Pkthybridge and Lafferty — Dry-Rot of the Potato Tuber. 203 



In each of them F. cceruleum was proved to be the cause of the rot, and eleven 

 strains of the fungus have been isolated, and studied in very considerable 

 detail. Except for slight variations in their relative degrees of pathogenicity, 

 these strains have proved to be identical with each other, and also with a 

 strain of F. cceruleum procured from the Central Bureau of the International 

 Association of Botanists in Amsterdam. 



Two further cases of the rot were met with when our cultural work was 

 nearly completed; and in these cases also it was proved that the fungus 

 responsible for the decay was F. cceruleum. 



Occurrence of the Disease. — The most serious cases of the disease have 

 usually been met with in seed tubers of early varieties such as May Queen 

 and Ninety-fold, but it is by no means confined to such varieties. Complaints 

 regarding it are more commonly made in the early months of the year than 

 in the autumn, and this is probably correlated with the fact (to be discussed 

 later) that the maturity of the tuber has a good deal to do with the incidence 

 of the disease. We have never observed a case of the disease on tubers still 

 attached (as they are in summer and early autumn) to the parent plants, nor 

 have we seen it at the time of digging. The rot is essentially a storage one, 

 the visible effects of which are seen during the winter, and particularly in the 

 early spring. 



Description of the Rot. — The rot may begin in any external region of the 

 tuber, and is not associated with the heel or stem-end more than with any 

 other part. It very frequently originates at obvious wounds or lesions of 

 the skin, but often such injuries are not apparent where infection has 

 occurred, and, as will be shown later (see p. 213), infection can occur in the 

 absence of damage of this kind. A single tuber may be affected in several 

 distinct regions at one and the same time, but as time progresses the 

 decayed regions approach one another, and ultimately coalesce. 



In the earliest visible stages of attack the skin becomes somewhat 

 darker in colour than usual, and is slightly sunken. As time goes on the 

 diseased area spreads, the sunken dark skin becomes folded or ridged into 

 more or less parallel or concentric wrinkles, while fungus pustules break 

 out through it. During this time the rot also penetrates more deeply into 

 the inner tissues of the tuber. (See fig. 1, Plate VI.) 



The rot as a rule proceeds until the tuber is entirely destroyed. During 

 its progress much water is lost, the tuber shrinks, and finally becomes light 

 in weight, dry, and quite hard. In this condition it is quite difficult to cut it 

 with a knife. (See fig. 2, Plate VI.) 



If a partially rotted tuber be cut open, it will be observed that the older 

 decayed tissues are rather dry and brown in colour, but the browning has 



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