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I. MOLDS, MILDEWS, MUSHROOMS & MAN TOP D. Common Groups of Fungi and Fungal-like Organisms 4. Zygomycota 5. Ascomycota
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4. Zygomycota:
This class contains fungi that are usually highly filamentous, forming extensive mold growth. The bread molds and allied groups belong here; as well as entomogenous and mycorrhizal fungi. They reproduce asexually be forming sporangia with many nonmotile spores (Fig. 2-6).
Fig. 2-6. Life cycle of Rhizopus stolonifer. (© Alexopoulos et.al. 1996) They
reproduce sexually by gametangial fusion resulting in a normally
thick-walled, pigmented zygospore. While most members are saprobic, some are parasites of
plants or animals, including man. Sometimes they are even found growing
on mushrooms in the field. We will note later in
our study of medical mycology, a number of Zygomycetes such as Absidia,
Mucor, and Rhizopus are serious pathogens to man. 5. Ascomycota:
This is the largest group of fungi and all form their sexual spores in microscopic sacs that are called asci. They also reproduce asexually by the formation of conidia. Asci may develop naked (Fig. 2-7), as in the yeasts in which the ascospores are formed in small globose asci, or in containers called cleistothecia which are usually globose and enclosed; perithecia which are usually flask-shaped with an apical opening through which spores are ejected, pseudothecia which are usually hard, stromatic structures with a cavity containing asci, or apothecia in which asci are borne in a cupulate or disc-shaped structure (Fig. 2-8).
Fig. 2-8. Asci are borne: A) naked, B) in cleistothecia, C) in perithecia or D) in apothecia.
The
Ascomycota contains some of our most important plant pathogens and a
number of parasites of insects and other animals, including man. Their
industrial use is extensive and will be discussed in later chapters. The asexual state of many cleistothecial groups are better recognized than the sexual state and contain such genera as Aspergillus and Penicillium which are used extensively in the commercial production of drugs and other compounds. Many human pathogens also belong here (Fig. 2-9).
Fig. 2-9. A cleistothecium as formed by some species of Aspergillus.
Powdery mildews on plants also form cleistothecia (Fig. 2-10; Fig. 2-11).
Fig. 2-10. Cleistothecia and conidia of powdery mildews.
Fig. 2-11. Powdery mildew on a leaf surface. Perithecial forming Ascomycota contain most of our plant pathogens (Fig. 2-12), including the famous chestnut blight (Fig. 2-13) fungus, Cryphonectria parasitica. CLICK HERE TO GO TO TOP OF RIGHT SIDE COLUMN
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Fig. 2-12. Light micrograph of a perithecium.
Fig. 2-13. A large chestnut tree killed by chestnut blight, caused by the fungus Cryphonectria parasitica. Some Ascomycota, like the “dead man’s fingers”, become quite large (Fig. 2-14).
Fig. 2-14. The ascomycete 'dead man's fingers'. If thin sections are made through these “fingers” one finds numerous perithecia embedded within (Fig. 2-15).
Fig. 2-15. A perithecium just below the surface in a fruiting body of 'dead man's fingers'. Apothecial fungi are commonly referred to as “cup-fungi” or discomycetes because of their shape. A prime example of an apothecial fungus is Monilinia fructicola, the cause of brown rot of peach. Infected fruits become “mummified”, overwinter, and in the spring months give rise to apothecia (Fig. 2-16).
Fig. 2-16. Apothecia of Monilinia fruticola, caused of brown rot of peach. Apothecial fungi can also become large and highly variable and have beautiful apothecia (Fig. 2-17), some of which are highly contorted like the highly prized edible morels (Fig. 2-18).
Fig. 2-17. Apothecia of Phillipsia with bright orange-red surfaces.
Fig. 2-18. Morels, with highly contorted apothecia. Stromatic ascomycetes, like species of Venturia that causes “apple scab” (Fig. 2-19), have pseudothecia borne in a crust (Fig. 2-20).
Fig. 2-19. Lesions on apples caused by the apple scab fungus Venturia inequalis.
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