Studies on species of Septoria causing leaf spots of pistachio (Pistacia vera).

Abstract

Two pathogens Septoria pistaciarum and S. pistacina, causing leaf spot diseases of Pistachio (Pistacia vera) were studied. The morphology and ultrastructure of the two pathogens and also the ultrastructure of the host-pathogen interface were investigated. The conidia spores of both pathogens were narrowly elongate with several transverse septae and each cell contained one nucleus. Spore length and cell number were positively correlated. Spore germination was shown in detail in S. pistaciarum. Spore germination, initially an increase in volume followed by an increase in length by cell division, and germination by the production of twin-pairs of secondary septate spores and/or hyphal germ tubes. A pattern of cell division, polarity and germination was established and an interactive model was described. The spore growth initially followed linear kinetics and then became exponential. The production of hyphal tips hecame continuous exponential at the same time and the hyphal growth unit stabilised at 40-60 urn. The germination pattern could be altered by manipulation of the nitrogen or phosphate source and levels in the medium. In the resting spore the wall was composed of three layers an outer layer (o) and two inner layers I1 and I2. Each primary septum was a duplex layer derived from I1 and I2 with an intermediate electron lucent layer. Perforation was simple with paired Woronin bodies. During increase in lengththe development of new (secondary) septa showed the crosswall material was derived from layer I2 and development was centripetal. The germ-tube wall was formed from wall layer I2 and wall layers 0 + I1 were disrupted. Secondary spores were formed enter oblastically. S. pistaciarum and S. pistacina were used in the ultrastructure of host-pathogen investigation, infection of the host by S. pistaciarum was by inoculation with conidia produced on culture whereas with S. pistacina inoculation was obtained from previously infected leaves. The spores germinated on the leaf surface and penetration was made by fine hyphae through the stomatal aperture in both species, growth of pathogen within the leaf was limited to the intercellular spaces. Penetration of the host cell wall did not occur. The hyphae were attached to mesophyll and palisade cells by an accumulation of extra-hyphal sheathing material. The initial host response to infection was shown by an accumulation of electron-dense material and disruption of the tonoplast, followed by disruption of chloroplasts and disorganisation of the host cell cytoplasm. Pycnidia were produced within the lesion. Pycnidial initials were formed in sub-stomatal spaces and spore tendrils were released on the leaf surface.