Abstract
Systemic aceial infections of secondary Canada thistle shoots were observed 4-6 weeks after inoculation of root cuttings with teliospores of Puccinia punctiformis. Pretreatment with a stimulatory thistle root extract sometimes increased infection. Inoculation of dormant buds was the most effective way to induce infection of root cuttings. Aqueous soil extracts from potted Canada thistle plants stimulated teliospore germination. The active component in the soil was soluble in hexane, volatile, and was similar in nature to the hexane extract of steam-distilled Canada thistle roots. Optimum temperature range for stimulated teliospore germination on 1% agar was 16-20 C at 7 days.