Evaluation location: Iowa, United States
Six hundred and twenty-seven maize accessions were tested for resistance to Stewart's wilt, caused by Pantoea (Erwinia) stewartii. Plots were planted on 05-May-2003 at the NCRPIS farm at Ames, IA. Accessions were grown in three replicate plots with 15-18 plants per plot (row). Inoculations were done by the pinprick method on 11-JUL-2003 at the 6 to 8-leaf stage and again on 25-JUL-2003. Inoculum was produced from a mixture of two P. stewartii isolates (Iowa origin). Ratings were done between 13-AUG-2003 and 25-SEP-2003, based on plant maturity (silks brown and dry). Individual plant scores (10-12 plants) were averaged to obtain a disease rating for each plot. Some plots were lost due to spring flooding. A composite disease average was calculated remaining plots of each accession. Disease reactions were scored on a 1-9 scale where: 1= no appreciable spread of symptoms from pinpricks (within 1 cm); 2= limited spread of lesions from pinpricks (within 3 cm); 3= limited spread from pinpricks with water-soaking, chlorosis, or necrosis; spread occurring mainly toward tips of leaves; 4= abundant spread from pinpricks with water-soaking, chlorosis, or necrosis toward both ends of inoculated leaves, but no evidence of systemic infection; 5= lesions spread to the main stalk; minimal systemic infection observed; 6= moderate systemic infection with 5-25% symptomatic leaf area on non-inoculated leaves; 7= abundant systemic infection and stunting with 25-50% of the leaf area symptomatic; 8= severe systemic infection and stunting with 50-90% of the plant infected; and 9= 90-100% infection, severe necrosis or dead. The checks were B37Ht (mod. res.), A632 (mod. susc.), and CM105 (susc.) with average ratings of 3.3, 4.3, and 4.5, respectively. The late summer drought and corn borer infestation halted disease progress, resulting in noticeably less systemic infection in the susceptible accessions.