I. MOLDS, MILDEWS, MUSHROOMS & MAN

1. Mycetozoa

2. Chytridiomycota

3. Oomycota   

D. Common Groups of Fungi and Fungal-like Organisms            

The groups of fungi that are recognized in these outlines are those of Alexopoulos et al. (Introductory Mycology, John Wiley & Sons, NY. 1996). There are several groups of fungi and fungal-like organisms (02-01). Several of the groups are aquatic and have different types of zoospores (02-02). Trained mycologists are able to recognize the groups by the types of zoospore they produce.  

1. Mycetozoa:

These are cellular to stalked fungal-like organisms belonging to the Kingdom Protoctista. They include the slime molds and related groups. Most are saprobes, but one group, the Plasmodiophorales, are important plant pathogens and will be discussed later under FUNGI AS PARASITES. They form plasmodia within plant cells, cause swelling of the tissues, eventually forming resting spores that survive periods of dormancy, and then liberate zoospores into the soil that will reinfect.  

  2. Chytridiomycota:

Commonly referred to as chytrids, these highly reduced, microscopic aquatic fungi that are saprobic or parasitic on plants, animals or other fungi in the water or moist soil (Fig. 02-03).

Fig. 02-03. Life cycle of a chytrid.

They form zoosporangia that liberate uniflagellate zoospores (Fig. 02-04).

Fig. 02-04. Zoosporangia of Rhizolpidium on pine pollen

Under certain conditions zoospores function as gametes, fuse and the zygote develops into a resistant sporangium, also called meiosporangium. While most chytrids are saprobic of decaying organic matter, some like Physoderma cause serious plant diseases (02-05).  

3. Oomycota:

Oomycota are filamentous water molds and soil-borne groups now considered in the Kingdom Stramenopila. Economically important groups include members of the Peronosporales that cause downy mildews, late blight of potatoes, and white rusts of plants. These organisms form a diploid mycelium within the host and sporangia to the outside that liberate either biflagellate zoospores or windborne nonflagellated spores. They go through periods of dormancy usually by the production of oogonia that contain oospores (Fig. 02-06).

Fig. 02-06. Oogonium containing oospores.

Many have elongate sporangia filled with spores (Fig. 02-07), while others have globose sporangia (02-08).

Fig. 02-07. Elongate sporangium containing spores.

Many are saprobes on decaying plant material in bodies of water (02-09), but one of our the most destructive plant pathogens, Phytophthora infestans, the cause of late blight of Irish potatoes is one of the Oomycota (Fig. 02-10).

Fig. 02-10. Sporangia of Phytophthora infestans, the causal agent of late blight of potatoes.