614 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



3. Planting disease-free sets, specially obtained by Messrs. Sutton, 

 raised on land, in 191 1, upon which streak is unknown. 



Whilst not a single case of typical streak appeared in the crop 

 raised at Wisley in 1912, a small percentage of tubers with specked, 

 spotted, or blotched flesh occurred, the marks resembling those charac- 

 teristic of internal disease. Marks of this kind were absent in 1913, 

 when the only disease prevalent was that caused by Phytophthora. 



4. The effect of washing. 



It may first of all be remarked that washing diseased sets would in 

 no case free the tuber from the risk of conveying infection, since the 

 disease is internal. Washing, indeed, might tend to increase the un- 

 healthy condition of these diseased sets, since it was done after they 

 were cut, and render the yield more unsatisfactory. This supposition 

 seems to be supported by the results obtained. On the one hand, 

 the yield from washed diseased sets was only 52 lb., the smallest 

 recorded in the experiment ; and on the other hand 39 cases of streak 

 occurred among 258 tubers, originating from washed diseased sets, 

 whilst 6 cases only arose among 100 tubers produced by the unwashed 

 diseased sets. 



The result obtained in the case of the tubers selected free from 

 streak (lots B and D) is inconclusive. It is true that not a single case 

 of streak occurred among the produce from the washed tubers (628 

 examined), but, on the other hand, the produce from those unwashed 

 yielded only 13 cases (380 examined), or about three per cent. 



5. No increase nor spread of the disease during the storage period 

 could be detected. This result is probably due to the fact that the 

 tubers were kept dry and properly stored in trays through the winter. 



Explanation of Figure 197. 



A. Example of typical streak occurring in the sample of tubers 

 received from Liverpool (see Table V.). 



B. Peculiar spot markings in a tuber sent to Mr. Chittenden from 

 Liverpool in 1912 (see Table V.). This tuber was potted as in the case 

 of A, and yielded only a few small disease-free tubers. 



C. Peculiar spot markings occurring in the pith region, in a tuber 

 from the Liverpool sample (see Table V.). This tuber was potted as 

 in the case of A, and yielded only a few diseased tubers. 



D. Blotched tuber occurring among those received from Liverpool 

 (see Table V.). 



E. Tuber of the variety « Duke of York ' affected with streak. This 

 tuber originated from a disease-free sample grown in Aberdeenshire 

 in 1910, and planted in a farm in East Lothian in 1911. It was lifted 

 diseased in 191 1, potted and placed in the laboratory greenhouse at 

 Wisley, February 10, 191 2 ; the plant produced yielded a few small 

 disease-free tubers. 



