Origin of New Growth in Dormant Microsclerotial Masses of Verticillium Albo-Atrum

SUMMARY

One-year-old dried cultures of 4 microsclerotial isolates, 1 white mycelial variant (no microsclerotia), and 1 isolate of the DM form (no microsclerotia) of Verticillium albo-atrum were examined to determine the origin of new growth from shavings transferred to sterile distilled water or to water agar. The microsclerotial isolates produced 3 structures that gave rise to new growth after the dormant period. These were clusters of hyaline microsclerotial cells, hyaline chains of chlamydospore-like cells, and hyaline hyphae intertwined in microsclerotial masses. In no instance were the melanized, thick-walled microsclerotial cells observed to germinate. The white-mycelial variant produced hyaline chlamydospore-like cells, either occurring in chains or singly, and hyaline hyphae which give rise to new growth. New growth in the DM form of the fungus occurred only from hyaline hyphae. New growth in dormant cultures could not be attributed to germination of conidia. Conidia from microsclerotial forms completely lost viability when stored 19 days in water or when dried for 20 minutes on glass slides at 22 C. Attempts with tetrazolium salts and indigo carmine to determine viability of melanized, thick-walled microsclerotial cells were unsuccessful, but microscopic examination of old and young microsclerotial cells suggested that a large number of the old dark cells were not living. These observations suggest that the principal means of survival of the fungus V. albo-atrum under adverse conditions is in the various hyaline structures produced by the fungus rather than the melanized, thick-walled microsclerotial cells.