Formation of microsclerotia and chlamydospores from conidia of Alternaria brassicae

Abstract

Conidia of Alternaria brassicae are transformed into microsclerotia on some naturally infected, partially decayed rapeseed leaves stored at 3 °C in the dark. On V-8 juice agar medium, formation of microsclerotia occurs after 2-week-old cultures are exposed to 3 °C for at least 6–10 days. When rose bengal is added to the medium, growth is restricted and the transformation of conidia into microsclerotia is also initiated during gradual desiccation. Mature microsclerotia are darkly pigmented, many-celled, unevenly spheroidal, and about 60–150 μm in diameter. Microsclerotia are resistant to desiccation and freezing. On a 1% malt extract solution, germination of frozen–thawed microsclerotia results almost exclusively in the direct production of conidiophores and conidia, and germination of untreated microsclerotia results primarily in the production of germ tubes that extended into hyphae.Chlamydospores also form within conidium cells after cold temperature (3 °C) treatment or during gradual desiccation in cultures. Conidia placed on natural soil at room temperature become empty or form endocellular chlamydospores.