Class: Loculoascomycetes

Fungi in this class are distinguished by having bitunicate asci. These have two distinct, separable walls. The outer wall is thin and inextensible and the inner thick and extensible. At maturity, the outer wall ruptures at the apex allowing the inner wall to expand under hydrostatic pressure. The ascospores,

which are usually coloured and multicellular, are released one at  a time through an apical pore in the     inner wall. The ascocarp is usually a pseudothecium which closely  resembles a perithecium in form and function but develops quite differently.In pseudothecial formation, vegetative hyphae aggregate into a spherical structure with heavily melanized outer layer. Asci develop in a single  cavity or loculus which form within this.

The pore or ostiole, through which the asci are discharged, arise by breakdown of pre-existing hyphae at a predetermined site.

Order: Dothideales

Family: Venturiaceae

Venturia inaequalis

causes apple scab, an economically important disease affecting apple leaves and fruit. The fungus over winters mainly as developing pseudothecia on fallen apple leaves. The cold of winter triggers ascus development and as the new apple leaves unfold in the spring, asci mature and discharge their ascospores up into the turbulent air after a good shower of rain. They impact on leaves and germinate. Bundles or strands of hyphae develop in a plate- like manner under the cuticle. At a later stage the cuticle is ruptured by the growth of short, brown conidiophores, giving the scab a velvety, brown to black appearance. The conidia are dispersed by rain splash and wind from spring to early autumn spreading the disease. At leaf fall in the autumn the fungus grows out from the leasions and forms pseudothecial initials. 

Live material:

To identify various locally collected specimens of mildew to genus and species. A variety of plant species infected with mildew will be available. Microscopic mounts will be prepared using a dissecting microscope to select mature fruiting bodies (cleistothecia) and

lifting one or a few cleistothecia off of the leaf into a drop of water or glycerine. Gently add a cover slip, and observe the characteristics of the cleistothecia before applying gentle pressure to the cover slip, and observe the characteristics of the cleistothecia before applying gentle pressure to the coverslip to force release of the asci. Using the attached keys, you are required to identify at least three of these specimens to species and then to have your identifications confirmed by one of the instructors.