A study on the ultrastructure and control of the gladiolus rust pathogen, Uromyces transversalis.

Abstract

The substomatal vesicle of Uromyces transversalis develops in five distinct stages after the formation of a single infection peg in the host plant Gladiolus spp. The primary hyphae of the substomatal vesicle orientate at right angles (rather than at acute angles or parallel) to the stomatal slit and the parallel veins of a gladiolus leaf or to those of the non-host, Zea mays. The transverse orientation of Uromyces transversalis uredia is apparently a genetically controlled phenomenon. The two primary hyphae normally develop asynchronously on opposite sides of the substomatal vesicle, and, after the formation of a haustorium mother cell, secondary hyphae branch off. These subsequently form basal cells each of which give rise to one or more holoblastic protruberances on its distal surface. A septum delimits the protruberance from the basal cell to form a urediospore initial, which in tum elongates and, by septum formation, forms a pedicel and a urediospore. The urediospore is seceded mechanically through the process of schizolysis. The pedicel of a spore thus formed remains on the basal cell and becomes a collar around the next protruberance. The basal layer of the two-layered septum, that delimited the pedicel from the basal cell, grows out to form the wall of the subsequent protruberance, in the process rupturing and laterally displacing the terminal septal layer. A new basipetal septum forms to delimit the subsequent urediospore initial. Therefore, successive urediospores are formed enteroblastically and give rise to a series of basipetal collars. Cultivars, naturally occurring species and breeding lines of gladiolus were evaluated for rust resistance. The cultivars showed no sign of resistance, whereas the populations of wild species and breeding lines had similar resistance profiles. The species G. daleni and G. tristis var. tristis showed promise for breeding. Infection of these species caused an almost immune, fleck reaction. The resistance of the species G. daleni was manifested by the abortion of primary hyphae prior to the formation of the haustorial mother cell. The intercellular hyphae of gladiolus rust only partially adhered to the mesophyll cell of the resistant host. The haustorial apparatus that did form in the mesophyll cells in the resistance reaction was inhibited at various stages of its development. The rust disease could be controlled chemically by either bitertanol or triadimefon. Triadimefon, however, caused a shortening of the internodes of the flower spike. The early development of U. transversalis infection structures in bitertanol-treated leaves was inhibited at, or shortly after, substomatal vesicle development. A number of mature substomatal vesicles with intercellular hyphae, however, did develop in the bitertanol-treated leaf tissue, probably because this fungicide undergoes limited translocation in the gladiolus lamina.