Samples
with cycle threshold (Ct) values less than 35 were considered to
be positive for A. citrulli. To ensure that there was no PCR inhibition
in samples that yielded negative results, 5 ng/μl of A.
citrulli genomic DNA was added to DNA preparations from 10
control seed samples and subjected to real-time PCR as described
The mean percentage of A. citrulli-positive seed sections was
determined for each seed lot. To confirm real-time PCR assay
results, attempts were made to isolate A. citrulli from seed sections.
Samples (n = 20 seeds/lot) were dissected into testa, PE
layer and embryo tissues as described above and the tissues were
macerated separately in PBS and 100-μl aliquots were spread onto
Nunhems semiselective medium. Plates were incubated for 5 days
at 28°C and the percentage of A. citrulli-positive seed sections
was compared for seeds inoculated by each method. For statistical
analysis the mean percentage of A. citrulli-positive seed tissues
for each lot (as determined by real-time PCR assay and plating on
semiselective media) were compared by factorial analysis using
the Student’s t test (P < 0.05) in SAS (version 8.1 for Windows;
SAS Institute Inc., Cary, NC).