i82 DISEASES OF CULTIVATED PLANTS 



belonging to the most complete cycle of development of the 

 fungus. 



If a tuber showing the incipient stage of the disease, before 

 the fungus has burst through the skin, is placed in a two per 

 cent, solution of formalin in water, for ten minutes, for the 

 purpose of destroying superficial germs, and afterwards rinsed 

 in distilled water and placed in a damp atmosphere at a tem- 

 perature of about 70 F., scattered, cushion-like, snow-white 

 tufts of the fungus will burst through the periderm or skin of 

 the potato on the shrunken patches within a few days, or a 

 week at most. These first tufts I invariably find to consist of 

 the Monosporium stage of the fungus (Fig. 2, life cycle of 

 Nectria solani). When this phase has been present for a 

 few days, the exact time depending on the amount of 

 chemical change effected on the host by the fungus, the 

 Monosporium stage ceases to develop, and the second or 

 Fusarium form of fruit appears on the same tufts that 

 previously produced Monosporium conidia (Fig. 4). The 

 Fusarium condition often remains, producing a succession of 

 conidia for several weeks, the period of time being mostly 

 determined by the relative activity of secondary organisms in 

 effecting the complete rotting of the tuber. When the 

 Fusarium conidia are germinated in a hanging-drop, one or 

 more irregularly branched germ-tubes are produced by each 

 conidium, and on these germ-tubes are produced at intervals, 

 .at the tips of short lateral branches, the clustered spores of 

 the Cephalosporium stage (Fig. 5). This is the last of the 

 conidial phases of the fungus. The pulvinate tufts or 

 stromata that produced the Monosporium and Fusarium 

 phases still remain embedded in the old and empty skin of 

 the potato, and during the following season produce the 

 ascigerous form of fruit, the spores of which on germina- 

 tion produce secondary spores. Whether these secondary 

 spores are capable of infecting young growing tubers I 

 do not know; all my experiments in this direction have 

 failed. When conidia of the Monosporium and Fusarium 

 stages are sown in culture media, each one continues to 

 reproduce its own kind. In such cultures the mycelium 

 is sometimes coloured deep blue, red, or salmon-colour. I 

 have succeeded in infecting young tubers with Fusarium 

 conidia, but not with those of Monosporium. The conidia 

 of the Cephalosporium stage persistently refused to ger- 

 minate. 



