Pethybkidge and Lafferty — Dry -Rot of the Potato Tuber. 213 



pathogenicity of members of the genus Fusarium towards the potato cognisance 

 must, as has already been explained, be taken of these factors. Possibly some 

 of the conflicting results obtained by earlier workers may be accounted for 

 by failure to appreciate the significance of these factors, although perhaps it 

 is just as likely that in some cases the species under investigation by the 

 different authors were not really identical. 



Mow Tubers become Infected. — Attention has already been called to the 

 fact that naturally infected tubers frequently exhibit wounds from which, it 

 is fairly clear, infection started. Seed potatoes are frequently transported in 

 sacks and, under the conditions of transit, the tubers are exposed to a good 

 deal of bruising. Such bruising frequently results in small cracks being 

 produced in the skin. It was proved by experiments with three varieties of 

 potato that the fungus could infect the tubers through wounds deliberately 

 produced by bruising and thus cause the rot; and it is believed that bruising 

 in transit may well be an important factor leading to infection. 



It was also proved that the wounds resulting from the breaking off of the 

 young shoots from sprouted tubers formed very favourable points of entrance 

 for the fungus. The careless handling and planting of sprouted tubers often 

 results in some of the sprouts being broken off. Some of the cases in which 

 presumably healthy seed-tubers have been observed to decay from dry-rot in 

 the ground after planting may probably be explained by the entry of the 

 fungus through wounds of this kind. 



Infection experiments were also carried out with tubers which, so far as 

 careful examination with a good lens could disclose, had perfectly intact 

 skins. In one case infection occurred at an eye, and in several cases at 

 lenticels. Since these results were somewhat unexpected, the experiments 

 were extended. Nine specially selected tubers of the Variety Early Puritan, 

 which, except for a few small scab spots, had perfectly intact skins, were 

 carefully washed in sterile water and allowed to dry. Some of them were 

 unsprouted and some had sprouts about 5mnb long. They were not treated 

 with any disinfectant, for fear of causing any damage to the eyes or the 

 sprouts. Five of these tubers were then steeped for a few seconds in a 

 suspension of the spores of F. cwruleicm in sterile water, while the remaining 

 four were steeped for the same length of time in sterile water alone. 



Both lots were then kept in a moist atmosphere in covered glass dishes 

 at laboratory temperature. The four tubers which were steeped in sterile 

 water remained absolutely free from any kind of rot. Dry-rot was set up, 

 however, in a short time at many points in the five tubers steeped in the 

 spore suspension. Ten cases of infection through scab spots and lenticels 

 were noticed, while infection occurred also in six cases through unsprouted 



