chitosanase detection medium and incubated at 30 ◦C
for 24 – 48 h. Chitosanase activity was detected by the
formation of a clear zone of degradation around the
growing colonies.
Greenhouse assay
The pathogenicity test was carried out using potato
tubers (Solanum tuberosum cv. Shepody) obtained
from glasshouse grown plants derived from tissue culture
with no visible sign of disease. Potato tubers were
planted in 250-mm diameter pots containing sand and
vermiculite (2:1). S. scabies inoculum was prepared as
previously described by Faucher et al. (1992). S. scabies
strain EF-35 was grown for 2 weeks at 30 ◦C, in
50 mL tubes containing 3 g vermiculite saturated with
Say-solution (Labruyère, 1971). This inoculum was
mixed with the plant growth substrate before planting.
At planting, talc or chitosan (0.5 g) with or without
S. melanosporofaciens strain EF-76 was sprinkled on
the top of each tuber. Spores were previously collected
from a 10-day-old culture on YME agar and then
mixed with talc or chitosan (108 spores/g of carrier).
The pathogenicity test was carried out in five replicates.
Pots were randomly dispersed in a greenhouse
and their contents were watered every three days. A
soluble fertilizer (20-20-20) was added every 2 weeks.
The progeny tubers were harvested after three months
and were then examined for common scab (Goyer et
al., 1998). Disease severity was visually estimated by
attributing a disease severity index (1 – 10) to each
tuber in regard of the extent of their surfaces covered
with scab lesions. A disease index of 1 was associated
to healthy tubers while a disease index of 10 represented
tubers covered by scab lesions on more than 90%
of their surface.