Pethybridge — The Verticillium Disease of the Potato. 81 



of disease began to appear in the earlier inoculated plants about one month 

 after inoculation ; but in those inoculated later, when the plants were much 

 larger, the symptoms first appeared within about seven days. The symptoms 

 were the appearance on the older leaves of pale green or yellow areas with 

 rather ill-defined margins. After a time these areas dried up and became 

 brown and dead. Gradually these symptoms repeated themselves in the 

 successively younger leaves, often but not always accompanied by an inrolling 

 of the margins of their leaflets. In some cases the leaves while still green 

 showed a true wilting due to loss of turgor in their cells ; and by degrees the 

 foliage of the inoculated plants died off by a process of desiccation proceeding 

 from below upwards. When the foliage was dead, a similar process of desicca- 

 tion took place in the stalks proceeding from above downwards. When the 

 inoculations were made while the plants were still young, the inoculated 

 plants remained much smaller than the corresponding controls ; but this 

 difference, although present, was not so marked in the case of plants 

 inoculated when already fairly advanced in growth. A good idea of the kind 

 of result obtained in this series of inoculation experiments is afforded by 

 figs. 3 and 4, Plate II. 



When the discolouring leaves showing the first symptoms of disease in 

 the plant were removed and placed in a moist atmosphere, they quickly 

 became covered externally with an aerial development of conidiophores of 

 Verticillium, thus proving that the fungus had already reached them. Sections 

 of the veins and petioles of such leaves showed the abundant presence of the 

 mycelium in the wood vessels. The same mycelium was also found to be present 

 in the stems, roots, and some of the new tubers of the inoculated plants at the 

 conclusion of the experiment. It was an easy matter to obtain the fungus in 

 culture from dead or dying plants, and to prove by the character of the black 

 submerged mycelium developed in due time that it was Verticillium albo- 

 atrum. 



The control plants were just as carefully watched and examined for the 

 presence of the fungus as the inoculated ones, but they showed no signs 

 whatever of disease, and no mycelium could be found in their vessels ; in 

 short, they remained perfectly sound, and produced sound tubers. 



The second series of inoculation experiments was carried out in one of the 

 gardens at the Albert Agricultural College, Glasnevin, in the open ground. 

 Six previously sprouted healthy tubers were inoculated in the sprouts before 

 planting, and six plants from healthy tubers were inoculated in their stalks 

 below ground in the manner previously described, when they were about 

 30 em. high. There were twelve controls. All of the inoculated plants 

 remained smaller than the corresponding controls. They soon began to show 



n 2 



