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Ascocarp  of Venturia inaequalis in Overwintered Apple Leaf

Images and text from MycoAlbum CD

LEFT: Note the asci developing inside the centrum of the fruitbody.  The ascospores are in a linear series inside the ascus and each ascospore is septate into two unequal cells (footprint shaped - see red arrow).  Ascospores are shot off under favourable conditions as the primary inoculum in the new growing season.

In most perithecial Ascomycota, the wall (peridium) develops as an independent  structure as the perithecium forms.  In Venturia a flask-shaped stroma of pseudoparenchyma is formed inside the leaf tissue.    Enzymes are produced by the fungus that lyse a cavity (locule) in the stroma.    Eventually the cavity is enlarged  and the residual layers of pseudoparenchyma not lysed by the enzymes function as a peridium.   i.e. the 'perithecium' in Venturia is a uniloculate stroma.

RIGHT:  Ascospores germinate and infect the young apple leaves and produce the Fusicladium conidial state as the secondary inoculum.  Conidia infect additonal leaves, blossoms and developing fruit causing severe damage and loss of quality.  There is also considerable defoliation of   heavily infected laves.  In ornamental Crab Apples in Ontario in wet years there is almost complete defoliation.