MECHANISM OF ACQUIRED IMMUNITY TO A PLANT RUST

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Abstract
When the uredinial stage of Uromyces phaseoli was used to inoculate entire bean leaves that already had a band of infected tissue, the tissue adjacent to the old infection did not become diseased, the width of the inhibiting zone varying directly with the age of the first infection. Spores added to macerated, non-infected tissue that had been adjacent to infected tissue had somewhat better germination than when the macerated tissue had been distant from the infection. Spore germination was less when agar plates were heavily seeded with spores or were kept closed than when they were lightly seeded or kept open, indicating the possible release of a gaseous substance by the rust spores which was toxic to other spores. The presence of rusted bean leaves in Petri dishes with spores seeded on plain agar inhibited spores germination, whereas healthy leaves gave no inhibition. Attachment of a rusted leaf to one side of a leaf piece inoculated with bean rust, broad bean rust or snapdragon rust, and of a healthy leaf to the opposite side, indicated that the rusted leaf reduced infection of the inoculated piece, the inhibiting action increasing with age of the rust infection. Rusted snapdragon leaves also inhibited infection of bean and snapdragon rust, but rusted bean leaves did not inhibit infection with bean anthracnose, bean powdery mildew, tobacco mosaic virus or tobacco necrosis virus, suggesting that the gas produced is toxic only to rust spores. The rust lesions close to old infections were not reduced in size, but spore germination was weaker close to old lesions. The toxic gas had no effect on the fungus after penetration had occurred.

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