Seed Infection Rate, but Not Pathogen Titer, Positively Correlates with Disease Index of Cephalosporium Stripe in...

Cephalosporium gramineum survives primarily in colonized plant residue but is also transmitted by seed at a low frequency. The purpose of this study was to correlate disease intensity in the field with percentage of infected seed and amount of pathogen DNA using a high-throughput PCR method. Field-grown seed of three wheat cultivars was collected over 4 years from plots with a known disease index. The culture-based seed infection rate was determined by isolation of C. gramineum from 2,016 seeds per seed lot. DNA of 380 seeds from each seed lot was extracted individually, and a PCR assay with a fluorescent-labeled forward primer for detecting C. gramineum was performed on each seed. C. gramineum was isolated from 0.12% of the seed on average (range 0 to 0.74%), whereas it was detected in 3.7% on average (range 1.3 to 7.6%) using PCR detection. The single-seed PCR assay was more sensitive than either the culture-based method or conventional PCR. DNA of 674 seeds that tested positive by this PCR was quantified using a real-time PCR with newly designed primers for the amount of pathogen per seed. Seed contained 0.017 to 77.1 pg/seed of C. gramineum DNA (mean 3.0 pg/seed). Disease index was positively correlated with seed infection rate but not with pathogen titer in seed. This fluorescent-labeled PCR, along with quantitative PCR, improved our understanding of seed transmission of C. gramineum in wheat.