326 FUNGOUS DISEASES OF PLANTS 



of conidia proceeds, in the older areas the spots vary from gray to 

 bright pink. Through such injuries the boll is usually seriously 

 damaged and may never open. Moreover, through the boll injuries 

 the fungus may probably penetrate the seed and thus be carried over 

 and distributed the following season. Seedlings may be affected 

 either upon cotyledons or stem, especially upon employing diseased 

 seed, and at this stage the plantlet is readily wilted as a result. 



Upon stems of the adult plant the Colletotrichum seems to be 

 largely a wound parasite, although in continued moist weather 

 direct injury may be induced. Upon mature leaves it is said to 

 take the form of a scald, or frost effect, and it may also accompany 

 other leaf diseases. 



Characters of the fungus. From a loose stroma within the tis- 

 sues conidiophores of two types break through the epidermis and 

 produce conidia abundantly. Small hyaline conidiophores, usually 

 less than twice the length of the conidia, are more numerous, and 

 they arise in a compact mass, each abscising one conidium after 

 another. The spores are (see Southworth) 4.5-7 X 1 5-20/n, oblong, 

 the diameter of the middle portion sometimes less than at the ends, 

 usually pointed at the base, and vacuoles may be present. The 

 other form of the conidiophores, termed setse, arise later from dark 

 colored cells of the stroma. These setae, which usually appear in 

 clusters of S to 10, are of a dark olive color, 100 to 250/x long, 

 tapering and septate. They bear ovate, basally pointed spores. 

 The mass of conidiophores and spores produced in this manner 

 constitute the acervuli. In this form an acervulus may be from 

 100 to 1 50/* in diameter. 



The spores germinate readily in almost any nutrient media, usu- 

 ally becoming once or twice septate during germination. The my- 

 celium, which grows vigorously in culture, is hyaline, flexuous, 

 and abundantly septate. Short conidiophores are promptly pro- 

 duced. The conidia are borne singly, but by virtue of a slightly 

 gelatinous envelope they may adhere in a crown about the tip 

 of the conidiophore. Appressoria are also produced by the myce- 

 lium, and these give rise to other similar structures, to an ordinary 

 hypha, or to a conidiophore. Such dark cells are also developed 

 from a germ tube when germination proceeds in water. The setae 

 have also been produced in culture. 



