ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Fig. 1 (left). — Fungus mycelium is composed of fine, threadlike strands 

 of entwined hyphae. 



Fig. 2 (center). — Fungi reproduce by means of spores, which are micro- 

 scopic in size and which vary in color, shape, and number of cells. The spores 

 in this picture are oval, colorless, single cells which are produced by the 

 fungus that causes Dothiorella canker of oak. 



Fig. 3 (right). — Bacteria which cause plant diseases are very small, rod- 

 shaped, single-celled organisms. 



Fungi grow as very fine, threadlike strands, called hyphae, 

 which branch and produce entwined masses of moldlike growth 

 called mycelium (Fig. 1). The mycelium may grow within the 

 host, or it may develop as powdery or moldlike growth on the 

 surfaces of leaves or other parts of the host. 



Fungi reproduce by means of spores, which are microscopic 

 in size and which vary in color and shape (Fig. 2). These spores 

 may be carried from diseased to healthy trees by wind, rain, 

 pruning tools, insects, and other agents. 



Bacteria. — Bacteria that cause diseases in trees are single- 

 celled, rod-shaped, microscopic organisms (Fig. 3). They repro- 

 duce by simple division — one individual divides to become two 

 individuals. Bacteria, like fungi, contain no chlorophyll and ob- 

 tain their nourishment from other living plants or from dead 

 organic matter. Also, like fungi, they may be carried from dis- 

 eased to healthy plants by wind, rain, pruning tools, insects, and 

 other agents. 



Viruses. — Viruses are infectious agents that are too small 

 to be seen with the highest power of the conventional compound 

 microscope. However, they can be seen with the electron micro- 

 scope. They appear to be protein in nature and are capable of re- 

 producing themselves in living hosts. Many viruses are carried 

 from diseased to healthy plants by insects. They may be trans- 

 mitted artificially when parts of diseased trees are grafted on 

 healthy trees. 



Higher Plants. — The only one of the higher plants known 

 to cause an infectious disease in Illinois trees is one of the mistle- 



