1905.] 



Blackleg in Potatoes. 



297 



the stem is examined when the leaves commence to droop its 

 surface will be found to be more or less covered with brownish 

 stains. This discolouration gradually extends up the stem, 

 which finally becomes black and rotten throughout. The 

 number of plants affected in a potato field varies very widely. 

 Diseased plants may be found growing among perfectly healthy 

 ones, but more frequently the disease spreads from one plant to 

 another. 



The disease is primarily caused by a bacterium called Bacillus 

 phytophthorus, but as decay proceeds various kinds of fungi, 

 e.g., moulds, &c, assist in the completion of the work. In some 

 samples of last year's crop (Scottish Triumph) forwarded from 

 Cheshire, which were badly diseased, Fusarium and other fungi 

 were present — apparently the outcome of neglect before storing. 

 The disease spreads with greatest rapidity during hot, damp 

 weather, and is most abundant during the months of June and 

 July. The death of the haulms at this early period of the 

 season, especially in the case of late varieties, means serious 

 loss, not only on account of the scanty crop, but because the 

 tubers also become infected by the bacteria that have been 

 washed into the soil from the rotten haulms. 



Direct infection of the land is caused in this way and by 

 allowing diseased tubers to rot on the land where they grew. 



Indirect infection results when diseased haulms and tubers 

 are placed on the manure heap, or are thrown into the pigstye. 

 In this case the quantity of bacteria returned to the land along 

 with the manure is far greater than in the case of direct 

 infection. 



To prevent infection in any form, all diseased plants should 

 be collected and burned as quickly as possible. The time and 

 labour involved in the collection of the rotten potatoes and the 

 haulm will be well repaid. 



The following measures have been suggested by Dr. Otto 

 Appel, who has studied the disease in Germany : — 



(1.) Potatoes, as well as beans, carrots, turnips, cucumbers, 

 vegetable marrows, sugar-beet, and mangolds, which are also 

 susceptible to the disease, should not be cultivated for two years 

 on land where the disease has occurred. (It has been experi- 

 mentally proved that cereals are not susceptible.) 



