2.4. Persistence of R. solanacearum in substrate and host tissue
Forty-gram samples of disinfected substrate (mixture of peat
moss and sand as used in the previous assay with tomato plants)
were placed into two sterile glass petri plates, and a 10 ml aliquot of
R. solanacearum inoculum (106 cfu ml1; produced as described
above) was added to each sample and mixed into the medium with
a sterilized spatula. The final water content was around 60% of the
water-holding capacity of the medium. A sample of 1 g was
immediately removed and serially diluted in 9 ml of physiological
water (8.5 g NaCl L1), and 100 mlwas spread onto the surface of the
selective medium (Triphenyl Tetrazolium Chloride, TTC) and the
non-selective medium (Yeast Dextrose Agar, YDA). Five dilutions
and two replications were used for each dilution and growing
medium. Two glass petri plates were sealed with parafilm to
prevent moisture loss and were incubated in the dark at 25 C and
45 C, respectively, for six weeks. At periodic intervals of seven
days, 1 g sample was removed, serially diluted and plated as
described above. All plates were inverted and incubated at 28 C for
one week and colony forming units (cfu) of bacteria and separately
of R. solanaceraum were recorded.