A scanning electron microscope study of developing sclerotia of Sclerotinia sclerotiorum

Abstract

The exterior features of the rind and the cut surface of sections of developing sclerotia of Sclerotinia sclerotiorum were examined with the scanning electron microscope. Before pigmentation, a developing sclerotium appears as a white mass of interwoven mycelial strands covered with numerous droplets of liquid, each of which is surrounded by membranous material. Subsurface mycelial cells swell, become pigmented, and result in the formation of the dark bulbous rind cells which darken with age. Membranous material which envelops the liquid exuded from developing sclerotia was always evident and appressed to the surface of the rind in old sclerotia. The interior of sclerotia consists of a mass of mycelia embedded in an amorphous matrix, which in young sclerotia is liquid but in older sclerotia appears solid. Openings in the outer rind cells, from sclerotia of old cultures and those removed from diseased sunflowers, indicate possible means by which other microorganisms could enter and degrade sclerotia. Multiple sclerotia were examined and the region where fusion occurs was evident.