Of the two potato glycoalkaloids, α-chaconine and α-solanine,
α-chaconine was the more inhibitory on spore germination in
Alternaria brassicicola and Phoma medicaginis, and on
growth
in liquid culture of these species and also Ascobolus crenulatus
and
Rhizoctonia solani. At 50 μM or less, solanine caused
little (<20%) or no inhibition but, in combination with comparable
concentrations of chaconine (which were sometimes also below the activity
threshold), synergistic inhibitory effects (up to 100%
inhibition) were apparent. A scanning electron microscopic examination
of
agar-grown cultures of P. medicaginis revealed that both
glycoalkaloids at 100 μM inhibited hyphal extension, but that
chaconine
particularly increased branch formation (three-fold) as well as
hyphal thickness and ‘beading’. Glycoalkaloid-containing liquid
medium from R. solani cultures did not contain glycoalkaloid
hydrolysis products. There was a 17% reduction in the solanine content
of the
medium, but the chaconine level was not significantly
different from that in uninoculated controls. Neither glycoalkaloids nor
their hydrolysis products were detectable in the washed
mycelium of such cultures.