Interactions between the glycoalkaloids solasonine and solamargine in relation to inhibition of fungal growth.

Inhibition of mycelium development in Phoma medicaginis and Rhizoctonia solani by solamargine and solasonine generally increased with increasing pH. P. medicaginis was the more susceptible species and solamargine the more potent compound. Solasonine was inactive against R. solani over the tested pH range (5-8). Dose-response curves confirmed these differential effects. Solamargine caused 50% growth inhibition in P. medicaginis at 60 microM (at pH 7) whereas no other treatment achieved this effect at 100 microM. Combinations of 50 microM of each glycoalkaloid produced synergistic effects against both fungi, especially R. solani which was essentially unaffected by either compound, by significantly inhibited by a 1:1 mixture of the two. The magnitude of the synergism was not affected by a pH change between 6 and 7. Spore germination in Alternaria brassicicola was markedly inhibited by 100 microM solamargine but unaffected by 100 microM solasonine or either compound at 50 microM. In P. medicaginis, neither glycoalkaloid was inhibitory up to 150 microM. In combination, the two compounds caused synergistic effects in both species, but to a much greater extent in A. brassicicola.