Evaluation location: Iowa, United States
Two hundred and twenty-two maize accessions representing popcorn germplasm from the NCRPIS collection were evaluated in 2000 for resistance to Stewart's bacterial wilt. Plots were planted on 11-May-2000 at the NCRPIS farm, Ames, IA. The trial included two replicates of single rows of approximately 15 plants per accession. Rows were spaced 0.92 m apart. Plants at the 8 to 10-leaf stage were inoculated with Erwinia stewartii on 29-JUN-2000 and 30-JUN- 2000 in the first and second replicates respectively using the pinprick method. Ten to twelve plants were inoculated per row, skipping two plants on each end. All plants were inoculated in plots where the stand count was 12 or less. Inoculum was produced from a mixture of two E. stewartii isolates, SS104 and ES9239, grown for 24 hrs on nutrient dextrose agar plates. Ratings were made between 25-AUG-2000 and 31-AUG-2000 on eight plants per row. Individual plant scores were averaged to obtain a disease rating for each plot. Scores for the two replications were averaged to obtain the overall rating for each accession. Stewart's wilt reactions were rated on a 1-9 scale where: 1 = no appreciable spread of symptoms from infection sites; lesions within 1 cm of pinpricks; 2 =limited spread from infection sites, lesions within 3 cm of pinpricks; 3 = limited spread from infection sites, predominantly toward tips of leaves; 4 = abundant lesion growth from infection sites, toward both ends of leaves; non-systemic; 5 = limited systemic infection, 15-25% of leaf area affected on inoculated leaves; 6 = Chlorosis and necrosis of 25-50% of leaf area on inoculated leaves; moderate systemic infection; 7 = Severe chlorosis and necrosis of 50-75% of leaf area on inoculated leaves; abundant systemic infection; 8 = Severe chlorosis and necrosis of 75-90% of leaf area on inoculated leaves; severe systemic infection; 9 = 90-100% infection, severe necrosis or dead plant.