Insect Pests and Fungoid Diseases. 429 



ONION MILDEW (Peronospora schleideni).This disease of the onion 

 is very well-known and wide-spread. It appears first in the form of small 

 yellowish patches on the leaves, which gradually increase in size until the 

 whole of the leaves assume a yellowish, wilted appearance. The surface of 

 the leaves becomes covered with a white powder, 

 which soon changes to a dingy lilac colour. 



The bulb itself is not attacked, but its growth is 

 completely arrested, whilst at the same time the 

 " neck " between the bulb and the base of the leaves 

 rapidly increases in length, this elongation being a 

 certain sign of the existence of the disease. The 

 attack is usually made in the early stages of growth, 

 and then unless remedial measures are at once taken 

 the crop is completely destroyed. 



Treatment : The disease first appears in one or 

 more isolated patches in the crop, which are indicated 

 by the yellowing of the leaves. If prompt measures 

 are adopted at this stage the disease can be held in 

 check. 



Remove entirety- all plants showing a trace of 

 disease and burn them ; be sure to destroy them as 

 if any are left lying about to decay the disease is 

 almost certain to appear again the following season. 

 After the diseased plants are removed spray the 

 remainder with Bordeaux mixture, or dust them with 

 quicklime and sulphur whilst the leaves are wet. 

 Either spraying or dusting should be repeated at 

 intervals of two weeks. 



Onion attacked by 



Mildew. 

 (Reduced.) 



POTATO BLACK-LEG or STEM-ROT (Bacillus phytophthorus).This 

 disease, which is becoming increasingly prevalent, is caused by a bacterium. 

 Affected plants become yellowish and shrivelled from the bottom upwards, 

 and finally die. The stem below the soil 'is first affected ; this turns brown 

 in patches, and the discolouration increases in size and extends up the 

 stem until the whole plant becomes black and rotten. Sometimes isolated 

 plants are affected in the midst of healthy ones, but more frequently the 

 disease spreads from one to another. It is most prevalent during June 

 and July, and appears to be encouraged by close damp weather. 



Treatment: So far, the only known lines of treatment consist of a 

 rotation of crops and the planting of clean seed. Where the disease has 

 occurred the sets should not be cut but should be planted whole, and the 

 use of nitrate of soda or sulphate of ammonia to force growth should be 

 avoided. 



POTATO BLACK SCAB or WART DISEASE (Synchytrium endobio- 

 ticum). This disease has been known for some years, chiefly as attacking 

 crops on allotments or other limited areas where potatoes have been grown 

 on the same ground year after year. Latterly, ordinary field crops have 



