder to smooth the seedbed and incorporate the herbicide.

Experimental design: A completely randomized block design with four replicates. The seeded plot size was 2.5 feet (center to center) by 20 feet long. There was one row per plot and the row spacing was 30 inches. Guard plots (10 feet wide) were established along all perimeters of the trial.

Planting details: The sunflowers were planted on May 29 using a cone seeder. The seeding rate was 3.83 seeds/linear foot of row = 60,000 seeds /acre (when sufficient seed was available). The final plant population was 1 plant every 10 inches of row = 21,000 plants/acre. The final population was established by manual thinning the sunflowers at the V2 to V4 growth stage (two to four true leaves).

Disease establishment: Inoculations were conducted over multiple days such that every head was inoculated twice - once at approximately R5.4 to R5.6 (40 to 60% of the disk flowers blooming or already bloomed) and once at approximately R5.5 to R5.9 (50 to 90% of the disk flowers blooming or already bloomed). To ensure that every plant was inoculated twice, plants were given a dot of spray paint on an upper leaf at each inoculation; once a plant received two dots of spray paint, the plant received no additional inoculations.

Spore solutions were prepared by adding laboratory-grown ascospores of Sclerotinia sclerotiorum to non-chlorinated water and adding one to two drops of Tween 20. Hand-held spray bottles were calibrated to determine how much liquid was released through each squirt of the bottle, and the spore solution was adjusted so that 5,000 spores were delivered through 3 squirts of the spray bottle. At each inoculation, 5,000 spores were applied to the front of each head (each head received a total of 10,000 spores over two inoculations).

Supplemental overhead irrigation was applied to this trial through a micro-sprinkler misting system, with the frequency and intensity of irrigation adjusted relative to weather conditions.

Agronomic notes: The flowering date for each plot was the date that 50% of the plants entered bloom.

Disease notes: Within each plot, disease assessments were conducted three times: at the R7 growth stage (back of the head has started to turn a pale yellow color), at the R8 growth stage (back of the head is yellow but the bracts remain green), and at the R9 growth stage (bracts yellow and brown, and heads ready for harvest; physiological maturity). Plants exhibiting damage from sunflower midge were excluded from the analysis; otherwise, all plants in each row were evaluated. A 0 to 5 scale was utilized: 0 = no Sclerotinia head rot, 1 = 1 to 25% of head exhibiting symptoms of Sclerotinia head rot, 2 = 26 to 50% of head exhibiting symptoms of Sclerotinia head rot, 3 = 51 to 75% of head exhibiting symptoms of Sclerotinia head rot, 4 = 76 to 99% of head exhibiting symptoms of Sclerotinia head rot, and 5 = 100% of head exhibiting Sclerotinia head rot. Disease assessments were taken as plots reached the target growth stage; R7 assessments were taken August 25 to September 19, R8 assessments were taken September 5 to 30, and R9 assessments were taken September 10 to October 6.

rAUDPC: The progression of disease over time relative to maximum disease; a rating of 100 equals 100% of plants with heads fully diseased with Sclerotinia head rot from bloom initiation until the R9 growth stage, and a rating of 0 equals 0% of plants showing symptoms of Sclerotinia head rot over the same interval. Calculated with the following formula:

where xi = disease severity index at the ith observation, ti = time in days at the ith observation, and n = number of observations.

Shattered heads: The incidence and severity of head shattering caused by Sclerotinia head rot was assessed on September 10 to October 6 as plots reached the R9 growth stage (bracts yellow and brown, and heads ready to harvest; physiological maturity). Severity was assessed on a 0 to 5 scale, with 0 = no shattering, 1 = 1 to 25% of head shattered, 2 = 26 to 50% of the head shattered, 3 = 51 to 75% of the head shattered, 4 = 76 to 99% of the head shattered, and 5 = 100% of the head shattered. Results were reported as shattering incidence and severity, where SHATTERING INCIDENCE denotes the percentage of heads exhibiting Sclerotinia head rot that were partially or fully shattered when heads reached the R9 growth stage and SHATTERING SEVERITY denotes the average severity of shattering (1 to 5 scale) observed in heads exhibiting Sclerotinia head rot.

This trial was not harvested.

Statistical analysis: Data were evaluated with analysis of variance. Assumptions of ANOVA: (1) The assumption of constant variance was assessed with Levene's test for homogeneity of variances and visually confirmed by plotting residuals against predicted values. (2) The assumption of normality was assessed with the Shapiro-Wilk test and visually confirmed with a normal probability plot. (3) The assumption of additivity of main-factor effects across replicates (no replicate-by-treatment interaction) was evaluated with Tukey's test for nonadditivity. To meet model assumptions, a systematic natural-log transformation was applied to the R7 Sclerotinia head rot incidence data; the R7, R8, and R9 disease severity index data; and the relative area-under-the-disease-progress-curve data. Analysis of variance was conducted on the transformed data. The shattering data exhibited moderate deviations from model assumptions, but a systematic transformation could not be found that addressed the problem, and the untransformed data were analyzed. All other data met model assumptions. Assessment of treatment differences: Analyses were conducted with replicate and treatment as main factor effects. Single-degree-of-freedom contrasts were performed for all pairwise comparisons of isolates; to control the Type I error rate at the level of the experiment, the Tukey multiple comparison procedure was employed. Analyses were implemented in PROC UNIVARIATE and PROC GLM of SAS (version 9.3; SAS Institute, Cary, NC).

FUNDED BY THE USDA NATIONAL SCLEROTINIA INITIATIVE AND SYNGENTA, NUSEED, NUSEED GLOBAL, GENOSYS, MYCOGEN, AND RED RIVER COMMODITIES.

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