232 



Perpetuation of Potato Disease. [JULY, 



rule, the disease spreads in the first instance from one or more 

 centres, which are indicated by the yellowing of the leaves. If 

 prompt measures are resorted to the disease can be held in 

 check. All plants showing a trace of disease should be removed 

 and burned, and the healthy surrounding plants, or preferably 

 the entire crop, should be dredged with a mixture of powdered 

 quicklime and sulphur, in the proportion of one of lime to two 

 of sulphur. The dredging may be done by placing the mixture 

 in a muslin bag and shaking it over the plants, or by specially 

 made bellows or other contrivances now on the market. The 

 work should be done when the plants are covered with dew. 



It is very important that as far as practicable every trace of 

 diseased onions should be collected and burned, and not thrown 

 on to the manure heap or into the piggery. If such are allowed 

 to decay on the land, a recurrence of the disease is almost 

 certain, as a second form of fungus-fruit is produced in the 

 decaying tissues of the leaves. This fruit remains unchanged 

 until the following season, or, if deeply buried, may remain so 

 for several years, and when again brought to the surface in the 

 ordinary routine of cultivation germination takes place, and if 

 onions happen to form the crop, infection follows. 



The following article by Mr. George Massee on the perpetua- 

 tion of Potato Disease and Potato Leaf Curl by means of 

 hybernating mycelium appears in the Kew 



Perpetuation of Bulletin (No. 4, 1906) :— 

 Potato Disease ^, , , 1 . , 



and Potato sudden and simultaneous appearance 



Leaf Curl. of " Potato-disease," caused by P hytophtho7\% 



infestans, De Bary, over widely extended 



areas in Britain and other countries has hitherto been attributed 



to the rapid production and diffusion of spores during a period 



when special meteorological conditions favoured the rapid 



development of the fungus. 



This explanation, however, when carefully considered, proves 



to be altogether inadequate. When a potato plant infected 



with the spores of Phytophthora is placed under a bell-jar in a 



very damp atmosphere, subdued light, and high temperature— 



