Murphy— On the Cause of Rolling in Potato Foliage. 165 



The trials were continued in 1922 with a slight variation in method. The 

 leaflets from each pair of plants of the same variety (diseased and healthy) were 

 cut from corresponding lower leaves in the evening (5 p.m.), and were kept in 

 the dark with their stalks in water for a period of from sixteen to forty hours. 

 The latter period was generally used, and was found to give a, better differentia- 

 tion. In these experiments, as in the previous year, leaflets of twenty-three 

 varieties were submitted to the iodine test during the months of June and July, 

 and in all cases without exception a pronounced difference between the diseased 

 and healthy leaflets was observed. In some eases whole leaves were used instead 

 of leaflets, with the same results. The healthy leaflets were generally free from 

 starch, but sometimes they showed a slight brownish mottling, indicating its 

 presence in small quantities ; the diseased leaflets, on the other hand, presented 

 an intense blue-black colour, which, as a rule, was practically uniform all over. 

 The bases of the mid-ribs and adjacent lateral veins sometimes contained no 

 starch, and more rarely practically clear patches alternated with deep blue-black 

 ones on other portions of the leaflets. 



Most of the better-known varieties of potato v/ere used in these experiments, 

 including those of long standing and others of more recent introduction. Some 

 of these were early, some mid-season, and some late sorts. 



Since the varieties from which positive results were secured in 1922 included 

 the only two exceptional ones of 1921 (when their condition in respect of leaf- 

 roll infection was felt to be open to some doubt on account of the exceptional 

 drought then prevailing), it seems safe to conclude that an abnormal accumula- 

 tion of starch in the lower leaves always accompanies leaf -roll ; and, incidentally, 

 this circumstance provides a convenient symptom for the accurate diagnosis 

 of the disease. Further, the iodine test has been tried and found to give satis- 

 factory results on diseased and healthy lower leaves of the same variety 

 purposely sent in by post in the ordinary way from the West of Ireland for 

 examination. In such cases the difference in the colour of the two leaves, after 

 boiling and treating with iodine, can be observed clearly by transmitted light 

 without preliminary extraction of the chlorophyll. It was found previously 

 that this could be done on material gathered direct and not sent through the 

 post; and the omission of the bleaching part of the test saves both time and 

 alcohol. 



III. — Effects of the Accumulation of Starcli on the Conformation and 

 Structure of the Leaves. 



A number of experiments were carried out to determine, if possible, the 

 relationship in respect of caiase and effect between starch accumulation and 

 rolling of the leaves. A beginning was made by the examination during May 

 and early June, 1922 (before any visible symptoms of leaf-roll appeared), of 

 the starch content of the lower leaves of plants derived from, diseased tubers.' 

 All the plants examined, as was to be expected, afterwards developed normal 

 symptoms of leaf-roll. With them were compared corresponding leaves from 

 healthy plants, and, in most cases, similar leaves which had already begun to 



'In the apparent absence of recognisable symptoms of the leaf-roll disease in the tubers, 

 or at least of such as are of constant occurrence, the presence of infection in any given 

 tuber can be assumed only when the plant of which it is the progeny was definitely known 

 to be diseased in the previous season. Throughout this paper the expression "diseased 

 tubers" is used in this sense. Similarly "healthy tubers" are such as were known to be 

 produced in the preceding season by plants which were apparently free from leaf-roll, and 

 lyJiich were as little as possible exposed to infection. 



2h2 



