Studies on sunflower rust. VI. Penetration and infection of sunflowers susceptible and resistant to Puccinia helianthi race I

Abstract

Spore germination, appressorium formation, and penetration by rust race 1 appeared identical on a susceptible and a resistant sunflower variety. Haustoria were formed in 24 h. They were round to elongated and numerous in the susceptible variety, and round and few in the resistant variety.Mycelial growth was faster and more extensive in susceptible than in resistant tissue. In susceptible tissues no injury was apparent 96 h after inoculation. Urediospore production started 144 h after inoculation. Host cells under incipient pustules collapsed. Chloroplasts appeared fewer in infected than uninfected mesophyll cells.Collapse of mycelium in resistant tissue was observed 96 h and death of invaded cells 120 h after inoculation. Chloroplasts degenerated in infected and neighboring cells.