1916] PARASITIC RHIZOCTONIAS IN AMERICA 287 



in cultures of the strains that produce such growth on the host plants. 

 The tufts are composed of brown hyphae, closely septate, constricted 

 at the septa, and often branching in an irregular manner. 



Sclerotia in cultures first appear as small, soft, white masses of 

 hyphae. Later they become larger and turn dark and hard. Study of 

 sclerotia at different ages shows that they are of uniform structure com- 

 posed entirely of masses of irregular and barrel-shaped cells which 

 break up into sections of one or several cells (Fig. 3). These shortened 

 hyphal cells function as conidia and germinate readily under suitable 

 conditions. Germination generally takes place by the protrusion of a 

 tube thru the septum of a cell where it has broken away from an adja- 

 cent cell. In some cases the hyphae of the germinating cells pass thru 

 adjacent cells, which are apparently empty. Occasionally these irreg- 

 ular and barrel-shaped cells germinate equatorially instead of at the 

 poles. After the germ tube has grown out some distance, it becomes 

 narrowed near the germinating cell and a septum is laid down. The 

 mycelium then develops in the usual manner (Fig. 4). 



The formation of sclerotia in nature is rather common on many 

 hosts. The best known examples are those formed on the potato tuber. 

 The size and shape of the sclerotia vary considerably. On potatoes 

 they are small, about 1 to 5 millimeters, and are generally flat. On 

 carnation plants they may reach a diameter of 5 to 8 millimeters. 

 When the fungus is grown on soil in pure culture, they become 5 to 6 

 centimeters in diameter (Fig. 5). 



The sporiferous stage of RJiizoctonia Solani was first observed in 

 this country by Rolfs 93 in 1903, on potato stems. It was described by 

 Hurt 94 as Corticium vagum B. & C., var. Solani* In Europe this same 

 fungus is generally known as Hypochnus Solani Prill. & Del." 



Altho the writer has observed RJiizoctonia Solani on seventy-five 

 species of plants, including weeds and field, vegetable, ornamental, and 

 floricultural crops, growing under diverse conditions and at different 

 times of the year, for the past three seasons, it was not until the spring 

 of 1915 that he found the Corticium stage. It was then observed in 

 his home garden on bean, beet, radish, potato, parsnip, carrot, chard, 

 spinach, pea, plantain, and pigweed. This stage was also found on 

 winter vetch growing on newly plowed land, on carnation plants, and 

 on a number of annual and perennial plants. In some cases patches of 

 soil well protected from desiccation were covered with the ashy gray 

 mycelium of the perfect stage. 



"In a recent letter from Dr. Burt, he states : " I do not now believe that there 

 is even a varietal difference between Corticium vagum B. & C. and that on the 

 potatoes; hence I shall drop var. Solani." 



b ln his monograph on the Thelephoracece, Burt 12 limits Hypochnus to resupi- 

 nate species with colored, echinulate spores, while under Corticium he includes 

 species always resupinate, which have colorless spores and lack cystidia. Accord- 

 ing to Burt's classification, Hypochnus Solani Prill. & Del. becomes a synonym 

 under Corticium vagum B. & C. 



