Cytological Studies on Rust Fungi (VIII) : Infection Puccinina horiana P. hennings on Three Chrysanthemum Cultivars of Different Resistance Levels

The infection process of Puccinia horiana P.Hennings in chrysanthemum cultivars of differ:ent resistance levels was investigated by scanning and transmission electron microscopy. Test fungus was harvested in a greenhouse of the National Vegetable and Ornamental Experiment Station in Tsu city. Test cultivars of chrysanthemum were Seiko-no-hana (highly susceptible to the test fungus) , Chiyozakura (susceptible) and Seizan (highly resistant). Numerous teliospores developed on Seiko-no-hana leaves, whereas only a few developed on Chiyozakura. No teliospores occurred on Seizan leaves. Germination of sporidia on three cultivars was compared by a scanning electron microscope. No differences in germination rate and morphology of germ tubes were found among the three cultivars. Infection process after the cuticle penetration was examined by a transmission electron microscope. Hyphae were obrserved in leaf epidermises of all test cultivars 1 day after inoculation. The hyphae in Seizan epidermises were encloscd by thick papillae, resulting in no further hyphal elongation in mesophyll cells. However, those in Seiko-no-hana and Chiyozakura epidermises further grew into mesophyll tissues and produced haustoria in mesophyll cells, ln specimens fixed 12 days after inoculation, hyphal masses were observed in intercellular spaces between epidermal and mesophyll cells of Seiko-no-hana and Chiyozakura leaves. Teliospores arose from such hyphal masses. A greater number of teliospores was produced on Seiko-no-hana than on Chiyozakura. These results suggest at least two steps of resistance mechanism: i) one associated with papilla deposition in epidermal celis, ramerkable in hiehly resisrant cultivars such as Seizan, and ⅱ) one which acts when teliospores are formed