Factors Affecting the Growth of Rust Fungi on Host Tissue Cultures

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Abstract
Tissue cultures were isolated from healthy tissues of snapdragon and sunflower. Several unsuccessful attempts were made to infect tissue cultures with urediospores of the appropriate rust fungi. Snapdragon tissue cultures produced an extracellular material which inhibited urediospore germination. The inhibitory material could be leached out in agar or in water. It was heat stable, dialysable, non-ionic, and ether and ethanol insoluble. Rust-infected sunflower cotyledons and red-cedar galls induced by cedar-apple rust were cultured. Although the host tissue proliferated, the rust mycelium did not invade the newly formed tissues. Some sunflower hypocotyl explants proliferated on a chemically defined medium and produced roots, leaves, and flowers. Leaf callus was isolated from shoots that developed in vitro. Attempts to infect sunflower leaf tissue cultures and shoots in vitro with urediospores of Puccinia helianthi were not successful. Development of aerial mycelium of P. anttrrhini was induced in infected snapdragon leaf tissue cultured on a defined medium. Aerial mycelium developed as white tufts, 1-3 mm long. This mycelium did not continue growth when transferred to a nutrient medium. It was not capable of infecting host tissue cultures. Inoculation of snapdragon tissue cultures covered by host leaf epidermis with urediospores did not produce infection. Attempts to infect callus tissue by placing in contact urediospores germinated on collodion membranes also failed.