4-i MISC. PUBLICATION 568, U. S. DEP'T. OF AGEICULTUEE 



Collecting 



Some degree of experience is required to collect good material, because 

 many of the fungi causing plant diseases are almost microscopic. A few 

 general hints, however, will enable any alert observer to contribute 

 interesting specimens. Collect generous quantities of material. 



The main or vegetative part of any fungus consists of a mass of fine 

 colorless strands, known as mycelium. A well-known example is the 

 cobwebby film which develops on moldy bread. In most cases these 

 strands develop below the surface of whatever substance is being in- 

 vaded by the fungus and consequently are not visible to the naked eye. 

 After a sufficient period of time, the mycelium undergoes modification 

 to produce spores. These spores serve exactly the same purpose for 

 fungi as seeds do for the higher green plants. Spores are formed in 

 various ways, depending upon the kind of fungus. Sometimes they 

 occur inside definite structures known as fruiting bodies, which may 

 vary in shape and size from minute, roundish pin points (figs. 30, E 

 and 32) to large objects like toadstools, or woody, bracketlike forma- 

 tions sometimes seen on tree trunks or pieces of rotted wood (fig. 30, A). 

 However, in some of the primitive fungi, the spores are not borne inside 

 a definite body but may appear to the naked eye as a minute feltlike 

 covering, or perhaps as tiny colored specks. 



The specialist in fungi bases his studies on the structure of these 

 bodies, and for that reason good specimens of fungi must include these 

 mature stages. The larger fruiting bodies will be easily recognized, but 

 for some of the minute kinds the collector may need a pocket lens of 

 about 7 x magnification (fig. 32). Diseased plants can be recognized by 

 a number of signs, but the symptoms vary according to the part affected 

 and according to the kind of fungus. 



Leaf spots (figs. 31, A, B, C) are a common manifestation and usually 

 are seen as roundish or irregular areas of dead tissue, which may or 

 may not be surrounded by a discolored margin. In some cases the dead 

 tissue falls out, leaving a shot-hole effect, a characteristic, also, of some 

 bacterial diseases. Leaf-blight (fig. 31, D) conditions occur when leaves 

 are involved entirely and killed. In such cases the leaves may become 

 detached and fall to the ground, or they shrivel up and remain in place. 

 In twig blights, the twig and all the leaves attached to it are affected. 

 Dieback is merely an intensification of a twig-blight condition, in that 

 a considerable portion of a branch becomes involved, from the tip 

 downward. Cankers (fig. 30, C) occur on branches, stems, or even on 

 roots, and usually are represented by dead or discolored areas on the 

 bark. If diseased bark tissue becomes enlarged to any extent, the con- 

 dition may be known as a gall (fig. 30, D). Rots include any condition 

 whereby a considerable amount of tissue is broken down by decay. A 

 common example is seen in fruit rots (fig. 30, E), but stem rots, root 

 rots, and wood rots (fig. 30, A) also occur. Mildews are recognized by 

 small, grayish patches, usually occurring on leaves. Smuts (fig. 30, B), 

 recognized by their black, sootlike appearance, usually attack the seed 

 of grasses or cultivated grain crops. Rusts may appear either as minute, 

 orange-colored cups, or as brownish or blackish slits, and commonly 

 develop on leaves, although they sometimes are seen also on fruits or 

 stems. 



In many of these diseased conditions the collector may expect to find 

 the fruiting bodies, or mature stages of the fungus, in the portion of the 



