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Sclerotinia sclerotiorum

Fruitbodies (= apothecia) of Sclerotinia sclerotiorum arise from a hard, black sclerotium (bottom left).     Sclerotia overwinter in the soil and, under the right conditions,  'germinate', to give rise to the apothecial fruitbodies you see here.  The ascospores are shot of and land on the blossoms of the host e.g. white bean, to initiate a new cycle of infection.

Many plant pathogenic fungi produce sclerotia.  Sclerotia tend to be relatively large, black, and hard with a thick, protective rind.    Sclerotia are designed to carry over large reserves of nutrient material from one growing season to the next.   These reserves allow the fungus to produce reproductive structures (spores) very quickly and give the parasite a flying start in the new season. 

Also, the fungus may have to persist in soil for many years before a suitable host plant appears in the vicinity.  Soil is a very competitive environment  and full of  microscopic flora and fauna looking for an easy mark.   Sclerotia have a relatively large nutrient content and  must protect themselves from predation by other life forms during this persistence phase which could last for many years.

  The protective outer rind is made up of small, thick-walled, dark pigmented, pseudoparenchymatous cells.  These thick-walled cells also contain phenolic and other compounds that make them inpenetrable, distasteful or lethal to would be attackers.