Which of the following is NOT an ascomycete of importance?

The correct choice is identified as C, Ustilago sp. This is because Ustilago species belong to a different group of fungi known as basidiomycetes, rather than ascomycetes. Basidiomycetes are characterized by their reproductive structures, which are typically fruiting bodies that generate spores externally on specialized cells called basidia, while ascomycetes produce spores internally within sac-like structures called asci.

In contrast, Erysiphe necator, Taphrina sp., and Monilinia sp. are all classified as ascomycetes. Erysiphe necator is known for causing powdery mildew on grapevines; Taphrina includes species that are plant pathogens causing abnormalities such as galls; and Monilinia encompasses important pathogens that cause brown rot in stone fruits. All these fungi share the defining characteristics of ascomycetes, reinforcing their inclusion in that specific group. Hence, understanding the classification is crucial in identifying their significance in plant pathology.