June 189 5. J 



INJURIOUS INSECTS AND FUNGI. 



when the disease became more intensified, traces of decay were 

 seen upon the outside of some infected tubers. And, generally, 

 it was observed that directly the incipient shoots appeared 

 in the " eyes " of the tubers the disease began to spread. Before 

 this period it was more or less dormant. Even in mid-winter, 

 a few spots, small and much scattered, were found/ though 

 there was but little change or progress in the disease until 

 vegetation commenced. 



Examination with the microscope showed tbat the tissues of 

 the tuber in and around these infected spots were brown or 

 sherry- coloured, that the starch granules had been absorbed and 

 the cells broken down. 



Thin slices of tuber with the disease spots upon them were 

 placed on glass, covered with a bell glass, and kept at a tem- 

 perature of 70 degrees. They were examined after an interval 

 of about 60 hours, and, in a few instances, it was found that the 

 fungus had developed mycelium which was burying itself among 

 the intercellular passages. In one or two cases, branching 

 filaments, or conidiophores, were put ibrth fi'om the mycelium, 

 as shown in the illustration,* The cultivation of the mycelium 

 was, however, most difiicult, but sufficient evidence was obtained 

 to prove that the cause of the sherry-coloured spots in the 

 tubers was the Fhytopfithora infestana. There may be other 

 kinds of dark spots m potato tubers due to other fungi or other 

 causes, but it is (juite clear that in all the cases examined this 

 season the disease was occasioned by the Pkytopldhora. 



In these circumstances it is important, especially after a 

 season when potato disease occasioned by this fungus was so 

 widespread, to remind potato growers that, as pointed out by 

 De Bary in 1863, the disease can be conveyed by these spotty, 

 infected tubers, in two ways. First, by the mycelium of the 

 fungus within the tuber sending out spore-bearing branches 

 whose spores may be carried up by the growing plant, and 

 communicated to plants near, or conveyed by insects and animals. 

 This, De Bary admits, may not be of frequent occurrence. But 

 the second is the most important method of infection, viz. : — 

 by the mycelium growing up in due time into the shoots from the 

 tubers, and producing conidia, whose spores may infect many 

 plants near and far by means of wind, insects, and other agencies. 

 It does not always follow that an infected tuber communicates 

 disease to the plant it produces or that it propagates disease 

 in any way, though of course the risk is great when infected 

 tubers are planted. 



The disease, as is now well known, is also propagated by 

 means of conidia evolved from "resting spores " which liave 

 hibernated in old rotten potato tubers, or haulm, or in other 

 retreats, but it cannot be ascertained whether infection is more 

 largely due to these or to tubers containing the germs of disease. 



* X)raim by Mr. Worthington G. ^mitji. 



