22 MASS. EXPERIMENT STATION BULLETIN 183. 



The longevity of conidia, then, appears to be very Hmited when kept 

 in a dry condition. When the atmosphere is kept very humid the}" live 

 longer, at least several weeks, but no careful investigation has been 

 undertaken to determine just how long with each degree of humidity. 

 If water stands on them, even in the culture dish, they germinate and 

 then quickly die if dried out at once. 



Parasitic Life of the Fungus. 

 Pathogenicity. 

 In order to prove that an organism is the causal factor of a certain 

 disease there are four requirements — called the four rules of proof — 

 which pathologists all agree must be fulfilled. These are: (1) find the 

 organism constantly associated with the disease; (2) isolate the organism, 

 grow and study it in pure cultures; (3) produce the disease again by 

 inoculation from these pure cultures; (4) reisolate the organism and prove 

 by culture its identity with the organism which was first found. These 

 four rules were comphed with by Massey (1917), and the pathogenicity 

 of Cylindrocladium scopariwm established. The present writer has also 

 given the four rules repeated test, and obtained results similar to those 

 of Massey. These experiments are not described in detail here, but 

 only certain notes on each of the four steps recorded. 



1. Constant association of --the pathogene with the canker is not so 

 easy to establish as in most fungous diseases because the fungus can rarely 

 be seen with the naked eye on cankers in rose houses. Nevertheless, the 

 writer has occasionally been able to find a white band of conidia around 

 cankers on yovmg shoots just at the surface of the grovmd. Almost always 

 when a canker is kept in a moist chaml^er for twenty-four hours or longer 

 the mycelium grows out as long, straight, white hypha?, which can readily 

 be recognized as peculiar to Cyhndrocladium by one who has become 

 acquainted with the appearance of this fungus. Also, after a few days 

 in the moist chamber, conidia usually begin to develop on the surface. 

 The presence of the pathogene in old cankers is also often betrayed by 

 sclerotia, ■ — small, flat, shining black specks just under the epidermis. 

 Yet the writer has often found cankers in which the organism could not 

 be determined in any of the above ways. There seems to be only one 

 absolutely sure way of determining association of the pathogene in all 

 cases, and that is by making isolations, which is really a part of the sec- 

 ond rule of proof. 



2. The following has been found the most satisfactory method of 

 isolation: — 



Method. — The surface of the canker is first sponged with mercuric chloride 

 1-1,000. Scalpels and steel needles are kept in a jar of 95 per cent, alcohol. The 

 epidermis, or at least a thin outer layer of the canker, is then peeled off with a 

 scalpel from which the alcohol has been burned over a Bunsen. Another scalpel 

 sterilized in the same way is used to cut out a portion of the peeled canker. It is 



